Stiff person syndrome is associated with:

This disorder is characterized by progressive muscle rigidity, stiffness, and painful spasms triggered by auditory, sensory, or emotional stimuli. Rigidity mainly involves the lower trunk and legs, but it can affect the upper extremities and neck. Symptoms improve with sleep and general anesthetics. Electrophysiologic studies demonstrate continuous motor unit activity. Antibodies associated with the stiff-person syndrome target proteins involved in the function of inhibitory synapses utilizing -aminobutyric acid (GABA) or glycine as neurotransmitters. Ref: Harrison's principle of internal medicine 17th edition, chapter 97.