Mechanisms and Normal Data

This study provides a comprehensive description of upper eyelid movement in normal human subjects. Using the magnetic search coil technique to monitor lid position and modified skin electrodes to record orbicularis oculi electromyographic (EMG) activity, the authors found that the basic eyelid movements, blinks, and saccadic lid movements, can be uniquely and reliably characterized by their amplitude-maximum velocity relationships. The data show that similar increases in levator palpebrae activity produce the upward lid movements that accompany upward saccadic eye movements as well as the upward phase of a blink. The lid movements that accompany downward saccadic eye movements arise almost exclusively from the passive downward forces and relaxation of the levator palpebrae muscle. In contrast, active orbicularis oculi contraction and the passive downward forces act together to generate lid closure with a blink. These normative data and techniques provide the basis for the clinical analysis of lid motility by which abnormal lid movements can be compared with normal lid kinematics. Invest Ophthalmol Vis Sci 32:387-400,1991 Movement of the upper eyelid is a consequence of three active forces, two skeletal muscles and a smooth muscle-and a passive force produced by the mechanical arrangement of the eyelid. The two skeletal muscles provide the dominant forces that act on the lid. The tonically active levator palpebrae muscle raises the eyelid and holds it up; the normally quiescent orbicularis oculi muscle lowers the eyelid. Mueller's muscle, a smooth muscle that originates from the levator palpebrae and inserts onto the upper margin of the tarsal plate, provides an additional upward force on the eyelid. The arrangement of the ligaments and the insertion points of the skeletal muscles produce a constant, passive downward force on the upper eyelid. The neural signals required to generate lid movements reflect the mechanical simplicity of the lid. To blink, the nervous system turns off the tonically active levator palpebrae, and a burst of orbicularis oculi activity leads to a rapid lowering of the upper eyelid. To raise the eyelid again, the orbicularis oculi activity ceases and levator palpebrae activity resumes. 1 " 9 In contrast, the levator palpebrae muscle