Although less common than the Duchenne and myotonic dystrophies, FSH is not rare (an estimated yearly incidence rate of 5:100,000) and we have seen 1 or more cases yearly. The age of onset is usually between 6 and 20 years, but cases beginning in early adult life are occasionally encountered. Weakness and atrophy of the involved muscles are the major physical findings; pseudohypertrophy occurs only rarely and is slight. As a rule, the first manifestations are difficulty in raising the arms above the head and winging of the scapulae, although bifacial weakness may have initially attracted attention, even in early childhood. There is involvement especially of the orbicularis oculi, the zygomaticus, and the orbicularis oris, whereas the masseters, as well as the temporalis, extraocular, pharyngeal, and respiratory muscles are spared. There is an inability to close the eyes firmly, to purse the lips, and to whistle; the lips have a peculiar looseness and tendency to protrude. The lower parts of the trapezius muscles and the sternal parts of the pectorals are almost invariably affected. By contrast, the deltoids may seem to be unusually large and strong, an appearance that may be mistaken for pseudohypertrophy. The advancing atrophic process involves the sternocleidomastoid, serratus magnus, rhomboid, erector spinae, latissimus dorsi, and eventually the deltoid muscles as well. The bones of the shoulders become prominent; the scapulae are winged and elevated (“angel-wing” appearance), and the clavicles stand out. The anterior axillary folds slope down and out as a result of wasting of the pectoral muscles. Usually the biceps waste less than the triceps, and the brachioradialis muscles even less, so that the upper arm may be thinner than the forearm (“Popeye” effect). Pelvic muscles are involved later and to a milder degree, giving rise to a slight lordosis and pelvic instability. The pretibial muscles weaken, and foot-drop is added to the waddling gait. The Beevor sign, an upward movement of the umbilicus on flexing the neck as a result of weakness of the lower abdominal muscles, is reportedly common (Awerbuch et al), but we have not seen it in early cases. Life expectancy is said to not be shortened.
Initially, and even throughout the course, the muscular weakness may be asymmetrical (winging of only one scapula). Many of the patients with milder degrees of this form of dystrophy are unaware that they have the disease. This was true of nearly half of the large series of patients described by Tyler and Stephens in the Utah Mormon population. At any point, the disease may become virtually arrested. Nevertheless, 15 to 20 percent of patients eventually require a wheelchair (Tawil et al).
An interesting feature of this group of diseases is the occasional congenital absence of a muscle (amyoplasia of one pectoral, brachioradialis, or biceps femoris) or part of a muscle in patients who later develop the typical features of the disease. The external ocular muscles are known to occasionally become affected late in the illness. Although cardiac involvement is rare, in a few cases tachycardia, cardiomegaly, and arrhythmias have occurred. Mental function is normal. Serum CK values are normal or slightly elevated.