Abnormal H-reflexes in periodic limb movement disorder; impact on understanding the pathophysiology of the disorder

OBJECTIVE: To get more insight in the pathophysiological basis of periodic limb movement disorder (PLMD) with or without restless legs syndrome (RLS), we investigated whether these patients have spontaneous changes in H-reflexes or show altered reflex patterns after (external) inhibition or excitation of the relevant spinal segment.

METHODS: The ratio of the peak-to-peak values of the maximal soleus H-reflex and the maximal direct muscle potential (H/M ratio), H-reflex recruitment curves, vibratory inhibition and recovery curves of the soleus H-reflex in double stimulus experiments were measured in 9 PLMD patients and 11 controls.

RESULTS: In comparison to controls the vibratory inhibition, predominantly reflecting pre-synaptic inhibitory action, was depressed in PLMD patients. The soleus H-reflex recovery curves showed increased late facilitation and depressed late inhibition, both reflecting diminished inhibition due to post-synaptic central activity.

CONCLUSIONS: Our data indicate diminished inhibition at spinal level in PLMD patients. This is probably due to altered function of the descending spinal tracts, peripheral influence or changes at the inter-neural circuitry at spinal level itself, or combinations of these 3 possibilities.

SIGNIFICANCE: The results of this study give further insight in the pathophysiology of PLMD and RLS by stressing the importance of diminished central inhibition.