There are several different types of eye movements coordinated with one another and regulated by an integrated control system. Refixation saccadic movements serve to redirect the eye, to bring images of interest to the center of the retina. Pursuit movements and optokinetic nystagmus prevent or reduce slippage of the image over the retina. The vestibulo-ocular reflex acts to stabilize the eye's position during head movements. Convergence and divergence movements allow objects at various distances to be imaged on the retinas at corresponding locations. The various types of movement are classified in ■ Fig. 11.1. Supranuclear structures that act through the motor nuclei of the third, fourth, and sixth cranial nerves have the task of coordinating the movements of both eyes, on the one hand, and on the other hand, they must generate the encoded innervational signal patterns for each of the various types of eye movement. The innervational signals are integrated within the cells of the motor nuclei. Consequently, infranuclear (peripheral) lesions compromise the final common pathway - all the eye movement types summarized in ■ Fig. 11.1 - whereas supranuclear lesions can cause separate impairments of individual types of eye movement.
Fast conjugate eye movements (saccades)
Volitional, targeted, spontaneous saccades
Versions = gaze movements, conjugate eye movements, associated ocular movements
Fast phases of optokinetic and vestibular nystagmus
Slow conjugate eye movements
Smooth pursuit and slow phases of optokinetic nystagmus
Vergences = disjugate or disjunctive eye movements
Vestíbulo - ocular reflex and slow phases of vestibular nystagmus
Fig. 11.1. Classification of eye movements
Differential Diagnosis of Supranuclear Disorders of Ocular Motility
The portrayal of supranuclear disorders of ocular motility in this chapter is intended to be a systematic introduction. A patient who shows signs of an eye movement disorder, however, confronts the physician in an entirely different way. For diagnosis, the first step is to determine the location of the lesion. The second step is concerned with identifying the type of lesion. To accomplish this purpose, the following discussion summarizes the most important rules by which supranuclear disorders can be differentiated from other types of disordered eye movement.
For the diagnosis of neuro-ophthalmic disorders the computed tomography (CT) and, more importantly, magnetic resonance imaging (MRI) scans have become indispensable tools. In many cases, these imaging procedures will immediately reveal the location and the nature of the disease process. CT and MRI scans, however, do not release the physician from his/her obligation to classify disorders according to their clinical presentations and to judge whether an imaging procedure is necessary. The neuroradiologist must determine the anatomic region within which high-resolution instruments should be used. Only through precise clinical inquiry can the potential advantages of the CT and MRI be fully realized. In ■ Fig. 11.2, rules are summarized by which eye movement disorders can be attributed to problems in one of the following four regions (oval fields): (1) ocular muscles; (2) orbit; (3) the subarachnoid or cavernous sinus segments of cranial nerves III, IV, and VI, and (4) the brainstem or posterior cranial fossa. The topical classification of eye movement disorders leads the examiner to corresponding groups of differential diagnoses (rectangular fields). Myasthenia (a great mimic) can produce all forms of eye movement disturbances, but will not affect the pupil, damage the optic nerve, or cause changes in the soft tissues of the orbit.
To be able to follow the decision process outlined in the flow diagram, note that four types of eye movements have to be tested:
1. Fast eye movements (volitional refixation saccades)
2. Pursuit movements (and/or optokinetic nystagmus [OKN])
3. Vestibulo-ocular reflex (VOR)
4. Near convergence
Eye movement disorders
Binocular 1 1
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