Stiff Person Syndrome (SPS) is a progressive neurological autoimmune disorder which leads to involuntarily muscle contractions (and subsequently rigidity) which can severely affect range of motion.

It's classed as a rare disease, nevertheless at any one time, it's estimated that around 1 million people are affected by the syndrome - and many more are mis-diagnosed as suffering from other problems such as Multiple sclerosisplugin-autotooltip__plain plugin-autotooltip_bigMultiple sclerosis

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At any given time, it affects around 3 million people worldwide., Parkinson's diseaseplugin-autotooltip__plain plugin-autotooltip_bigParkinson's disease

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"After osteoarthritis, fibromyalgia is the second most common 'rheumatic' disorder. Depending on the diagnostic criteria used, the prevalence is from 2% to 8% of the population." [...] Patients with fibromyalgia are likely to have a history of headaches, dysmenorrhea, temporomandibular joint disorder, chronic fatigue, irritable bowel syndrome and other functional gastrointestinal disorders,interstitial cystitis/painful bladder syndrome, endometriosis, and other regional pain syn…, psychosomatic illness, anxiety and phobia.

It was first described in the mid 1950s and is frequently associated with other autoimmune diseases such as type-I diabetes, thyroiditis, vitiligo, and pernicious anemia.

The cause is not known.

One significant (though unexplained) factor is that sufferers often have very elevated levels of antibodies to an enzyme known as glutamic acid decarboxylase or GAD. (GAD is an important component of the mammalian nervous system, with a crucial role in the nerve signalling process). Although these high GAD antibody levels are quite rare in the general population, most of those who do have them do not go on to develop SPS. Suggesting that the levels of antibodies might be a symptom of the disease rather than a cause.

Further information: US NIH