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Blepharospasm refers to excessive involuntary closure of the eyelids. It is generally (but not always) due to spasm of the orbicularis oculi (OO) muscles and occurs in a variety of disorders of the CNS, or may occur secondarily to ocular disorders. The most common form of blepharospasm, benign essential blepharospasm (BEB), is of unknown origin and is generally considered a form of focal dystonia. Before a full discussion of the clinical and pathophysiologic aspects of blepharospasm, we review the normal anatomy and physiology of eyelid control.
Anatomy
The movements of the eyelids are controlled by two muscles. The OO muscle closes the eyelid, and the levator palpebrae opens the eyelid by elevating the upper lid. The OO muscle is composed of three portions. The deepest (closest to the eyeball) is the pretarsal portion that covers the tarsus (the eyelid proper). Next is the preseptal portion, and the most superficial is the orbital portion. With normal blinking, the pretarsal portion is primarily recruited. The preseptal and orbital regions are recruited with more forced voluntary closure of the eyelids.
The muscle fibers of the OO muscle do not run the full length of the muscle, and interdigitate. 1 In the rabbit eyelid, fiber length averages 36% of the length of the entire muscle. The motor units of the OO are small, with approximately 25 muscle fibers per unit. Anatomically, 90% of the muscle fibers is fast or type II. Physiologic observations of the time to peak tension and half relaxation times are consistent with this, but the muscle appears to be more fatigue resistant than would be expected. 2 Hence, the character of the muscle is unique compared with skeletal muscle. There are occasional muscle spindles in OO muscle, but these are sparse and are not well studied.
The kinematics of blinks has …
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