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

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

Multiple sclerosis (MS) is a severe persistent condition of the central nervous system in which the coating around nerve fibres (myelin) is damaged, causing a wide range of distressing symptoms varying in severity.

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

Also known as Parkinson disease, Parkinson's, idiopathic Parkinsonism, primary Parkinsonism, PD, or paralysis agitans - is a degenerative disorder of the central nervous system resulting from the death of dopamine-containing cells in the, fibromyalgia, 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