Hereditary Neuralgic Amyotrophy (HNA) is a rare, inherited neurological disorder that affects peripheral nerves, especially those in the shoulders, arms, and hands. It is characterized by recurring episodes of intense pain, followed by muscle weakness and atrophy. It specifically affects the brachial plexus, a network of nerves extending from the spine through the neck into the armpit and down the arms. These nerves control movement and sensation in the shoulders, arms, and hands.
The Genetic Cause and Attack Triggers
HNA is an autosomal dominant condition, meaning a single altered gene copy is sufficient to cause the disorder. An affected parent has a 50% chance of passing the gene variant to each child. Mutations or duplications in the SEPT9 gene have been identified as the cause in many, though not all, cases of HNA. The SEPT9 gene provides instructions for making septin-9, a protein involved in cell division and the structural integrity of nerve cells, particularly in the brachial plexus.
While the SEPT9 gene creates a predisposition, HNA episodes are often set off by environmental triggers, including:
Recent infections (viral or bacterial illnesses, immunizations).
Physical stressors (surgery, childbirth, strenuous exercise).
Exposure to cold temperatures.
Periods of emotional disturbance.
Recognizing the Symptom Progression
An HNA attack unfolds in distinct phases, beginning with sudden, severe pain. This acute pain is often debilitating, described as sharp, burning, or aching, and commonly affects one shoulder and/or arm. The pain intensifies at night, making sleep difficult, and can persist for a few hours to several weeks. Controlling this initial pain with medication can be challenging.
As the intense pain gradually subsides over days or weeks, the weakness and atrophy phase becomes apparent. Significant muscle weakness and wasting develop in affected areas, leading to difficulties with movement. This muscle wasting can result in changes in posture or the appearance of the shoulder, back, and arm, such as “scapular winging” where the shoulder blade protrudes. Individuals may also experience decreased sensation or abnormal sensations like numbness or tingling.
The recovery phase for HNA is a slow and gradual process of nerve regeneration and muscle recovery. This period can extend from several months to as long as five years. While many patients experience substantial improvement, recovery may not always be complete. Some individuals are left with residual weakness, chronic pain, or reduced exercise tolerance in the affected limb.
The Diagnostic Pathway
Diagnosis of HNA begins with a thorough clinical evaluation. This involves a detailed review of symptoms, including sudden pain followed by weakness, and a comprehensive family medical history. Physical examination helps identify affected muscles and any signs of atrophy.
Electrodiagnostic tests evaluate nerve and muscle damage. Electromyography (EMG) assesses the electrical activity of muscles, while nerve conduction studies (NCS) measure how well electrical signals travel along nerves. These tests reveal patterns of nerve injury and muscle denervation consistent with HNA, helping to rule out other conditions.
Genetic testing, which identifies a SEPT9 gene mutation, often provides a definitive diagnosis. This genetic confirmation distinguishes HNA from the non-hereditary Parsonage-Turner Syndrome, which shares similar clinical features but lacks the familial genetic link. While SEPT9 mutations are found in most HNA cases, some families with the condition do not have identified SEPT9 variants, indicating other genetic causes may exist.
Managing Attacks and Long-Term Recovery
Managing acute HNA attacks focuses on controlling severe pain. Medications such as corticosteroids are prescribed early in an attack, as they may shorten pain duration and potentially hasten recovery. Analgesics are also used to alleviate discomfort.
Long-term recovery emphasizes comprehensive rehabilitation. Physical therapy is a key part of this recovery, aiming to maintain range of motion and prevent stiffness. Therapists guide patients through exercises to strengthen recovering muscles. Occupational therapy assists individuals in adapting daily living activities and regaining functional abilities. If significant muscle weakness remains, adaptive equipment may be recommended.