Pathophysiology of blepharospasm and oromandibular dystonia.

The pathophysiology of reflexes mediated by the fifth and seventh cranial nerves has been studied in 16 patients with blepharospasm and oromandibular dystonia compared with normal age-matched subjects. The EMG activity of the dystonic spasms in the periocular and jaw muscles was similar to that described in other muscles in patients with generalized torsion dystonia. The latency of the R1 and R2 components of the blink reflex and of the corneal reflex was normal. However, the amplitude and the duration of the R1 and R2 and the duration of the corneal reflex were increased. In some patients the R1 component was also present on the side contralateral to the stimulus, while in normal subjects it was present only on the ipsilateral side. The excitability cycle of recovery of the R2 component of the blink reflex after a prior conditioning shock was enhanced in the patients. There were no EEG potentials preceding blepharospasms in the patients, although a Bereitschaftspotential was seen beginning some 500 ms prior to voluntary blinks in the same individuals. Exteroceptive suppression in the contracting masseter and orbicularis oculi muscles was absent in 40 to 50 per cent of the patients. The jaw jerk was present in all the patients with normal latency. These results indicate that the neuronal arcs of the facial reflexes in blepharospasm and oromandibular dystonia are normal. However, there is an abnormal excitatory drive, perhaps from the basal ganglia, to the facial motoneurons and the interneurons which mediate the facial reflexes in the brainstem.