Facioscapulohumeral Dystrophy

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Facioscapulohumeral Dystrophy
Inheritance Autosomal dominant (FSHD 1), digenic (FSHD 2)
Genetics Heterozygous pathogenic contraction of D4Z4 repeat array in chromosome 4q35 (FSHD 1), hypomethylation of D4Z4 repeat array (FSHD 2)
Clinical Features Slowly progressive weakness of facial muscles, scapular stabilisers, arms, and dorsiflexors of the foot.
Treatment Supportive care
Prognosis 20% end up in wheelchair, life expectancy not shortened

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most prevalent forms of muscular dystrophy. The name refers to how FSHD affects muscles of the face (facio), scapula (scapulo), and humerus (humeral). It caan present with a wide range of onset and severity. Treatment options for FSHD are limited, and supportive care is often the main approach. FSHD has two recognized forms, FSHD type 1 (autosomal dominant, 95% of cases) and FSHD type 2 (digenic, 5% of cases) which present similarly. The pathophysiology of FSHD is not fully understood, but it is thought to involve mutations leading to the activation of a normally inactive transcription factor, DUX4.

Clinical Features

FSHD1 and FSHD2 exhibit comparable clinical features, albeit with variable onset and severity. While the most common onset is during adolescence or early adulthood, the disorder may manifest in infancy in FSHD1 or remain asymptomatic in carriers. Although the condition progresses gradually, some patients may experience a sudden loss of specific muscle groups in a few months.

Initial symptoms usually involve the facial and scapular muscles, although MRI findings suggest that the hamstring and abdominal muscles may also be affected early on. Patients commonly present with eyelids that remain partially open during sleep, an inability to pucker their lips, and difficulty whistling. Moreover, they often display an asymmetrical smile. In infants with early-onset FSHD, reduced suckling ability and the absence of a social smile may be observed.

The scapular fixation muscles, namely the rhomboids and serratus anterior, exhibit weakness, leading to scapular medial border winging and upward rise of the rostral border. This, in turn, causes a "poly-hill" sign on arm abduction. Patients with FSHD often have difficulty in gradually abducting or extending their arms to 180 degrees, but they may do so by abruptly throwing their arms up. Pectoralis weakness results in horizontal clavicles and deep axillary creases. Biceps and triceps are disproportionately affected compared to the deltoid and forearm flexors. Weakness of the tibialis anterior causes footdrop. Paraspinal and abdominal muscle weakness leads to lordosis and a bulging abdomen. The Beevor sign occurs when the umbilicus moves rostrally during attempts to sit up. Eventually, muscles such as the forearm flexors and extensors and knee flexors and extensors may become weak. The weakness and wasting are often asymmetric.

Chronic pain and fatigue was found to be present in the majority of individuals with FSHD1 in one study. The regions of pain are shoulders (72%), arms (59%), low back (71%), legs (64%), hips (51%), hands (10%), and feet (34%). Opioid use was common.[1]

Respiratory involvement occurs in up to one-third of nonambulatory FSHD patients, with the most significant impairment in those who are wheelchair-dependent and have kyphoscoliosis. Nocturnal hypoventilation is an early manifestation, and ventilator support is rarely necessary. FSHD is not associated with cardiomyopathy, but conduction defects or arrhythmias may be more prevalent, with incomplete right bundle-branch block found in one-third of patients in one study. Retinal vasculopathy is associated with FSHD1, with half of patients exhibiting mild retinal abnormalities. Severe retinal vasculopathy is rare and is most commonly found in early-onset FSHD patients with large 4q35 deletions. Hearing loss is not more common in typical FSHD patients, but high-frequency hearing loss is more common in those with large 4q35 deletions, similar to retinal vasculopathy.

Differential Diagnosis

The clinical features can be similar to limb-girdle muscular dystrophy (especially calpainopathy with scapular winging), Pompe Disease, and myositis.

Investigations

CK levels are normal or mildly elevated. Muscle biopsy and EMG typically show only non-specific features. Genetic testing is the basis for diagnosis.

Resources

GeneReviews - FSHD

References

  1. Morís, Germán; Wood, Libby; FernáNdez-Torrón, Roberto; González Coraspe, José Andrés; Turner, Chris; Hilton-Jones, David; Norwood, Fiona; Willis, Tracey; Parton, Matt; Rogers, Mark; Hammans, Simon (2018-03). "Chronic pain has a strong impact on quality of life in facioscapulohumeral muscular dystrophy". Muscle & Nerve. 57 (3): 380–387. doi:10.1002/mus.25991. ISSN 1097-4598. PMC 5836962. PMID 29053898. Check date values in: |date= (help)

Literature Review