Abnormal late responses upon transcranial magnetic stimulation in patientswith Stiff Man Syndrome

Introduction: Stiff Man Syndrome (SMS) is a rare disorder characterised by fluctuating stiffness of axial and proximal limb muscles. The diagnosis is made on clinical signs and symptoms. Elevated titers of antibodies against glutamic acid decarboxylase support the diagnosis. Abnormal late responses (LRs) upon transcranial stimulation (TMS) were found in a patient with SMS. We report on the follow-up of this patient and on TMS findings in another patient with SMS. Methods: Cortex and lumbar plexus were stimulated with a magnetic stimulator. Compound muscle action potentials from anterior tibial muscles were recorded with surface electrodes in the patients and in 22 he althy controls. Results: Upon TMS, in both patients primary responses were normal, but there were LRs with a latency of 69 ms and 67 ms, respectively. The LRs were independent of an attack of stiffness and did not habituate. In the first patient, treatment with diazepam was clinically effective. The LRs were abolished after treatment and did not recur during the following 2 years. In the second patient, treatment with immunoglobulins in addition to benzodiazepines was clinically effective. After this treatment LRs disap peared only in the right leg. Discussion: The LRs were abnormal because the y occurred within the physiological silent period. The correspondence betwe en the presence of the LRs and the patients, symptoms indicates that the LR s are part of SMS. In contrast to other electrophysiological findings in SM S, such as spasmodic reflex myoclonus, the LRs showed no habituation. TMS m ay be a simple test for SMS.