Stiff person syndrome is a rare (estimated 1-2 cases per million) disorder, "characterized by progressive muscle stiffness, rigidity, and spasm involving the axial muscles, resulting in severely impaired ambulation." ("Stiff person syndrome," UpToDate)
60-80% of patients have autoantibodies against glutamic acid decarboxylase (GAD), though amphiphysin and gephyrin autoantibodies may also be present in the paraneoplastic variant.
A recent and concise review (with images, differential, and treatment) can be found at Stiff Person Syndrome.
Two additional case reports (with MRI and photographs respectively), can be found at:
Sudden spasms following gradual lordosis--the stiff-person syndrome
Stiff person syndrome presenting with sudden onset of shortness of breath and difficulty moving the right arm: a case report