Introduction: The eyelid movements are mediated mainly by the orbicularis oculi (OO) and the levator palpebrae superioris (LPS) muscles. Dissociated upper lid functions exhibit different counterbalanced action of these muscles, and in blinking they show a strictly reciprocal innervation. The disturbance of this close LPS-OO relationship likely leads to many of the central lid movement disorders.
Development: Two major lid postural disorders, the blepharospasm (BSP) and the blepharocolysis (BCO), share two different clinical aspects: the involuntary eyelids' closure along with the inability to open the eyes. BSP consists of an involuntary overactivity of the OO, with LPS co-contraction activity, and is expressed as frequent and prolonged blinks, clonic bursts, prolonged tonic contraction or a blend of all of them. BCO (commonly named 'so-called lid opening apraxia') is an involuntary overinhibition of the LPS muscles with no evidence of ongoing OO activity; it exists a co-inhibition of these muscles. BSP and BCO occur in many instances of idiopathic dystonias and basal ganglia diseases and, less frequently, in rostral brainstem lesions. Both may coincide in the same patient.
Conclusions: BSP and BCO should be considered different expressions of the dystonic focal eyelid movement disorders, related to the increase of the two counteracting components of the normal eyelid motor and blinking patterns: the excessive OO activation in the BSP; the excessive LPS inhibition in the BCO.