Physiologic studies of spinal inhibitory circuits in patients with stiff-person syndrome

Objective: To test inhibitory spinal circuits in patients with stiff-person syndrome (SPS). Background: Patients with SPS have fluctuating muscle stiffness and spasms, and most have antibodies against GABAergic neurons. We predicted they would also have abnormalities of spinal GABAergic circuits. Design/Methods: Physiologic methods using H-reflexes were used to test reciprocal inhibition in the forearm and thigh, vibration-induced inhibition of flexor carpi radialis and soleus H-reflexes, recurrent inhibition, and nonreciprocal (Ib) inhibition of soleus H-reflexes. Results: Vibration-induced inhibition of H-reflexes was diminished in eight of nine patients tested, but the presynaptic period of reciprocal inhibition was normal in most patients. Bath circuits are presumed to involve presynaptic inhibition and GABAergic interneurons. Presumed glycinergic circuits, including the first period of reciprocal inhibition and nonreciprocal (Ib) inhibition, showed occasional abnormalities. Recurrent inhibition was normal in all five patients tested. Conclusion: Differences between the two presumptive GABAergic circuits may indicate that not all populations of GABAergic neurons are uniformly affected in SPS. The involvement of presumptive glycinergic circuits in some patients could point to impairment of nonGABAergic neurons, unrecognized involvement of GABAergic neurons in these inhibitory circuits, or, more likely, alterations of supraspinal systems that exert descending control over spinal circuits.