Autoantibodies in MG

Autoantibodies in thymoma-associated myasthenia gravis with myositis or neuromyotonia.

Thymomas are associated with red cell aplasia and in 50 %, with MG. Some patients with MG have inflammatory myopathy of striated and cardiac muscle. Diagnosis is with prosimal muscle weakness, high CPK levels, and myopathic EMG. Cardiac myositis leads to CHF, arrythmia, and sudden death. Patients with thymoma also may have neuromyotonia (NMT) with hyperactive peripheral motor nerves, myokymia, muscle stiffness, cramps, hypertrophy. EMG shows bursts of high frequency motor unit discharges. Antibodies against VGKC have been detected in NMT with or without thymoma. Other antibodies seen are against skeletal muscle calcium release channel (ryanodine receptor RyR) of sarcoplasmic reticulum; and antibodies to cytoplasmic filaments titin or neurofilaments.

There are 3 subgroups of thymoma associated MG with different spectrum of antibodies. 19/19 had AchR antibodies and 17/19 had anti-titin antibodies that did not correlate with symptoms (although anti-titin was higher in patients with antiRyR). Antititin is more or less synonymous with striated muscle antibodies and correlates with the presence of a thymoma.

10/19 had anti RyR, of which five had myositis and 1 had NMT. Myositis is associated with a poorer prognosis, occassionally rippling muscle disease, electrically silent muscle contractions.

7/16 patients tested had anti VGKC, and 4 had NMT, 1 myositis and 2 nothing. RyR and VGKC coexisted only in 2 patients.

Thymic pathology was not correlated, and included thymic ca and carcinoid.