Face

Weakness of the muscles of the face is the most distinguishing sign of FSHD.^[4] It is typically the earliest sign, although it is rarely the initial complaint.^[4] At least mild facial weakness can be found in 90% or more with FSHD.^[29][24] One of the most common deficits is inability to close the eyelids, which can result in sleeping with the eyelids open and dry eyes.^[4] The implicated muscle is the orbicularis oculi muscle.^[4] Another common deficit is inability to purse the lips, causing inability to pucker, whistle, or blow up a balloon.^[4] The implicated muscle is the orbicularis oris muscle.^[4] A third common deficit is inability raise the corners of the mouth, causing a "horizontal smile," which looks more like a grin.^[4] Responsible is the zygomaticus major muscle.^[4]

Weakness of facial muscles contributes to difficulty pronouncing words.^[30] Facial expressions can appear diminished, arrogant, grumpy, or fatigued.^[4] Muscles used for chewing and moving the eyes are not affected.^[24][14] Difficulty swallowing is not typical, although can occur in advanced cases.^[31][30] FSHD is generally progressive, but it is not established whether facial weakness is progressive or stable throughout life.^[32]

Shoulder, chest, and arm

Bilateral scapular winging, right moreso than left. Left image showing wall push test, right image showing attempted shoulder flexion.

After the facial weakness, weakness usually develops in the muscles of the chest and those that span from scapula to thorax. Symptoms involving the shoulder, such as difficulty working with the arms overhead, are the initial complaint in 80% of cases.^[24][14] Predominantly, the serratus anterior and middle and lower trapezii muscles are affected;^[4] the upper trapezius is often spared.^[14] Trapezius weakness causes the scapulas to become downwardly rotated and protracted, resulting in winged scapulas, horizontal clavicles, and sloping shoulders; arm abduction is impaired. Serratus anterior weaknesss impairs arm flexion, and worsening of winging can be demonstrated when pushing against a wall. Muscles spanning from the scapula to the arm are generally spared, which include deltoid and the rotator cuff muscles.^[33][34] The deltoid can be affected later on, especially the upper portion.^[4]

Severe muscle wasting can make bones and spared shoulder muscles very visible, a characteristic example being the "poly-hill" sign elicited by arm elevation.^[4] The first "hill" or bump is the upper corner of scapula appearing to "herniate" up and over the rib cage. The second hill is the AC joint, seen between a wasted upper trapezius and wasted upper deltoid. The third hill is the lower deltoid, distinguishable between the wasted upper deltoid and wasted humeral muscles.^[4] Shoulder weakness and pain can in turn lead to shoulder instability, such as recurrent dislocation, subluxation, or downward translation of the humeral head.^[35]

Also affected is the chest, particularly the parts of the pectoralis major muscle that connect to the sternum and ribs. The part that connects to the clavicle is less often affected. This muscle wasting pattern can contribute to a prominent horizontal anterior axillary fold.^[36][4] Beyond this point the disease does not progress further in 30% of familial cases.^[24][14] After upper torso weakness, weakness can "descend" to the upper arms (biceps muscle and, particularly, the triceps muscle).^[24] The forearms are usually spared, resulting in an appearance some compare to the fictional character Popeye,^[4] although when the forearms are affected in advanced disease, the wrist extensors are more often affected.^[24]

Lower body and trunk

After the upper body, weakness can next appear in either the pelvis, or it "skips" the pelvis and involves the tibialis anterior (shin muscle), causing foot drop. One author considers the pelvic and thigh muscles to be the last group affected.^[24] Pelvic weakness can manifest as a Trendelenburg's sign.^[4] Weakness of the back of the thigh (hamstrings) is more common than weakness of the front of the thigh (quadriceps).^[4] In more severe cases, especially infantile FSHD, there can be anterior pelvic tilt, with associated hyperextension of the knees.^[37]

Weakness can also occur in the abdominal muscles and paraspinal muscles, which can manifest as a protuberant abdomen and lumbar hyperlordosis.^[2][4] Abdominal weakness can cause inability to do a sit-up or the inability to turn from one side to the other while lying on one's back.^[4] Of the rectus abdominis muscle, the lower portion is preferentially affected, manifesting as a positive Beevor's sign.^[4][2] In advanced cases, neck extensor weakness can cause the head to lean towards the chest, termed head drop.^[24]

Non-muscular

Funduscopy of the retinal: (A) normal blood vessels (B) tortuous blood vessels, as often seen with FSHD

The most common non-musculoskeletal manifestation of FSHD is abnormalities in the small arteries (arterioles) in the retina. Tortuosity of the arterioles is seen in approximately 50% of those with FSHD. Less common arteriole abnormalities include telangiectasias and microaneurysms.^[38][39] These abnormalities of arterioles usually do not affect vision or health, although a severe form of it mimics Coat's disease, a condition found in about 1% of FSHD cases and more frequently associated with large 4q35 deletions.^[2][40] High-frequency sensorineural hearing loss can occur in those with large 4q35 deletions, but otherwise is no more common compared to the general population.^[2] Large 4q35 deletion can lead to various other rare manifestations.^[41]

Scoliosis can occur, thought to result from weakness of abdominal, hip extensor, and spinal muscles.^[42][43] Conversely, scoliosis can be viewed as a compensatory mechanism to weakness.^[42] Breathing can be affected, associated with kyphoscoliosis and wheelchair use; it is seen in one-third of wheelchair-using patients.^[2] However, ventilator support (nocturnal or diurnal) is needed in only 1% of cases.^[2][44] Although there are reports of increased risk of cardiac arrhythmias, general consensus is that the heart is not affected.^[14]

DUX4 and the D4Z4 repeat array

D4Z4 array with three D4Z4 repeats and the 4qA allele.^[27]

CEN	centromeric end	TEL	telomeric end
NDE box	non-deleted element	PAS	polyadenylation site
triangle	D4Z4 repeat	trapezoid	partial D4Z4 repeat
white box	pLAM	gray boxes	DUX4 exons 1, 2, 3
arrows
corner	promoters	straight	RNA transcripts
black	sense	red	antisense
blue	DBE-T	dashes	dicing sites

DUX4 resides within the D4Z4 macrosatellite repeat array, a series of tandemly repeated DNA segments in the subtelomeric region (4q35) of chromosome 4.^[47] Each D4Z4 repeat is 3.3 kilobase pairs (kb) long and is the site of epigenetic regulation, containing both heterochromatin and euchromatin structures.^[48][49] In FSHD, the heterochromatin structure is lost, becoming euchromatin,^[48] which consists of less methylation of DNA, and altered methylation of histones.^[50] Histone methylation patterns differ slightly between FSHD1 and FSHD2.^[50]

The subtelomeric region of chromosome 10q contains a tandem repeat structure highly homologous (99% identical) to 4q35,^[8][46] containing "D4Z4-like" repeats with protein-coding regions identical to DUX4 (D10Z10 repeats and DUX4L10, respectively).^[8][51] Because 10q usually lacks a polyadenylation sequence, it is usually not implicated in disease. However, chromosomal rearrangements can occur between 4q and 10q repeat arrays, and involvement in disease is possible if a 4q D4Z4 repeat and polyadenylation signal are transferred onto 10q,^[52][8][53] or if rearrangement causes FSHD1.

DUX4 consists of three exons. Exons 1 and 2 are in each repeat. Exon 3 is in the pLAM region telomeric to the last partial repeat.^[8][7] Multiple RNA transcripts are produced from the D4Z4 repeat array, both sense and antisense. Some transcripts might be degraded in areas to produce si-like small RNAs.^[27] Some transcripts that originate centromeric to the D4Z4 repeat array at the non-deleted element (NDE), termed D4Z4 regulatory element transcripts (DBE-T), could play a role in DUX4 derepression.^[27][54] One proposed mechanism is that DBE-T leads to the recruitment of the trithorax-group protein Ash1L, an increase in H3K36me2-methylation, and ultimately de-repression of 4q35 genes.^[55]

FSHD1

FSHD involving deletion of D4Z4 repeats (termed 'D4Z4 contraction') on 4q is classified as FSHD1, which accounts for 95% of FSHD cases.^[2] Typically, chromosome 4 includes between 11 and 150 D4Z4 repeats.^[48][8] In FSHD1, there are 1–10 D4Z4 repeats.^[8] The number of repeats is roughly inversely related to disease severity. Namely, those with 8 - 10 repeats tend to have the mildest presentations, sometimes with no symptoms; those with 4 - 7 repeats have moderate disease that is highly variable; and those with 1 - 3 repeats are more likely to have severe, atypical, and early-onset disease.^[56] Deletion of the entire D4Z4 repeat array does not result in FSHD because then there are no complete copies of DUX4 to be expressed, although other birth defects result.^[57][8] One contracted D4Z4 repeat array with an adjoining 4qA allele is sufficient to cause disease, so inheritance is autosomal dominant. De novo (new) mutations are implicated in 10 - 30% of cases,^[4] up to 40% of which exhibit somatic mosaicism.^[14] In an individual with mosaic FSHD, the severity of disease is correlated to the proportion of their cells carrying the mutation.^[14]

It has been proposed that FSHD1 undergoes anticipation, a phenomenon primarily associated with trinucleotide repeat disorders in which disease manifestation worsens with each subsequent generation.^[58] As of 2019, more detailed studies are needed to definitively show whether or not anticipation occurs.^[59] If anticipation does occur in FSHD, the mechanism is different than that of trinucleotide repeat disorders, since D4Z4 repeats are much larger than trinucleotide repeats, an underabundance of repeats (rather than overabundance) causes disease, and the repeat array size in FSHD is stable across generations.^[60]

D4Z4 array examples, with each D4Z4 repeat represented by a triangle. The circles above the triangles represent DNA methylation, which determine DNA packaging as represented by the circles in line with the triangles.

FSHD2

FSHD without D4Z4 contraction is classified as FSHD2, which constitutes 5% of FSHD cases.^[2] Various mutations cause FSHD2, all of chromatin modifier genes that result in D4Z4 hypomethylation, at which the genetic mechanism converges with FSHD1.^[61][10] Approximately 80% of FSHD2 cases are due to deactivating mutations in the gene SMCHD1 (structural maintenance of chromosomes flexible hinge domain containing 1) on chromosome 18. SMCHD1 is responsible for DNA methylation, and its deactivation results in hypomethylation of the D4Z4 repeat array.^[2] Specific mutations of SMCHD1 are also associated with Bosma arhinia and microphtalmia syndrome.^[50] Another cause of FSHD2 is mutation in DNMT3B (DNA methyltransferase 3B), which also plays a role in DNA methylation.^[62][63] Mutations in DNMT3B can also cause ICF syndrome.^[50] As of 2020, early evidence indicates that a third cause of FSHD2 is mutation in both copies of the LRIF1 gene, which encodes the protein ligand-dependent nuclear receptor-interacting factor 1 (LRIF1).^[64] LRIF1 is known to interact with the SMCHD1 protein.^[64] As of 2019, there are presumably additional mutations at other unidentified genetic locations that can cause FSHD2.^[2]

Mutation of a single allele of SMCHD1 or DNMT3B can cause disease. Mutation of both copies LRIF1 has been tentatively shown to cause disease in a single person as of 2020.^[64] As in FSHD1, a 4qA allele must be present for disease to result. However, unlike the D4Z4 array, the genes implicated in FSHD2 are not in proximity with the 4qA allele, and so they are inherited independently from the 4qA allele, resulting in a digenic inheritance pattern. For example, one parent without FSHD can pass on an SMCHD1 mutation, and the other parent, also without FSHD, can pass on a 4qA allele, bearing a child with FSHD2.^[61][63]

Two ends of a disease spectrum

FSHD1 and FSHD2 have been traditionally viewed as separate entities with distinct genetic causes (albeit, the downstream genetic mechanisms merge).^[65] Alternatively, the genetic causes of FSHD1 and FSHD2 can be viewed as risk factors, each contributing to an FSHD disease spectrum.^[65] Not rarely, an affected individual seems to have contributions from both.^[56] For example, in those with FSHD2, although they have do not have a 4qA allele with D4Z4 repeat number less than 11, they still often have one less than 17 (relatively short compared to the general population), suggesting that a large number of D4Z4 repeats can prevent the effects of an SMCHD1 mutation.^[56] Further studies are needed to determine the upper limit of D4Z4 repeats in FSHD2.^[56]

In those with FSHD1 and FSHD2, that is, having 10 or fewer repeats with an adjacent 4qA allele and an SMCHD1 mutation, the disease manifests more severely, illustrating that the effects of each mutation are additive.^[66] A combined FSHD1/FSHD2 presentation is most common in those with 9 - 10 repeats. A possible explanation is that a sizable portion of the general population has 9 - 10 repeats with difficult to detect or no disease, yet with the additive effect of an SMCHD1 mutation, symptoms are severe enough for a diagnosis to be made.^[56] The 9 - 10 repeat size can be considered as an overlap zone between FSHD1 and FSDH2.^[56]

Molecular

DUX4 signaling in FSHD-affected skeletal muscle.

As of 2020, there seems to be a consensus that aberrant expression of DUX4 in muscle is the cause of FSHD.^[67] DUX4 is expressed in extremely small amounts, detectable in 1 out of every 1000 immature muscle cells (myoblast), which appears to increase after myoblast maturation, in part because the cells fuse as they mature, and a single nucleus expressing DUX4 can provide DUX4 protein to neighboring nuclei from fused cells.^[68]

It remains an area of active research how DUX4 protein causes muscle damage.^[69] DUX4 protein is a transcription factor that regulates many other genes. Some of these genes are involved in apoptosis, such as p53, p21, MYC, and β-catenin, and indeed it seems that DUX4 protein makes muscle cells more prone to apoptosis. Other DUX4 protein-regulated genes are involved in oxidative stress, and indeed it seems that DUX4 expression lowers muscle cell tolerance of oxidative stress. Variation in the ability of individual muscles to handle oxidative stress could partially explain the muscle involvement patterns of FSHD. DUX4 protein downregulates many genes involved in muscle development, including MyoD, myogenin, desmin, and PAX7, and indeed DUX4 expression has shown to reduce muscle cell proliferation, differentiation, and fusion. DUX4 protein regulates a few genes that are involved in RNA quality control, and indeed DUX4 expression has been shown to cause accumulation of RNA with subsequent apoptosis.^[67] Estrogen seems to play a role in modifying DUX4 protein effects on muscle differentiation, which could explain why females are lesser affected than males, although it remains unproven.^[70]

The cellular hypoxia response has been reported in a single study to be the main driver of DUX4 protein-induced muscle cell death. The hypoxia-inducible factors (HIFs) are upregulated by DUX4 protein, possibly causing pathologic signaling leading to cell death.^[71]

Another study found that DUX4 expression in muscle cells led to the recruitment and alteration of fibrous/fat progenitor cells, which helps explain why muscles become replaced by fat and fibrous tissue.^[68]

A single study implicated RIPK3 in DUX4-mediated cell death.^[72]

Muscle histology

Microscopic cross-sectional views of FSHD-affected muscle fibers. Visible is inflammation and fibrosis, as well as muscle fiber shape change, death, and regeneration.

Unlike other muscular dystrophies, early muscle biopsies show only mild degrees of fibrosis, muscle fiber hypertrophy, and displacement of nuclei from myofiber peripheries (central nucleation).^[27] More often found is inflammation.^[27] There can be endomysial inflammation, primarily composed of CD8+ T-cells, although these cells do not seem to directly cause muscle fiber death.^[27] Endomysial blood vessels can be surrounded by inflammation, which is relatively unique to FSHD, and this inflammation contains CD4+ T-cells.^[27] Inflammation is succeeded by deposition of fat (fatty infiltration), then fibrosis.^[73][27] Individual muscle fibers can appear whorled, moth-eaten, and, especially, lobulated.^[74]

Muscle involvement pattern

Why certain muscles are preferentially affected in FSHD remains unknown. There are multiple trends of involvement seen in FSHD, possibly hinting at underlying pathophysiology. Individual muscles can weaken while adjacent muscles remain strong.^[75] The right shoulder and arm muscles are more often affected than the left upper extremity muscles, a pattern also seen in Poland syndrome and hereditary neuralgic amyotrophy; this could reflect a genetic, developmental/anatomic, or functional-related mechanism.^[24][25] The deltoid is often spared, which is not seen in any other condition that affects the muscles around the scapula.^[32]

Medical imaging (CT and MRI) have shown muscle involvement not readily apparent otherwise^[33]

• A single MRI study shows the teres major muscle to be commonly affected.^[34]
• The semimembranosus muscle, part of the hamstrings, is commonly affected,^[25][76][77] deemed by one author to be "the most frequently and severely affected muscle."^[2]
• Of the quadriceps muscles, the rectus femoris is preferentially affected^[76]
• Of the gastrocnemius, the medial section is preferentially affected;^[76][77]
• The iliopsoas, a hip flexor muscle, is very often spared.^[77][2]

Retinopathy

Tortuosity of the retinal arterioles, and less often microaneurysms and telangiectasia, are commonly found in FSHD.^[38] Abnormalities of the capillaries and venules are not observed.^[38] One theory for why the arterioles are selectively affected is that they contain smooth muscle.^[38] The degree of D4Z4 contraction correlates to the severity of tortuosity of arterioles.^[38] It has been hypothesized that retinopathy is due to DUX4-protein-induced modulation of the CXCR4–SDF1 axis, which has a role in endothelial tip cell morphology and vascular branching.^[38]

Genetic testing

Genetic testing is the gold standard for FSHD diagnosis, as it is the most sensitive and specific test available.^[2] Commonly, FSHD1 is tested for first.^[2] A shortened D4Z4 array length (EcoRI length of 10 kb to 38 kb) with an adjacent 4qA allele supports FSHD1.^[2] If FSHD1 is not present, commonly FSHD2 is tested for next by assessing methylation at 4q35.^[2] Low methylation (less than 20%) in the context of a 4qA allele is sufficient for diagnosis.^[2] The specific mutation, usually one of various SMCHD1 mutations, can be identified with next-generation sequencing (NGS).^[79]

Assessing D4Z4 length

Measuring D4Z4 length is technically challenging due to the D4Z4 repeat array consisting of long, repetitive elements.^[80] For example, NGS is not useful for assessing D4Z4 length, because it breaks DNA into fragments before reading them, and it is unclear from which D4Z4 repeat each sequenced fragment came.^[4] In 2020, optical mapping became available for measuring D4Z4 array length, which is more precise and less labor-intensive than southern blot.^[81] Molecular combing is also available for assessing D4Z4 array length.^[82] These methods, which physical measure the size of the D4Z4 repeat array, require specially prepared high quality and high molecular weight genomic DNA (gDNA) from serum, increasing cost and reducing accessibility to testing.^[83]

Diagram showing restriction enzyme sites used to differentiate between D4Z4 repeat arrays of 4q and 10q.

Restriction fragment length polymorphism (RFLP) analysis was the first genetic test developed and is still used as of 2020, although it is being phased out by newer methods. It involves dicing the DNA with restriction enzymes and sorting the resulting restriction fragments by size using southern blot. The restriction enzymes EcoRI and BlnI are commonly used. EcoRI isolates the 4q and 10q repeat arrays, and BlnI dices the 10q sequence into small pieces, allowing 4q to be distinguished.^[4][46] The EcoRI restriction fragment is composed of three parts: 1) 5.7 kb proximal part, 2) the central, variable size D4Z4 repeat array, and 3) the distal part, usually 1.25 kb.^[84] The proximal portion has a sequence of DNA stainable by the probe p13E-11, which is commonly used to visualize the EcoRI fragment during southern blot.^[46] The name "p13E-11" reflects that it is a subclone of a DNA sequence designated as cosmid 13E during the human genome project.^[85][86] Considering that each D4Z4 repeat is 3.3 kb, and the EcoRI fragment contains about 5 kb of DNA that is not part of the D4Z4 repeat array, the number of D4Z4 units can be calculated.^[70]

D4Z4 repeats = (EcoRI length - 5) / 3.3

Sometimes 4q or 10q will have a combination of D4Z4 and D4Z4-like repeats due to DNA exchange between 4q and 10q, which can yield erroneous results, requiring more detailed workup.^[46] Sometimes D4Z4 repeat array deletions can include the p13E-11 binding site, warranting use of alternate probes.^[46][87]

Auxiliary testing

MRI showing asymmetrical involvement of various muscles in FSHD

Other tests can support the diagnosis of FSHD, although they are all less sensitive and less specific than genetic testing.^[89][4] Nonetheless, they can rule out similar-appearing conditions.^[14]

• Creatine kinase (CK) blood level is often ordered when muscle damage is suspected. CK is an enzyme found in muscle, leaking into the blood when muscles become damaged. In FSHD, CK level is normal to mildly elevated,^[2] never exceeding five times the upper limit of normal.^[4]
• Electromyogram (EMG) measures the electrical activity in the muscle. EMG can show nonspecific signs of muscle damage or irritability.^[2]
• Nerve conduction velocity (NCV) measures the how fast signals travel along a nerve. The nerve signals are measured with surface electrodes (similar to those used for an electrocardiogram) or needle electrodes.
• Muscle biopsy is the surgical removal and examination of a small piece of muscle, usually from the arm or leg. Microscopy and a variety of biochemical tests are used for examination. Findings in FSHD are nonspecific, such as presence of white blood cells or variation in muscle fiber size. This test is rarely indicated.^[2]
• Muscle MRI is sensitive for detecting muscle damage, even in mild cases. T1-weighted MRI imaging can visualize fatty infiltration of muscles, and T2-weighted MRI imaging can visualize muscle edema. Because of the particular muscle involvement patterns of FSHD, MRI can help differentiate FSHD from other muscle diseases, directing genetic testing.^[33][34]

Differential diagnosis

Included in the differential diagnosis of FSHD are limb-girdle muscular dystrophy (especially calpainopathy),^[2] scapuloperoneal myopathy,^[90] mitochondrial myopathy,^[2] Pompe disease,^[2] and polymyositis.^[2] Calpainopathy and scapuloperoneal myopathy, like FSHD, present with scapular winging.^[90] Features that suggest FSHD are facial weakness, asymmetric weakness, and lack of benefit from immunosuppression medications.^[2] Features the suggest an alternative diagnosis are contractures, respiratory insufficiency, weakness of muscles controlling eye movement, and weakness of the tongue or throat.^[14]

Screening and monitoring of complications

The American Academy of Neurology (AAN) recommends several medical tests to detect complications of FSHD.^[91] A dilated eye exam to look for retinal abnormalities is recommended in those newly diagnosed with FSHD; for those with large D4Z4 deletions, an evaluation by a retinal specialist is recommended yearly.^[92][2] A hearing test is recommended for individuals with early-onset FSHD prior to starting school, or for any other FSHD-affected individual with symptoms of hearing loss.^[92][2] Pulmonary function testing (PFT) is recommended in those newly diagnosed to establish baseline pulmonary function,^[2] and recurrently for those with pulmonary insufficiency symptoms or risks.^[92][2] Routine screening for heart conditions, such as through an electrocardiogram (EKG) or echocardiogram (echo), is considered unnecessary in those without symptoms of heart disease.^[91]

Physical and occupational therapy

Aerobic exercise has been shown to reduce chronic fatigue and decelerate fatty infiltration of muscle in FSHD.^[93][94] Physical activity in general might slow disease progression in the legs.^[10] The AAN recommends that people with FSHD engage in low-intensity aerobic exercise to promote energy levels, muscle health, and bone health.^[2] Moderate-intensity strength training appears to do no harm, although it has not been shown to be beneficial.^[95] Physical therapy can address specific symptoms; there is no standardized protocol for FSHD. Anecdotal reports suggest that appropriately applied kinesiology tape can reduce pain.^[96] Occupational therapy can be used for training in activities of daily living (ADLs) and to help adapt to new assistive devices. Cognitive behavioral therapy (CBT) has been shown to reduce chronic fatigue in FSHD, and it also decelerates fatty infiltration of muscle when directed towards increasing daily activity.^[93][94]

Braces are often used to address muscle weakness. Scapular bracing can improve scapular positioning, which improves shoulder function, although it is often deemed as ineffective or impractical.^[97] Ankle-foot orthoses can improve walking, balance, and quality of life.^[98]

Pharmacologic management

No pharmaceuticals have definitively proven effective for altering the disease course.^[91] Although a few pharmaceuticals have shown improved muscle mass in limited respects, they did not improve quality of life, and the AAN recommends against their use for FSHD.^[91]

Reconstructive surgery

Scapular winging is amenable to surgical correction, namely operative scapular fixation. Scapular fixation is restriction and stabilization of the position of the scapula, putting it in closer apposition to the rib cage and reducing winging. Absolute restriction of scapular motion by fixation of the scapula to the ribs is most commonly reported.^[99] This procedure often involves inducing bony fusion, called arthrodesis, between the scapula and ribs. Names for this include scapulothoracic fusion, scapular fusion, and scapulodesis. This procedure increases arm active range of motion, improves arm function, decreases pain, and improves cosmetic appearance.^[100][101] Active range of motion of the arm increases most in the setting of severe scapular winging with an unaffected deltoid muscle;^[11] however, passive range of motion decreases. In other words, the patient gains the ability to slowly raise their arms to 90+ degrees, but they lose the ability to "throw" their arm up to a full 180 degrees.^[2] The AAN states that scapular fixation can be offered cautiously to select patients after balancing these benefits against the adverse consequences of surgery and prolonged immobilization.^[91][10]

Another form of operative scapular fixation is scapulopexy. "Scapulo-" refers to the scapula bone, and "-pexy" is derived from the Greek root "to bind." Some versions of scapulopexy accomplish essentially the same result as scapulothoracic fusion, but instead of inducing bony fusion, the scapula is secured to the ribs with only wire, tendon grafts, or other material. Some versions of scapulopexy do not completely restrict scapular motion, examples including tethering the scapula to the ribs, vertebrae, or other scapula.^[99][102] Scapulopexy is considered to be more conservative than scapulothoracic fusion, with reduced recovery time and less effect on breathing.^[99] However, they also seem more susceptible to long-term failure.^[99] Another form of scapular fixation, although not commonly done in FSHD, is tendon transfer, which involves surgically rearranging the attachments of muscles to bone.^{[99][103][104]} Examples include pectoralis major transfer and the Eden-Lange procedure.

Various other surgical reconstructions have been described. Upper eyelid gold implants have been used for those unable to close their eyes.^[105] Drooping lower lip has been addressed with plastic surgery.^[106] Select cases of foot drop can be surgically corrected with tendon transfer, in which the tibialis posterior muscle is repurposed as a tibialis anterior muscle, a version of this being called the Bridle procedure.^{[107][108][96]} Severe scoliosis caused by FSHD can be corrected with spinal fusion; however, since scoliosis might be a compensatory change in response to muscle weakness, correction of spinal alignment can result in further impaired muscle function.

• Scapular winging management
• 
  Kinesiology tape applied across the scapulas.
• 
  A cloth brace to hold the scapulas in retraction to reduce shoulder symptoms, such as collarbone pain.
• 
  Scapula-to-scapula scapulopexy, pre- and post-operation. The scapulas are tethered together into a retracted position with an Achilles tendon graft, which, in this case, rendered the rhomboid major muscles distinguishable.

Pregnancy

Pregnancy outcomes are overall good in mothers with FSHD; there is no difference in rate of preterm labor, rate of miscarriage, and infant outcomes.^[111] However, weakness can increase the need for assisted delivery.^[111]

A single review found that weakness worsens, without recovery, in 12% of mothers with FSHD during pregnancy, although this might be due to weight gain or deconditioning.^[111]

Chronology of important FSHD-related genetic research

1861	Person with muscular dystrophy depicted by Duchenne. Based on the muscles involved, this person could have had FSHD.
1884	Landouzy and Dejerine describe a form of childhood progressive muscle atrophy with a characteristic involvement of facial muscles and distinct from pseudohypertrophic (Duchenne's MD) and spinal muscle atrophy in adults.[118]
Two brothers with FSHD followed by Landouzy and Dejerine Photograph of one brother at age 21. The right scapula is protracted, downwardly rotated, and laterally displaced. Drawing of another brother at age 17. Visible is lumbar hyperlordosis. The upper arm and pectoral muscles appear atrophied.
1886	Landouzy and Dejerine describe progressive muscular atrophy of the scapulo-humeral type.[119]
1950	Tyler and Stephens study 1249 individuals from a single kindred with FSHD traced to a single ancestor and describe a typical Mendelian inheritance pattern with complete penetrance and highly variable expression. The term facioscapulohumeral dystrophy is introduced.[120]
1982	Padberg provides the first linkage studies to determine the genetic locus for FSHD in his seminal thesis "Facioscapulohumeral disease."[23]
1987	The complete sequence of the Dystrophin gene (Duchenne's MD) is determined.[121]
1991	The genetic defect in FSHD is linked to a region (4q35) near the tip of the long arm of chromosome 4.[122]
1992	FSHD, in both familial and de novo cases, is found to be linked to a recombination event that reduces the size of 4q EcoR1 fragment to < 28 kb (50–300 kb normally).[85]
1993	4q EcoR1 fragments are found to contain tandem arrangement of multiple 3.3-kb units (D4Z4), and FSHD is associated with the presence of < 11 D4Z4 units.[84] A study of seven families with FSHD reveals evidence of genetic heterogeneity in FSHD.[123]
1994	The heterochromatic structure of 4q35 is recognized as a factor that may affect the expression of FSHD, possibly via position-effect variegation.[124] DNA sequencing within D4Z4 units shows they contain an open reading frame corresponding to two homeobox domains, but investigators conclude that D4Z4 is unlikely to code for a functional transcript.[124][125]
1995	The terms FSHD1A and FSHD1B are introduced to describe 4q-linked and non-4q-linked forms of the disease.[126]
1996	FSHD Region Gene1 (FRG1) is discovered 100 kb proximal to D4Z4.[127]
1998	Monozygotic twins with vastly different clinical expression of FSHD are described.[20]
1999	Complete sequencing of 4q35 D4Z4 units reveals a promoter region located 149 bp 5' from the open reading frame for the two homeobox domains, indicating a gene that encodes a protein of 391 amino acid protein (later corrected to 424 aa[128]), given the name DUX4.[129]
2001	Investigators assessed the methylation state (heterochromatin is more highly methylated than euchromatin) of DNA in 4q35 D4Z4. An examination of SmaI, MluI, SacII, and EagI restriction fragments from multiple cell types, including skeletal muscle, revealed no evidence for hypomethylation in cells from FSHD1 patients relative to D4Z4 from unaffected control cells or relative to homologous D4Z4 sites on chromosome 10. However, in all instances, D4Z4 from sperm was hypomethylated relative to D4Z4 from somatic tissues.[130]
2002	A polymorphic segment of 10 kb directly distal to D4Z4 is found to exist in two allelic forms, designated 4qA and 4qB. FSHD1 is associated solely with the 4qA allele.[131] Three genes (FRG1, FRG2, ANT1) located in the region just centromeric to D4Z4 on chromosome 4 are found in isolated muscle cells from individuals with FSHD at levels 10 to 60 times greater than normal, showing a linkage between D4Z4 contractions and altered expression of 4q35 genes.[132]
2003	A further examination of DNA methylation in different 4q35 D4Z4 restriction fragments (BsaAI and FseI) showed significant hypomethylation at both sites for individuals with FSHD1, non-FSHD-expressing gene carriers, and individuals with phenotypic FSHD relative to unaffected controls.[133]
2004	Contraction of the D4Z4 region on the 4qB allele to < 38 kb does not cause FSHD.[134]
2006	Transgenic mice overexpressing FRG1 are shown to develop severe myopathy.[135]
2007	The DUX4 open reading frame is found to have been conserved in the genome of primates for over 100 million years, supporting the likelihood that it encodes a required protein.[136] Researchers identify DUX4 mRNA in primary FSHD myoblasts and identify in D4Z4-transfected cells a DUX4 protein, the overexpression of which induces cell death.[128] DUX4 mRNA and protein expression are reported to increase in myoblasts from FSHD patients, compared to unaffected controls. Stable DUX4 mRNA is transcribed only from the most distal D4Z4 unit, which uses an intron and a polyadenylation signal provided by the flanking pLAM region. DUX4 protein is identified as a transcription factor, and evidence suggests overexpression of DUX4 is linked to an increase in the target paired-like homeodomain transcription factor 1 (PITX1).[137]
2009	The terms FSHD1 and FSHD2 are introduced to describe D4Z4-deletion-linked and non-D4Z4-deletion-linked genetic forms, respectively. In FSHD1, hypomethylation is restricted to the short 4q allele, whereas FSHD2 is characterized by hypomethylation of both 4q and both 10q alleles.[138] Splicing and cleavage of the terminal (most telomeric) 4q D4Z4 DUX4 transcript in primary myoblasts and fibroblasts from FSHD patients is found to result in the generation of multiple RNAs, including small noncoding RNAs, antisense RNAs and capped mRNAs as new candidates for the pathophysiology of FSHD.[139]
2010	A unifying genetic model of FSHD is established: D4Z4 contractions only cause FSHD when in the context of a 4qA allele due to stabilization of DUX4 RNA transcript, allowing DUX4 expression.[8] Several organizations including The New York Times highlighted this research[140] (See FSHD Society). Dr. Francis Collins, who oversaw the first sequencing of the Human Genome with the National Institutes of Health stated:[140] "If we were thinking of a collection of the genome's greatest hits, this would go on the list," Daniel Perez, co-founder of the FSHD Society, hailed the new findings saying: "This is a long-sought explanation of the exact biological workings of [FSHD]" The MDA stated that: "Now, the hunt is on for which proteins or genetic instructions (RNA) cause the problem for muscle tissue in FSHD." One of the report's co-authors, Silvère van der Maarel of the University of Leiden, stated that "It is amazing to realize that a long and frustrating journey of almost two decades now culminates in the identification of a single small DNA variant that differs between patients and people without the disease. We finally have a target that we can go after." DUX4 is found actively transcribed in skeletal muscle biopsies and primary myoblasts. FSHD-affected cells produce a full-length transcript, DUX4-fl, whereas alternative splicing in unaffected individuals results in the production of a shorter, 3'-truncated transcript (DUX4-s). The low overall expression of both transcripts in muscle is attributed to relatively high expression in a small number of nuclei (~ 1 in 1000). Higher levels of DUX4 expression in human testis (~100 fold higher than skeletal muscle) suggest a developmental role for DUX4 in human development. Higher levels of DUX4-s (vs DUX4-fl) are shown to correlate with a greater degree of DUX-4 H3K9me3-methylation.[7]
2012	Some instances of FSHD2 are linked to mutations in the SMCHD1 gene on chromosome 18, and a genetic/mechanistic intersection of FSHD1 and FSHD2 is established.[61] The prevalence of FSHD-like D4Z4 deletions on permissive alleles is significantly higher than the prevalence of FSHD in the general population, challenging the criteria for molecular diagnosis of FSHD.[141] When expressed in primary myoblasts, DUX4-fl acted as a transcriptional activator, producing a > 3-fold change in the expression of 710 genes.[142] A subsequent study using a larger number of samples identified DUX4-fl expression in myogenic cells and muscle tissue from unaffected relatives of FSHD patients, per se, is not sufficient to cause pathology, and that additional modifiers are determinants of disease progression.[143] Mechanism proposed of DBE-T (D4Z4 Regulatory Element transcript) leading to de-repression of 4q35 genes.[55]
2013	Mutations in SMCHD1 are shown to increase the severity of FSHD1.[66] Transgenic mice carrying D4Z4 arrays from an FSHD1 allele (with 2.5 D4Z4 units), although lacking an obvious FSHD-like skeletal muscle phenotype, are found to recapitulate important genetic expression patterns and epigenetic features of FSHD.[144]
2014	DUX4-fl and downstream target genes are expressed in skeletal muscle biopsies and biopsy-derived cells of fetuses with FSHD-like D4Z4 arrays, indicating that molecular markers of FSHD are already expressed during fetal development.[145] Researchers "review how the contributions from many labs over many years led to an understanding of a fundamentally new mechanism of human disease" and articulate how the unifying genetic model and subsequent research represent a "pivot-point in FSHD research, transitioning the field from discovery-oriented studies to translational studies aimed at developing therapies based on a sound model of disease pathophysiology." They describe the consensus mechanism of pathophysiology for FSHD as an "inefficient repeat-mediated epigenetic repression of the D4Z4 macrosatellite repeat array on chromosome 4, resulting in the variegated expression of the DUX4 retrogene, encoding a double-homeobox transcription factor, in skeletal muscle."[27]

Past pharmaceutical development

Early drug trials, before the pathogenesis involving DUX4 was discovered, were untargeted and largely unsuccessful.^[19] Compounds were trialed with goals of increasing muscle mass, decreasing inflammation, or addressing provisional theories of disease mechanism.^[19] The following drugs failed to show efficacy:

• Prednisone, a steroid, based on its antiinflammatory properties and therapeutic effect in Duchenne muscular dystrophy.^[146]
• Oral albuterol, a β2 agonist, on the basis of its anabolic properties. Although it improved muscle mass and certain measures of strength in those with FSHD, it did not improve global strength or function.^{[147][148][149]} Interestingly, β2 agonists were later shown to reduce DUX4 expression.^[150]
• Diltiazem, a calcium channel blocker, on the bases of anecdotal reports of benefit and the theory that calcium dysregulation played a part in muscle cell death.^[151]
• MYO-029 (Stamulumab), an antibody that inhibits myostatin, was developed to promote muscle growth. Myostatin is a protein that inhibits the growth of muscle tissue.^[152]
• ACE-083, a TGF-β inhibitor, was developed to promote muscle growth.^[153]

Media

• In the Amazon Video series The Man in the High Castle, Obergruppenführer John Smith's son, Thomas, is diagnosed with Landouzy-Dejerine syndrome.^[154]
• In the biography Stuart: A Life Backwards, the protagonist was affected by muscular dystrophy,^[155] presumably FSHD.

Patient and research organizations

• The FSHD Society (named "FSH Society" until 2019)^[156] was founded in 1991 on the East Coast by two individuals with FSHD, Daniel Perez and Stephen Jacobsen.^[157][158] The FSHD Society claims to have advocated for the standardization of the disease name facioscapulohumeral muscular dystrophy and its abbreviation FSHD.^[156] The FSHD Society claims to have raised funding for seed grants for FSHD research and co-wrote the MD-CARE Act of 2001, which provided federal funding for all muscular dystrophies.^[157][158] The FSHD Society has grown into the world's largest grassroots organization advocating for patient education and scientific and medical research for FSHD.^[159][160] One notable spokesperson for FSHD Society has been Max Adler, an actor on the TV series Glee.^[161]
• Friends of FSH Research is a research-oriented nonprofit organization founded in 2004 by Terry and Rick Colella from Kirkland, Washington after their son was diagnosed with FSHD.^{[162][163][164][165]}
• The FSHD Global Research Foundation was founded in 2007 by Bill Moss in Australia, a former Macquarie Bank executive affected by FSHD.^{[166][167][168]} It is currently directed by Moss's daughter.^[167] It was named Australian charity of the year for 2017.^[167] It is the largest funder of FSHD medical research outside of the United States as of 2018.^[168]
• FSHD EUROPE was founded in 2010.^[169] Spanning multiple countries in Europe, it has launched the European Trial Network.^[169]

Notable cases

• Chip Wilson is a Canadian billionaire and founder of Lululemon. He has pledged 100 million Canadian dollars to research through the venture 'Solve FSHD'.^[170]
• Chris Carrino is the radio voice of the Brooklyn Nets. He founded the 'Chris Carrino Foundation for FSHD', oriented towards research funding.^[171]
• Morgan Hoffman is an American professional golfer. He started the Morgan Hoffman Foundation, oriented towards research funding.^[172]

Pharmaceutical development

After achieving consensus on FSHD pathophysiology in 2014, researchers proposed four approaches for therapeutic intervention:^[27]

1. enhance the epigenetic repression of the D4Z4
2. target the DUX4 mRNA, including altering splicing or polyadenylation;
3. block the activity of the DUX4 protein
4. inhibit the DUX4-induced process, or processes, that leads to pathology.

Small molecule drugs

Most drugs used in medicine are "small molecule drugs," as opposed to biologic medical products that include proteins, vaccines, and nucleic acids. Small molecule drugs can typically be taken by ingestion, rather than injection.

• Losmapimod, a selective inhibitor of p38α/β mitogen-activated protein kinases, was identified by Fulcrum Therapeutics as a potent suppressor of DUX4 in vitro.^[174] Results of a phase IIb clinical trial, revealed in June 2021, showed statistically significant slowing of muscle function deterioration. Further trials are pending.^[175][176]
• Casein kinase 1 (CK1) inhibition has been identified by Facio Therapies, a Dutch pharmaceutical company, to repress DUX4 expression and is in preclinical development. Facio Therapies claims that CK1 inhibition leaves myotube fusion intact, unlike BET inhibitors, p38 MAPK inhibitors, and β2 agonists.^[177][178]

Gene therapy

Gene therapy is the administration of nucleotides to treat disease. Multiple types of gene therapy are in the preclinical stage of development for the treatment of FSHD.

• Antisense therapy utilizes antisense oligonucleotides that bind to DUX4 messenger RNA, inducing its degradation and preventing DUX4 protein production. Phosphorodiamidate morpholinos, an oligonucleotide modified to increase its stability, have been shown to selectively reduce DUX4 and its effects; however, these antisense nucleotides have poor ability to penetrate muscle.^[2]
• MicroRNAs (miRNAs) directed against DUX4, delivered by viral vectors, have shown to reduce DUX4, protect against muscle pathology, and prevent loss of grip strength in mouse FSHD models.^[2] Arrowhead pharmaceuticals is developing an RNA interference therapeutic against DUX4, named ARO-DUX4, and intends to file for regulatory clearance in third quarter of 2021 to begin clinical trials.^{[179][180][181]}
• Genome editing, the permanent alteration of genetic code, is being researched. One study tried to use CRISPR-Cas9 to knockout the polyadenylation signal in lab dish models, but was unable to show therapeutic results.^[182]

Potential drug targets

• Inhibition of the hyaluronic acid (HA) pathway is a potential therapy. One study found that many DUX4-induced molecular pathologies are mediated by HA signaling, and inhibition of HA biosynthesis with 4-methylumbelliferone prevented these molecular pathologies.^[183]
• P300 inhibition has shown to inhibit the deleterious effects of DUX4^[184]
• BET inhibitors have been shown to reduce DUX4 expression.^[150]
• Antioxidants could potentially reduce the effects of FSHD. One study found that vitamin C, vitamin E, zinc gluconate, and selenomethionine supplementation increased endurance and strength of the quadriceps, but had no significant benefit on walking performance.^[185] Further study is warranted.^[2]

Outcome measures

Ways of measuring the disease are important for studying disease progression and assessing the efficacy of drugs in clinical trials.

• Quality of life can be measured with questionnaires, such as the FSHD Health Index.^[186][2]
• How the disease affects daily activities can measured with questionnaires, such as the FSHD‐Rasch‐built overall disability scale (FSHD-RODS)^[187] or FSHD composite outcome measure (FSHD-COM).^[188]
• Electrical impedance myography is being studied as a way to measure muscle damage.^[2]
• Muscle MRI is useful for assessment of all the muscles in the body. Muscles can be scored based on the degree of fat infiltration.^[2]