matory disorders, such as syphilis. The typical findings include a somewhat enlarged, hyperemic optic disc with blurred margins, enlarged veins, and usually hemorrhages (Fig. 11.2). Inexperienced clinicians often have difficulty distinguishing papilledema from other changes of the optic disc.

Optic nerve atrophy is a permanent residual finding after lesions of the optic n. The extent of visual loss, however, need not reflect the degree of visible atrophy. The optic disc is pale all the way to the disc margin, which remains sharp. These findings are typically seen after retrobulbar neuritis, for example (Fig. 3.4, p. 18), but also after optic nerve compression (whether from outside, as by a meningioma, or from inside by an optic glioma). Further causes of optic nerve atrophy include chronic papilledema, syphilis, Leber hereditary optic nerve atrophy (LHON, a mitochondrial disease occurring in men), many types of spinocerebellar degeneration, ischemia, and exogenous intoxication.

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Fig. 11.2 Acute papilledema (left eye) in a patient with a brain tumor. The optic disc is swollen, with blurred margins and small, linear hemorrhages.

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Fig. 11.3 Anatomical substrate of conjugate eye movements.

The diagram shows the anatomical pathways for a conjugate movement to the right: neural impulses flow from the cortical eye fields of the left hemisphere to the right PPRF and onward to the nucleus of the right abducens n. Impulses in the abducens n. induce contraction of the lateral rectus m. of the right eye. Meanwhile, cortical impulses also travel by way of the medial longitudinal fasciculus to the nucleus of the left oculomotor n., and impulses in this nerve induce contraction of the medial rectus m. of the left eye. Thus, lesions of the hemispheres or of the PPRF result in a palsy of conjugate horizontal gaze (hemispheric lesion: contralateral gaze palsy, PPRF lesion: ipsilateral gaze palsy). On the other hand, lesions of the medial longitudinal fasciculus cause an isolated loss of adduction of one eye during horizontal eye movement (internuclear ophthalmoplegia). Vertical eye movements are generated by the midbrain reticular formation (riMLF, p. 188), which receives input from both the cerebral cortex and the PPRF.

visual cortex in the occipital lobe, impulses travel to the eye fields of the temporal lobe (“medial superior temporal visual area,” MST) and the neighboring parietal cortex. These areas are interconnected with the paramedian pontine reticular formation (PPRF) and with the cerebellum. Impulses from the PPRF control the nuclei of the eye muscles either directly or by way of interneurons.

Disturbances of the pursuit system cause pursuit movements to break up into saccades. If the saccade system is also damaged, gaze palsy can result (see below).

Convergence movements serve to fix a nearby object in view and involve simultaneous adduction of both eyes.

Oculomotor Disturbances

Nystagmus

In purely descriptive terms, nystagmus is an involuntary, repetitive, rhythmic movement of the eyes. Nystagmus is often, but not always, pathological.

! Nystagmus is sometimes physiological.

Examples of physiological nystagmus include optokinetic nystagmus (p. 186) and the type of vestibular nystagmus that is induced by rotation in a swivel chair. End-gaze nystagmus (p. 185) is also physiological, as long as it occurs symmetrically in both directions. Pathological nystagmus, on the other hand, indicates the presence of a lesion in the anatomical structures subserving eye movements. A large number of components in this system can be damaged and nystagmus has a correspondingly wide spectrum of possible causes (see below).

Phenomenological classification of nystagmus. As already discussed to some extent in Chapter 3, nystagmus can be characterized according to various criteria:

Jerk vs. pendular nystagmus: most types of nystagmus are either of the “jerking” type, i. e., with a fast and a slow phase, or pendular (back-and-forth).

Direction of beat in relation to the three major axes of eye movement: one speaks of horizontal, vertical, or rotatory nystagmus.

Direction of beat in relation to the midline of the eye: nystagmus may beat to the left, to the right, upward, downward, or diagonally.

In saltatory nystagmus, the direction of beat is defined, by convention, as that of the rapid phase, even though the slow phase is actually the pathological component and the rapid phase is a physiological correction for it, serving to return the eyes to their original position.

Nystagmus can be spontaneous (p. 185) or else present only in response to specific precipitating stimuli

(e. g., position, change of position, a rotatory or thermal stimulus to the vestibular system, or a particular direction of gaze gaze-evoked nystagmus, p. 185).

The examiner must also determine whether nystagmus is equally severe in both eyes, or whether it is weaker or perhaps nonexistent in one eye. Nystagmus that is unequal in the two eyes is also called dissociated nystagmus.

A mainly phenomenologically oriented listing and illustration of the most important types of nystagmus and their causes, is found in Table 11.1 and Fig. 11.4.

There are a few rarer types of nystagmus whose phenomenology is quite complex and not easily described by the criteria listed above. These types of nystagmus are summarized in Table 11.2.

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Topical classification of pathological nystagmus.

Often, the type of nystagmus that is present already provides a clue to the site of the lesion:

Gaze-paretic nystagmus. This type of nystagmus may be due to disease of the eye muscles themselves, or to a lesion of the cranial nerves innervating the eye muscles or of the corresponding brainstem nuclei.

Gaze-paretic nystagmus is usually slow, coarse, and in the direction of the impairment of gaze.

Vestibular nystagmus is due to a lesion of the vestibular organ itself or of the vestibular n. or its nuclei in the brainstem. It typically appears as a spontaneous nystagmus beating away from the side of the lesion, regardless of the direction of gaze (nystagmus in a fixed direction, cf. Table 11.1). Vestibular nystagmus

is typically inhibited by fixation; it is sometimes observable only if fixation is abolished by having the patient wear Frenzel goggles or shake the head rapidly.

Gaze-evoked nystagmus beats in the direction of gaze and indicates a lesion in the brainstem or its afferent or efferent connections with the cerebellum. If caused by a unilateral cerebellar lesion, it can be highly asymmetrical or even beat only to the side of the lesion. In such patients, gaze-evoked nystagmus can be difficult to distinguish from vestibular nystagmus.

Nystagmus due to brainstem lesions. Vestibular spontaneous nystagmus, gaze-evoked nystagmus, upbeat or downbeat vertical nystagmus and posi-

tional and/or positioning nystagmus can all indicate the presence of a brainstem lesion. These types of nystagmus are often rotatory or dissociated (as in internuclear ophthalmoplegia).

Positioning nystagmus is a predominantly rotatory nystagmus lasting several seconds after changes of position of a particular type; it is found in benign paroxysmal positioning vertigo, a disorder of the peripheral portion of the vestibular system (p. 202).

Congenital pendular nystagmus is characterized by conjugate, pendular eye movements that increase with attention or monocular fixation. It is normally well compensated. There is no underlying, pathological structural lesion.

Physiological nystagmus. The most important example is optokinetic nystagmus. This normal phenomenon serves to stabilize the visual image of a moving object on the retina and thus has the same purpose as the vestibulo-ocular reflex.

Optokinetic nystagmus consists of slow pursuit movements alternating with rapid return movements (saccades). The return movements occur whenever the moving object “threatens” to leave the visual field. If the object is moving very rapidly, optokinetic nystagmus can be voluntarily suppressed. Absent, asymmetrical, or dissociated optokinetic nystagmus is pathological.

Vestibulo-ocular reflex (VOR) is a function of the labyrinth that serves to stabilize gaze fixation on rapid movement of the head: it produces a compensatory eye movement in the direction opposite the head movement. Slower head movements do not need to be compensated for by the vestibular system, as the ocular pursuit system suffices to keep gaze fixated in this case (see above, p. 183). Vestibular nystagmus can be suppressed by fixation on an object moving in tandem with the head (nystagmus or VOR suppression test, see below). An inability to suppress the VOR by fixation is pathological.

Nystagmus suppression test (= VOR suppression test).

In this test, the subject stretches both arms forward, holds his or her thumbs up, and fixates gaze on them. When the subject is then rapidly rotated around the long axis of the body, there is normally no nystagmus, because vestibular nystagmus can be suppressed by visual fixation (Fig. 11.5). If nystagmus does appear, this indicates a lesion in the cerebellum or its connections with the vestibular apparatus of the brainstem.

Table 11.2 Rare types of nystagmus

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