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.

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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

Epidemiology

Frequency

United States

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]}

International

For the same reasons outlined for the US, 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

See the list below:

• 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.

Sex

See the list below:

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

Age

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

Clinical Presentation