"Stiff person syndrome": A clinical mystery ?

Stiff person syndrome (SPS), first noted in 1956, is a rare neurologic entity of unclear etiology. It is characterized by progressive rigidity and stiffness mainly affecting the axial trunk muscles thought it can affect the limb(s), one or more.  It looks much like a sensory-motor neuropathic gait, and similar to a CIDP (chronic inflammatory demyelinating polyneuropathy) client we have seen. Chronic pain, impaired mobility, and lumbar hyperlordosis are common symptoms. Spasms can be severe and leave the patient very sensitive to touch and sound, which can sometimes trigger spasms. Some studies have noted that patients generally tend to have anti-glutamic acid decarboxylase (GAD) antibodies, anti-amphiphysin, anti-gephyrin, and anti-gamma-aminobutyric acid A receptor-associated protein (GABARAP) antibodies leaving one to assume this is a variation of an autoimmune disorder expressing neurologically. This is complicated by lack of adequate or specific lab tests to pin point the diagnosis though some CSF and EMG tests are helpful. Generally this is a progressive disorder eventually leaving many clients with movement disabilities.  Enough patients tend towards cancer over time that there may be a paraneoplastic underpinning.
This is a bizarre one, what do you think ?  Have you seen it ?

Pract Neurol. 2011 Oct;11(5):272-82. doi: 10.1136/practneurol-2011-000071. Stiff person syndrome. Hadavi S1, Noyce AJ, Leslie RD, Giovannoni G.

Brain Nerve. 2010 Apr;62(4):377-85. [Stiff-person syndrome and other myelopathies constitute paraneoplastic neurological syndromes]. [Article in Japanese] Ishii A1.