Background

Focal atrophy of an individual muscle or group of muscles, often encountered clinically, may create diagnostic and therapeutic challenges.

A wide variety of neurologic disorders may present with focal muscular atrophy (FMA). FMA also may be secondary to nonneurologic conditions, leading to disuse of part of a limb.

Pathophysiology

The organ ultimately affected is the muscle, although the pathology may be anywhere along the lower motor neuron (LMN) or, at times, secondary to nonneurologic disorders.

Etiologic factors include the following:

Infection
Trauma
Inflammation
Spinal cord disorders
Vasculitis
Entrapment
Altered immune mechanisms
Toxins
Physical agents, such as electrical or radiation injury
Genetic and enzyme defects

Frequency

Focal muscular atrophy (FMA) is a heterogenous disorder with diverse etiologies, so overall prevalence rates are not available.

An estimated 1.63 million polio survivors reside in the US; 28–50% of them will develop postpolio progressive muscular atrophy (PPMA).^{[1, 2]}

For the same reasons outlined for the United States, international incidence and prevalence data are not available. Even for the individual diseases, considerable geographic variation exists. A population-based study revealed a PPMA prevalence of 92 per 100,000 population in a Swedish county.^[3]The prevalence of post-polio syndrome in Kitakyushu, Japan was 18 per 100,000 population.^[4]The frequencies of PPMA among survivors of polio in other countries (not community-based studies) are 60% in the Netherlands^[5], 58% in Norway^[6], 68% in Germany, and 22% in India^[7]. An estimated 3000-5000 persons with PPMA reside in New Zealand.^[8]These figures are likely to be overestimated.^[9]

Many of the infectious causes of FMA (eg, polio, leprous neuropathy) are more frequent in developing countries.

Monomelic amyotrophy has been reported more often in India^[10], Korea^[11], and Japan than in other countries. In a hospital-based study from India, among 110 patients with anterior horn cell disease, 10.9% had progressive muscular atrophy; 1.8%, PPMA; and 22.7%, monomelic amyotrophy.^[12]

Mortality/Morbidity

Most disorders that cause FMA are benign and do not lead to higher-than-normal mortality rates. Most patients do not suffer significant disability, except when the FMA involves an entire limb, becomes generalized, or has an acute onset.

Race-, age-, and sex-related demographics

Most conditions that cause FMA do not have any racial predilection. The geographic variations of some of these disorders probably reflect environmental conditions rather than genetic predisposition.

Bulbospinal muscular atrophy (an X-linked disorder) involves only males. Monomelic amyotrophy is more common in men. PPMA is more frequent in women.

Disorders such as polio and monomelic amyotrophy are more common in younger people.

Prognosis

The diseases that cause focal muscle wasting are mostly self-limiting and benign. They do not affect the lifespan of the individual.

Clinical Presentation

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Media Gallery

A man with neuralgic amyotrophy presenting with wasting of deltoids involving the right side more than the left.
A middle-aged man with (atypical) anterior horn cell disease presenting with wasting of the right quadriceps.
EMG at rest from the right quadriceps muscle of a patient with atypical anterior horn cell disease and isolated atrophy of the right quadriceps; EMG shows spontaneous activity.
EMG on voluntary effort from the right quadriceps muscle of a patient with atypical anterior horn disease and isolated atrophy of the right quadriceps; EMG shows motor unit potentials that exhibit prolonged duration and polyphasia.
EMG on maximal effort from the right quadriceps muscle of a patient with atypical anterior horn disease and isolated atrophy of the right quadriceps; EMG shows an impaired interference pattern.
Clinical photograph of a subject with monomelic amyotrophy showing wasting of left forearm. Note the characteristic feature of oblique atrophy, where a normal brachioradialis dominates the atrophied forearm.
Wasting of right forearm and both hand muscles in a patient with Hirayama Disease. Note the oblique atrophy of right forearm.
Wasting of small muscles of the hands in a patient with Hirayama Disease.
T2-weighted cervical spine MRI of a patient with Hirayama disease showing focal cord hyperintensity at C5-C6 level.
T2-weighted cervical spine MRI of the same patient during neck flexion showing anterior displacement of the posterior dural wall with flattening and compression of the cord against the bodies of the vertebrae with prominent dorsal epidural flow voids.