From MedlinePlus Genetics
Hereditary neuralgic amyotrophy is a disorder characterized by episodes of severe pain and muscle wasting (amyotrophy) in one or both shoulders and arms. Pain is felt along the path of one or more nerves (neuralgia) and often has no obvious physical cause. The network of nerves involved in hereditary neuralgic amyotrophy, called the brachial plexus, controls movement and sensation in the shoulders and arms.

People with hereditary neuralgic amyotrophy usually begin experiencing pain episodes in their twenties, but pain episodes have occurred in children as young as 1 year old. The episodes may be spontaneous, or they may be triggered by stress such as strenuous exercise, childbirth, surgery, exposure to cold, infections, immunization, or emotional disturbance. While the frequency of the episodes tends to decrease with age, affected individuals are often left with residual problems, such as chronic pain and impaired movement, that accumulate over time.

Typically, an attack begins with severe pain on one or both sides of the body; the pain most commonly occurs on the right side of the body. About one-third of individuals with hereditary neuralgic amyotrophy have pain in both shoulders or arms. The pain typically starts out sharp and may become more of an ache over time, and the pain lasts about a month. It may be difficult to control the pain with medication. Over a period of a few hours to a couple of weeks, the muscles in the affected area begin to weaken and waste away (atrophy), and movement becomes difficult. Muscle wasting may cause changes in posture or in the appearance of the shoulder, back, and arm. In particular, weak shoulder muscles tend to make the shoulder blades (scapulae) stick out from the back, a common sign known as scapular winging. Additional features of hereditary neuralgic amyotrophy may include decreased sensation (hypoesthesia) and abnormal sensations in the skin such as numbness or tingling (paresthesias). Areas other than the shoulder and arm may also be involved.

In a few affected families, individuals with hereditary neuralgic amyotrophy also have unusual physical characteristics including short stature, excess skin folds on the neck and arms, an opening in the roof of the mouth (cleft palate), a split in the soft flap of tissue that hangs from the back of the mouth (bifid uvula), and partially webbed or fused fingers or toes (partial syndactyly). They may also have distinctive facial features including eyes set close together (ocular hypotelorism), a narrow opening of the eyelids (short palpebral fissures) with a skin fold that covers the inner corner of the eye (epicanthal fold), a long nasal bridge, a narrow mouth, and differences between one side of the face and the other (facial asymmetry).  https://medlineplus.gov/genetics/condition/hereditary-neuralgic-amyotrophy