Ocular movement and vision are virtually inseparable. A moving
object evokes movement of the eyes and almost simultaneously arouses
attention and initiates the perceptive process. To look searchingly,
i.e., to peer, requires stable fixation of the visual image on the
center of the two retinas. One might say that the ocular muscles
are at the service of vision.
Abnormalities of ocular movement are of three basic types. One
category can be traced to a lesion of the extraocular muscles themselves,
the neuromuscular junction, or to the cranial nerves that supply
them (nuclear or infranuclear palsy).
The second type is a derangement in the highly specialized neural
mechanisms that enable the eyes to move together (supranuclear and internuclear
palsies). This distinction, in keeping with the general
concept of upper and lower motor neuron paralysis, hardly portrays
the complexity of the neural mechanisms governing ocular motility;
nevertheless, it constitutes an essential step in the approach to
the patient with defective eye movements. A knowledge of the anatomic
basis of normal movement is essential to an understanding of abnormal
movement. Perhaps more common but not primarily neurologic is a
third group, strabismus, in which there is a congenital
imbalance of the yoked muscles of extraocular movement that leads
to a developmental reduction in monocular vision, as discussed at
the end of the previous chapter.
In no aspect of human anatomy and physiology is the sensory guidance
of muscle activity more instructively revealed than in the neural
control of coordinated ocular movement. Moreover, the entirely predictable
and “hard-wired” nature of the central and peripheral
oculomotor apparatus allows for a very precise localization of lesions
within these pathways. To focus the eyes voluntarily, to stabilize
objects for scrutiny when one is moving, to bring into sharp focus
near and far objects—all require the perfect coordination
of six sets of extraocular muscles and three sets of intrinsic muscles
(ciliary muscles, sphincters, and dilators of the iris). The neural
mechanisms that govern these functions reside mainly in the midbrain
and pons but are greatly influenced by centers in the medulla, cerebellum,
basal ganglia, and the frontal, parietal, and occipital lobes of
the brain. Most of the nuclear structures and pathways concerned
with fixation and stable ocular movements are now known, and much
has been learned of their physiology both from clinical-pathologic
correlations in humans and from experiments in monkeys. Different
diseases give rise to particular defects in ocular movement and
pupillary function, and these are of diagnostic importance.
Accurate binocular vision is actually achieved by the associated
action of all the ocular muscles. The symmetrical and synchronous
movement of the eyes is termed conjugate movement or gaze (conjugate meaning
yoked or joined together). The simultaneous movement of the eyes
in different directions, as in convergence, is termed dysconjugate or disjunctive.
These two forms of normal ocular movement are also referred ...