Distal Myopathies

The distal myopathies are a rare heterogeneous group of disorders characterized by progressive muscular weakness and atrophy beginning in the hands or feet ( ... Table.36-7 ). Several types of distal myopathies have been differentiated clinically, distinguished either by site of onset or mode of inheritance or by differences seen on muscle biopsy.

The late adult-onset distal autosomal dominant myopathy that first appears in the hands, also called Welander distal myopathy or Swedish-type distal myopathy, has been linked to chromosome 14.[76| It has a mean onset of 47 years (range, 20 to 77 years), with weakness in the hands that generally presents as clumsiness in the thumb and forefinger that spreads to other fingers. Eventually, patients develop distal leg weakness, more often in the extensors. Fewer than 20 percent of patients develop proximal extremity weakness, while more than 90 percent complain of vasomotor disturbances, primarily a sensation of cold, in the hands and feet. Another late adult-onset autosomal dominant disease found in Finland first involves weakness in the anterior tibial muscles, followed by weakness of the long toe extensor muscles and proximal leg weakness in about half the patients. There is no weakness in the hands. Yet another late-onset autosomal dominant distal myopathy,

TABLE 36-7 -- DISTAL MYOPATHIES: DISTINCTIONS IN DIFFERENT TYPES

Age of Onset

Type

Mode of Site of Onset

Inheritance

Late adulthood

Welander

Hands

AD

Finnish

Legs

AD

Markesberry- Griggs

Legs

AD

Early adulthood

type I: Nonaka

Anterior compartment of legs

AR

type II: Miyoshi

Posterior

AR

compartment of legs

type III: Laing

Anterior compartment of legs, distal weakness with sternoeleidomastoid weakness

AD

Childhood (ages 5 to 15) Infancy (<2 years)

Desmin storage

Juvenile

Infantile

Distal weakness with respiratory and bulbar involvement Feet, then hands Hands and feet

AD AD AR

AD. Autosomal dominant: AR, autosomal recessive known as Markesberry-Griggs myopathy, has been described in which proximal weakness is also common and the intrinsic foot muscles are affected. Cardiomyopathy in non-Scandinavian middle-aged adults can also occur.

The early adult-onset autosomal recessive myopathies begin with leg weakness. Type I starts in the anterior compartment, usually presenting with foot drop, whereas type II starts in the gastrocnemius muscles. At this time, these types are assumed to be different entities owing to differences seen on muscle biopsy. There are two other early adult-onset forms that are autosomal dominant. Laing's myopathy, linked to chromosome 14, includes facial and shoulder weakness in addition to distal weakness. Desmin storage myopathy has respiratory and bulbar musculature involvement as well as prominent hand and foot involvement.

Two early onset forms have also been described in Dutch and British families. The infantile onset form spreads from the feet to the hands but does not progress after age 18. The juvenile onset form starts simultaneously in the hands and feet and slowly progresses until about age 50 years. [7?i

The differential diagnosis includes myopathies and neuropathies. Whereas most myopathies predominantly affect proximal muscles, a few that can present distally include various metabolic myopathies and inclusion body myositis. Laing's myopathy can resemble FSH MD. Although distal weakness may suggest neuropathic disorders, EMG and muscle studies determine that these conditions are myopathic. Serum CK activity is markedly increased, often 50 times the normal value. Electrical studies demonstrate signs of myopathy in the involved areas of weakness, where nerve conductions are normal. Muscle biopsy shows dystrophic changes of variable severity. Rimmed vacuoles are

frequently observed, suggesting a link between distal myopathies and inclusion body myositis. Patients with Welander's disease also show angulated fibers, whereas desmin storage is typical of the early adult-onset form with respiratory muscle involvement. DM can present without clinically apparent myotonia, and the neuronal late-onset type of Charcot-Marie-Tooth disease (CMT) may look similar. ^

Thus far, no medical interventions have improved these conditions. Appropriate orthotics and physical and occupational therapy provide the most assistance. By and large, these are slowly progressive disorders that sometimes plateau, and they are not life threatening.

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