Benign Focal Amyotrophy: Causes, Symptoms, Treatment, Prognosis, Pathophysiology

Benign focal amyotrophy is a neurological condition characterized by gradual degeneration or loss of function of an individual or a group of voluntary muscles. It is also known as monomeric amyotrophy. It is caused primarily due to progressive degeneration in motor neurons, which are nerve cells present in the brain and spinal cord that control the functioning of voluntary muscles. The conditions usually present itself with weakness and wasting of muscles without pain in the limbs. It affects the upper limbs more frequently than the lower extremities. It may also be associated with sensory loss in the affected area. It starts unilaterally and after a while it may or may not progress to the opposite limb with a symmetric distribution. Focal atrophy often creates therapeutic and diagnostic challenges. Benign focal amyotrophy is often associated with other neurological conditions. In a few cases, focal amyotrophy may be secondary to non-neurological condition, which over a period of time may lead to disuse of a part of the entire extremity.

Benign focal amyotrophy is a lower motor neuron disorder primarily characterized by muscle weakness and wasting of muscles predominantly in the distal portion of the upper extremities. Benign focal amyotrophy generally starts during adolescence followed by spontaneous halt in symptoms with stabilization of symptoms. In rare cases, there may be progression of symptoms of Benign focal amyotrophy after the age of 40 years. Very rarely, it affects the opposite limb. Other rare symptoms of Benign focal amyotrophy include muscle cramps, cold hands, worsening of symptoms on exposure to cold, irregular episodes of tremors and contraction fasciculation.

Benign focal amyotrophy is more common in Asian countries, predominantly in Japan and India. A survey conducted between 1996-1998 revealed that there were about 333 cases of focal muscle atrophy in Japan with estimated prevalence being 1/33300 approximately. Very few cases of Benign focal amyotrophy have been reported in Europe and the United States. Overall prevalence rate of benign focal amyotrophy in the United States is unknown. It has been estimated that in 1.63 million polio survivors about 28-50% develop post-polio progressive muscular atrophy (PPMA).

Most of the cases of Benign focal amyotrophy, does not lead to higher than normal mortality rate. Disability rate is quite low in focal muscle atrophy. The condition does not have any racial predilection. The geographical variations show that there may be environmental factors that influence the condition in addition to genetic inheritance. Benign focal amyotrophy is more common in males than females and usually between the ages of 15 to 25 years.

The onset and progression of benign focal amyotrophy is slow. The progression of the symptoms of Benign focal amyotrophy is usually slow for the first 1 to 2 years before reaching a peak, thereafter it remains stable for many years. Rarely, Benign focal amyotrophy leads to disability or progress to the contralateral limb. There is a variety of benign focal amyotrophy called as O’Sullivan-McLeod syndrome. This is a slow progressing variety and it affects only a small group of muscles in the hands and the forearm.

The most common causes and risk factors of benign focal amyotrophy include:

• Repeated neck movement.
• Physical or mechanical trauma or injury.
• Underlying infection.
• Spinal cord diseases.
• Vasculitis.
• Inflammation.
• Entrapment of nerves.
• Exposure to toxins.
• Exposure to radiation or electrical injury.
• Genetic inheritance.
• Enzyme defects.
• Reduced immunity.

As mentioned earlier, the affected organ in benign focal amyotrophy is the muscle. This is usually due to damage or degeneration anywhere along the lower motor neuron or LMN. In certain cases, the muscle atrophy may be due to certain underlying non-neurological conditions. The exact pathophysiology of Benign focal amyotrophy is unknown. A large number of researches are being carried out to study this subject closely. One theory states that Benign focal amyotrophy is caused due to repeated neck movement, which leads to anterior shifting of the dural sac. This in turn leads to compression of the anterior portion of the spinal cord against the posterior portion of the vertebral column.

Benign focal amyotrophy does not lead to life threatening complications; however, over a period of time it can lead to temporary or permanent loss of function in the affected area. Disability can lead to difficulty in carrying out activities of daily living, social difficulties and emotional disturbances. Early intervention is recommended to limit progression.

Diagnosis of benign focal amyotrophy is done by an experienced neurologist. A detailed case history is obtained followed by clinical examination of the affected area.

• Neurological tests are carried out to determine the extent of the damage and to estimate the degree of loss of function.
• Investigative studies such as clinical imaging and electromyography (EMG) are done to confirm the diagnosis and rule out other possible causes of muscle atrophy such as multifocal motor neuropathy, cervical vertebral abnormalities, spinal cord tumors, brachial plexopathy, poliomyelitis, syringomyelia, anterior osseous/ deep ulnar neuropathy etc.
• Magnetic resonance imaging (MRI) usually show distinctive images of compression over the anterior horn when the individual is in maximum cervical anteflexion position. This is also known as the snake eyes sign. Atrophy may also be noted in the lower cervical spinal cord.
• CT scan is often helpful in diagnosis of benign focal amyotrophy.
• Nerve conduction tests are done to study the nerve functions. It usually show reduced compound muscle potential in the areas supplied by median and ulnar nerves.
• F-wave studies reveal prolonged minimum latency with reduced frequency in these muscles.

There is no specific treatment for benign focal amyotrophy. Treatment aims at symptomatic/conservative management of the symptoms of Benign focal amyotrophy. Treatments done at early stages of Benign focal amyotrophy usually result in significant improvement of symptoms. In some cases, the patient is advised to wear a cervical collar to help maintaining a neutral neck posture and prevent inappropriate neck flexion. Conservative treatment modality varies according the extent of Benign focal amyotrophy. Treatment generally consists of exercises for improving muscle strength and movement. Physical therapy and occupational therapy are beneficial in Benign focal amyotrophy patients with muscle atrophy. Psychological counseling and behavior management may be needed in Benign focal amyotrophy patients with emotional disturbances. In some cases, the muscle weakness may respond to corticosteroids. Surgical intervention is not beneficial in management of benign focal amyotrophy. However, in rare instances where symptoms are associated with intraspinal or extraspinal lesion, surgical manifestation may be considered.

Benign focal amyotrophy is a neurological condition that is characterized by weakness in limbs and wasting of muscles in the affected area. Benign focal amyotrophy is more common in hands than in legs and usually affects unilaterally. In rare cases, the condition gradually affects the opposite limb too. It is usually caused due to damage in the upper motor neurons, which control functioning of the voluntary muscles. Possible risk factors of Benign focal amyotrophy include physical trauma such as inappropriate neck flexion, exposure to radiation, exposure to toxins, auto-immunity, genetic inheritance etc. The symptoms are usually managed symptomatically. Physical therapy, occupational therapy, and exercises for strengthening the muscles are recommended. A large number of researches and studies are being carried out to understand focal muscle atrophy better. The NINDS supports and conducts these studies to help in finding out ways to treat, prevent and cure benign focal amyotrophy.