Lewis-Sumner syndrome (also known as multifocal acquired demyelinating sensory and motor neuropathy) is a neurological condition affecting primarily the arms and hands (upper limbs). The symptoms are a result of inflammation of the nerves leading to the upper body and the destruction of the fatty covering that protects the nerves (myelin sheath). Lewis Sumner syndrome is an acquired disorder, and the exact cause of the condition is not known. Lewis Sumner syndrome may be difficult to distinguish from other forms of demyelinating neuropathies, but diagnosis may be possible through nerve conduction studies or a specific type of imaging test, called MRI with T2 STIR. Treatment with intravenous immunoglobulin (IVIg) therapy has been successful in reducing the symptoms of the disease.

The signs and symptoms of Lewis-Sumner syndrome present during adulthood and include hand and arm weakness, a loss of reflexes in the upper limbs (areflexia), loss of feeling in the hands and fingers (neuropathy), and tingling in the hands and fingers (paresthesias). Some people affected by Lewis-Sumner syndrome also experience pain in the upper limbs. L-SS may also affect the legs and feet (lower limbs).

Lewis-Sumner syndrome is an acquired condition, meaning that the symptoms of the disease are not present at birth. The disease is not believed to be genetic, meaning that it is not known to be caused by changes (mutations) to our genes. In some cases, Lewis-Sumner syndrome is a side-effect of a type of medication called antagonists of TNF-alpha (anti-TNFα) This type of medication may be used for treatment of arthritis and inflammatory bowel disease.

The diagnosis of Lewis-Sumner syndrome consists of a clinical evaluation showing symptoms of Lewis-Sumner disease, as well as nerve conduction studies showing the involvement of the nerves leading to the upper limbs. Nerve conduction studies use electrical stimulation to measure how well the nerves are working. The diagnosis can be confirmed by performing a specific type of imaging test called MRI T2 STIR. This imaging technique allows for clearer viewing of the brachial plexus, which is one of the nerve networks that is often affected by Lewis-Sumner syndrome.

The treatment most commonly used for Lewis-Sumner syndrome is intravenous immunoglobulins (IVIg). This type of therapy uses a mixture of antibodies delivered through an IV to help alleviate the symptoms of the disease, and is successful in 80% of cases. For individuals who do not respond to IVIg, treatment with subcutaneous immunoglobulins or plasma exchange have proved effective. Up to 40% of individuals who are not responsive to treatment eventually experience a reduction in symptoms.