A rare, disabling neurological disease characterized clinically by progressive muscle rigidity and painful spasms commonly affecting the axial and limb musculature. It was initially described by Moersch and Woltman in 1956. Its prevalence is said to be around 1/1000000.

It is observed to be twice more common in females. It is presently recognized as having 3 different forms: classic stiff man syndrome-also known as autoimmune, affects most of the body; limited-also called paraneoplastic usually affects a particular region of the body (stiff limb, stiff body etc) and is usually associated with paraneoplastic cancer conditions; PERM (progressive encephalomyelitis with rigidity and myoclonus) is a rapidly progressive form with diffuse central nervous system findings. 

The autoimmune variety, accounting for around 60% of cases, is characterised by the presence of circulating anti-glutamic acid decarboxylase antibodies (anti-GAD). Although the exact etiology is unknown, the autoimmune nature of the disease is supported by its association with other autoimmune disorders such as diabetes (30%), thyroiditis (10%), pernicious anemia, cerebellar ataxia, and its response to immunotherapy. GAD is an essential, rate limiting enzyme in the synthesis of GABA (gamma amino butyric acid). Due to the disinhibition from higher centres there is exaggerated activity at the peripheral motor unit causing stiffness and rigidity.