Memory-contingent saccades and the substantia nigra postulate for essential blepharospasm.

Essential blepharospasm and cranial dystonia are related focal dystonias of unknown aetiology. Blepharospasm induced by acute dopamine depletion in parkinsonism restricts saccade initiation possibly via the substantia nigra pars reticulata (SNpr). If essential blepharospasm and cranial dystonia similarly restrict saccades, then a selective, somatotopically arranged pathway such as the SNpr may be involved. To test this possibility memory-contingent and visually-guided saccades were measured in patients with essential blepharospasm and cranial dystonia. The latency of both forms of saccades was either significantly prolonged or excessively variable, while the accuracy and peak velocity of these fast eye movements were similar to age-matched control subjects. Essential blepharospasm and cranial dystonia alter the initiation of saccadic eye movements. Subcortical brain regions or pathways where eyelid, saccade and cranial/cervical motor control are somatotopically approximated, such as the SNpr, may be involved in blepharospasm.