Better Than the Alternative
The patient, a law student, was 24 years old when she was bitten by a tick in the autumn of 2013. An expanding ring of erythema developed around the site of the bite. She was given oral doxycycline and the rash subsided.
In the summer of 2014, she woke up with a stabbing pain behind the right ear. By midday, she had difficulty drinking; whenever she tried, the beverage dribbled out of the right corner of her mouth, which she noticed was no longer straight. She immediately went to see a general practitioner, who examined her and found marked weakness of all muscles supplied by the right facial nerve, including the muscles of the forehead. Eye closure was incomplete on the right.
Here, as in all problems of neurologic diagnosis, the first task is to localize the lesion. The most common cause of facial weakness is damage to the facial nerve in the petrous canal: inflammatory swelling of the nerve, probably caused by a viral infection, leads to pressure on the nerve in the canal, with ensuing ischemia. In this disorder, called idiopathic or cryptogenic facial nerve palsy (also known by the eponym Bell palsy), lacrimation and taste are typically affected as well—a fact that can be useful in differential diagnosis. Weakness predominates, while lacrimation and taste are intact, if the facial nerve is affected more distally after its exit from the skull base via the stylomastoid foramen, or more proximally within the brainstem, either in its intramedullary fascicle (fascicular lesion) or in the facial nerve nucleus itself (nuclear lesion). Finally, the lesion may also lie above the facial nerve nucleus within the central nervous system (central facial palsy). The cells in the facial nerve nucleus that project to the forehead muscles receive afferent input from higher centers on both sides of the brain; thus, in central facial palsy, the forehead muscles are spared, and full eye closure is possible.
The doctor was able to exclude central facial palsy on the basis of his findings, but was nonetheless very concerned, fearing that the tick-borne borreliosis of the year before had not been fully eradicated and was now affecting the nervous system.
Nonidiopathic facial nerve palsy is often due to an infection with Borrelia burgdorferi (Lyme disease) or the varicella-zoster virus. Borreliosis, however, generally causes polyneuritic or polyradiculopathic weakness, rather than facial nerve mononeuropathy.
The doctor tested the patient’s sense of taste with various aqueous solutions: taste was impaired on the right side in the anterior two-thirds of the tongue. Moreover, he found by a Schirmer test that lacrimation was also diminished on the right. These findings are typical of idiopathic facial nerve palsy. Borreliosis serology revealed no evidence of a florid infection. There were no cutaneous vesicles indicating a reactivated varicella infection. Therefore, the doctor diagnosed idiopathic facial nerve palsy and prescribed a 5-day course of prednisone by mouth, as well as an ophthalmic ointment to prevent drying and damage of the cornea at night. The patient’s symptoms regressed completely within a few weeks.
12.1 Disturbances of Smell (Olfactory Nerve)
Neurologic disturbances of smell are usually due to traumatic or mechanical damage to the fila olfactoria or the olfactory bulb.
Anatomy The peripheral olfactory receptors can only be excited by substances dissolved in liquid. The receptors of the olfactory mucosa project their axons through the cribriform plate to the olfactory bulb (see ▶ Fig. 3.3), which lies on the floor of the anterior cranial fossa, beneath the frontal lobe. These axons make up the fila olfactoria, which collectively constitute the olfactory nerve. After a synapse onto the second neuron of the pathway in the olfactory bulb, olfactory fibers travel onward through the lateral olfactory striae to the amygdala and other areas of the temporal lobe. Olfactory fibers also travel by way of the medial olfactory striae to the subcallosal area and the limbic system (see section ▶ 5.5.4).
Clinical features Techniques for examining the sense of smell are discussed in section ▶ 3.3.2. Only the following types of olfactory disturbances are relevant to neurologic diagnosis:
Anosmia. A more or less complete loss of the sense of smell is most often due to a disorder of the nose, particularly rhinitis sicca. The most common neurologic cause of anosmia is a traumatically induced brain contusion and/or avulsion of the fila olfactoria as they traverse the cribriform plate. Anosmia regresses in one-third of patients, but distortions of olfactory perception, so-called parosmias, often persist, sometimes in the form of unpleasant kakosmia. Anosmia is the characteristic symptom of an olfactory groove meningioma and is often its initial manifestation. Rarer causes of hyposmia include Paget disease, Parkinson disease, prior laryngectomy, diabetes mellitus, and Kallmann syndrome (hyposmia or anosmia with hypogonadotropic hypogonadism, of genetic origin). Medications often alter or impair the sense of smell.
Anosmia always carries with it an impairment of the sense of taste (ageusia). The differential perception of gustatory stimuli requires not only an intact sense of taste but also an intact sense of smell.
Olfactory hallucinations—usually in the form of spontaneous kakosmia—are produced by epileptic discharges from a focus in the anteromedial portion of the temporal lobe. Such hallucinations are called uncinate fits (see “ ▶ 220.127.116.11” in section ▶ 5.5.1).
12.2 Neurologic Disturbances of Vision (Optic Nerve)
Visual disturbances can be caused by lesions of the retina or of its connections with the visual cortex, or of the cortex itself. Depending on the etiology, the clinical manifestation may be either impaired visual acuity (ranging to total blindness) or a visual field defect. The site of the lesion determines the type of visual abnormality that will be present and whether it will affect only one eye or both. As a rule, lesions of the retina and optic nerve cause monocular impairment of visual acuity and of the visual field; chiasmatic lesions impair visual acuity and the visual fields in both eyes; and retrochiasmatic lesions (from the optic tract to the visual cortex) cause visual field defects but spare visual acuity, unless there is more than one lesion and the lesions are located on both sides.
12.2.1 Visual Field Defects
A visual field defect is defined as the absence of some part of the normal visual field. The diagnostic assessment of a visual field defect involves, first, localization of the underlying lesion to a particular part of the visual pathway, and, second, determination of the etiology on the basis of the history, neurologic examination, and ancillary test findings.
Types of Visual Field Defect and Their Localization
The manual confrontation technique for examining the visual fields is described in section ▶ 3.3.2, and the use of special instrumentation for this purpose is described in section ▶ 4.5.3. Visual field defects may be either monocular or binocular.
Monocular visual field defects are caused by lesions of the retina and optic nerve; binocular visual field defects are caused by lesions in or behind the optic chiasm.
Fig. 12.1 Sites of lesions in the visual pathway and the visual field defects that they produce. 1 Left amaurosis due to a left optic nerve lesion. 2 Bitemporal hemianopsia due to a chiasmatic lesion. 3 Right homonymous hemianopsia due to a left optic tract lesion. 4–6 Lesions along the left optic radiation. 4 Right upper temporal quadrantanopsia. 5 Right lower temporal quadrantanopsia. 6 Right homonymous hemianopsia sparing central (macular) vision.
Blindness in one or both eyes, either congenital or acquired
A defect occupying one half of the visual field (right or left)
A defect occupying one quarter of the visual field
A defect occupying a small spot or patch within the visual field (a central scotoma is an impairment of central vision—and therefore a reduction of visual acuity—due to a lesion of the macula lutea or its efferent nerve fibers)
A near-hemianopic visual field defect sparing far lateral vision, caused by a contralateral occipital lesion that spares the rostral portion of the visual cortex on the banks of the calcarine fissure
Homonymous visual field defect
The same area of the visual field is affected in each eye (e.g., the right visual field of each eye)
Heteronymous visual field defect
Different areas of the visual field are affected in the two eyes (e.g., bitemporal hemianopsia)
Homonymous visual field defects If a binocular visual field defect involves a corresponding area of the visual field in both eyes (e.g., the right half of the visual field in both eyes), it is called a homonymous visual field defect.
A lesion of the right optic tract, lateral geniculate body, optic radiation, or visual cortex produces a left homonymous hemianopsia, while a lesion of any of these structures on the left produces a right homonymous hemianopsia ( ▶ Fig. 12.1).
A lesion along the course of the optic radiation or in the visual cortex may affect only part of these structures, causing a homonymous visual field defect that is less than a complete hemianopsia: thus, depending on the site and extent of the lesion, there may be a homonymous quadrantanopsia or a homonymous scotoma.
Heteronymous visual field defects These are defects involving noncorresponding areas of the visual field in the two eyes:
Most lesions of the optic chiasm affect the decussating fibers derived from the nasal half of each retina to produce a bitemporal hemianopsia or bitemporal quadrantanopsia ( ▶ Fig. 12.1). The defect lies in the temporal half of each visual field, that is, in the right half of the visual field of the right eye and the left half of the visual field of the left eye.
If a tumor, such as a pituitary adenoma, compresses the optic chiasm from below, there is initially an upper bitemporal quadrantanopsia only later followed by bitemporal hemianopsia. If a tumor compresses the optic chiasm from above (e.g., a craniopharyngioma), there is initially a lower bitemporal quadrantanopsia, and later a bitemporal hemianopsia.
If a tumor compresses the optic chiasm from one side, it will affect not only the decussating medial fibers but also the uncrossed fibers from the retina on that side. The resulting visual field defect involves the entire visual field on the side of the lesion and the temporal hemifield on the opposite side.
The Localization of Lesions That Impair Visual Acuity
Half of the efferent neurons of the macula project to the ipsilateral cerebral hemisphere, and the other half to the contralateral hemisphere. An eye can see with full acuity if at least half of the neural output from the macula is intact. Thus, lesions of the retina, optic nerve, or chiasm affecting more than half of the fibers from one eye impair visual acuity, but unilateral retrochiasmatic lesions do not.
Lesions of the retina, optic nerve, or optic chiasm impair visual acuity, while unilateral retrochiasmatic lesions do not.
Etiologic Classification of Visual Field Defects
A visual field defect that arises suddenly is generally due to either ischemia or trauma. The shape of the visual field defect sometimes provides a clue to its etiology; thus, a temporal crescent is highly characteristic of a vascular lesion. A slowly progressive visual field defect suggests the presence of a brain tumor. In such patients, the patient may fail to notice the visual field defect, particularly if the tumor lies in the right parietal lobe. There may be visual hemineglect accompanying, or instead of, a “true” visual field defect. The patient ignores visual stimuli in the affected hemifield, even though he or she may still be able to see them, and is unaware of the deficit. Neuroimaging generally reveals the site and nature of the underlying lesion ( ▶ Fig. 12.2).
Fig. 12.2 Infarct in the territory of the right posterior cerebral artery in a patient with left homonymous hemianopsia.
Special Types of Visual Field Defect
In the Riddoch phenomenon, the patient cannot see stationary objects in the affected area of the visual field, though movement can be perceived. In palinopsia, the perception outlasts the stimulus: the patient continues to “see” the presented image long after it has been removed. This phenomenon is produced by right temporo-occipital lesions.
12.2.2 Impairment of Visual Acuity
Impairment of visual acuity can be partial or total (blindness); it can arise suddenly or progress slowly, and it can affect one or both eyes. All of these distinctions, along with the ocular and retrobulbar findings, are relevant to the localization of the lesion and the determination of its etiology. To evaluate such problems, neurologists often collaborate with ophthalmologists.
Sudden Unilateral Loss of Vision
Sudden unilateral loss of vision, as long as its cause does not lie in the eye itself, is usually due to a lesion of the optic nerve. Sudden onset implicates ischemia as the cause. A defect of this type may be permanent, for example, in occlusion of the central retinal artery due to temporal arteritis or embolization from an atheromatous plaque in the carotid artery, or it may be temporary, in which case it is called amaurosis fugax (transient monocular blindness). Rarely, transient visual loss can be produced by a functional neurologic disturbance such as migraine (retinal migraine). Papilledema, too, can be accompanied by episodes of sudden visual loss (amblyopic attacks). The differential diagnosis includes trauma and ocular causes such as retinal detachment, preretinal hemorrhage, and central vein thrombosis. Correct diagnosis requires precise history-taking and meticulous examination of the optic disc and fundus.
Sudden Bilateral Loss of Vision
Bilateral visual loss of more or less sudden onset is usually due to simultaneous ischemia of both occipital lobes. Such events are often preceded by hemianopic episodes and loss of color vision as prodromal manifestations. The possible causes include embolization into the territory of the posterior cerebral arteries on both sides simultaneously, basilar artery thrombosis, and compressive occlusion of both posterior cerebral arteries by an intracranial mass. Patients often deny that they cannot see (anosognosia). Despite the severe visual loss, the pupillary light reflex is still present, because the pathway for visual impulses to the lateral geniculate body, where the fibers for the reflex branch off, remains intact. The visual evoked potentials (see section ▶ 4.3.3 and ▶ Fig. 4.26), however, are pathologic. In rare cases, sudden bilateral loss of vision is due to bilateral retinal ischemia, for example, on standing up in a patient with stenosis or occlusion of the cerebral vessels that arise from the aorta (aortic arch syndrome). Certain types of intoxication can also rapidly produce bilateral optic nerve lesions, for example, methanol poisoning, which causes blindness within hours.
Progressive Impairment of Visual Acuity in One or Both Eyes
Unilateral impairment is due to a process causing more or less rapid, progressive damage to the optic nerve or chiasm. Retrobulbar neuritis (see “ ” in section ▶ 8.2), that is, inflammation of the optic nerve between the retina and the chiasm, and optic papillitis, that is, inflammation of the optic nerve at the level of the optic disc, cause unilateral visual loss within a few days. Progressive, unilateral visual loss should also always prompt suspicion of a mass: optic glioma, for example, is a primary tumor within the optic nerve that is more common in children, while an optic sheath meningioma can compress the nerve from outside. Retrobulbar neuritis rarely occurs bilaterally, sometimes in combination with myelitis (cf. neuromyelitis optica, section ▶ 8.3.1). Further causes of bilateral loss of visual acuity are Leber hereditary optic neuropathy (LHON, a hereditary mitochondrial disease seen in men), and tobacco–alcohol amblyopia. In the latter condition, the most prominent initial finding is an inability to distinguish red from green. Vitamin B12 deficiency can cause progressive optic nerve atrophy in combination with polyneuropathy. Infections (syphilis, sarcoidosis) can also cause uni- or bilateral optic neuritis with impaired visual acuity.
Pathologic Findings of the Optic Disc
This is an area requiring close collaboration between the neurologist and the ophthalmologist.
Papilledema generally reflects intracranial hypertension but can also be seen in infectious or inflammatory disorders (e.g., syphilis). The typical findings include a somewhat enlarged, hyperemic optic disc with blurred margins, enlarged veins, and usually hemorrhages ( ▶ Fig. 12.3). Inexperienced clinicians often have difficulty distinguishing papilledema from other changes of the optic disc.
Fig. 12.3 Acute papilledema in a patient with a brain tumor. The optic disc is swollen, with blurred margins and a small hemorrhage at 3 o’clock.
Optic nerve atrophy is a permanent residual finding in the aftermath of an optic nerve lesion. The degree of visible atrophy does not necessarily correspond to the extent of visual loss. The optic disc is pale all the way to its margin, which remains sharp. These findings are typically seen after retrobulbar neuritis (see ▶ Fig. 3.4), but also after optic nerve compression (whether from outside, as by a meningioma, or from inside, as by an optic glioma). Further causes of optic nerve atrophy include chronic papilledema, syphilis, LHON, many types of spinocerebellar degeneration, ischemia, and exogenous intoxication.
12.3 Disturbances of Ocular and Pupillary Motility
Eye movements enable the centering of objects in the visual field and the ocular pursuit of moving objects. They are anatomically subserved by multiple neural structures, as discussed later. Lesions of the supranuclear structures (certain cortical areas, the brainstem gaze centers, and their connections to the brainstem nuclei that control eye movement) cause horizontal or vertical gaze palsy, or internuclear ophthalmoplegia. Such lesions must be distinguished from nuclear and infranuclear lesions of the third, fourth, and sixth cranial nerves. Lesions of all of these types can have a wide variety of causes. Moreover, abnormal eye movements and diplopia can also be due to diseases of neuromuscular transmission (e.g., myasthenia gravis), diseases of the extraocular muscles themselves, and orbital processes. Impaired pupillary motility has many different causes as well.
12.3.1 The General Principles of Eye Movements
Saccades, slow pursuit, and convergence are physiologic types of eye movement. Multiple anatomic structures are involved in their coordination and control: the extraocular muscles, the three cranial nerves that innervate them (the oculomotor, trochlear, and abducens nerves), and the corresponding brainstem nuclei. These nuclei, in turn, are under the influence of central impulses that originate in certain areas of the cerebral cortex and are transmitted to the brainstem nuclei via the brainstem gaze centers or the vestibular system. The brainstem nuclei that control eye movement are also linked to each other by the medial longitudinal fasciculus.
The Anatomic Substrate of Eye Movements
The anatomic substrate of eye movements consists of the following structures:
Cortical areas in the frontal, occipital, and temporal lobes, in which the impulses for voluntary conjugate eye movements and ocular pursuit are generated.
Several important gaze centers in the brainstem (particularly the paramedian pontine reticular formation [PPRF] and midbrain nuclei) that relay the cortical impulses onward to the motor nuclei innervating the extraocular muscles, which, in turn, effect coordinated movement around the three major axes (horizontal, vertical, and rotatory eye movements). Special white matter tracts play an important role in this process, particularly the medial longitudinal fasciculus (MLF, ▶ Fig. 12.4).
Finally, the motor nuclei and cranial nerves that innervate the extraocular muscles, and these muscles themselves (see ▶ Fig. 3.8).
The entire process is also affected by cerebellar impulses and by vestibular impulses that enter the central nervous system through the eighth cranial nerve.
Fig. 12.4 Anatomic substrate of conjugate eye movements. The diagram shows the anatomic 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 nerve. Impulses in the abducens nerve induce contraction of the lateral rectus muscle of the right eye. Meanwhile, cortical impulses also travel by way of the medial longitudinal fasciculus to the nucleus of the left oculomotor nerve, and impulses in this nerve induce contraction of the medial rectus muscle 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 = rostral interstitial nucleus of the medial longitudinal fasciculus, see “Internuclear Ophthalmoplegia” in section ▶ 12.3.3), which receives input from both the cerebral cortex and the PPRF.
Types of Eye Movement
Eye movements can be divided into the following types:
Saccades are rapid conjugate movements that are executed voluntarily or in reflex fashion in response to stimuli of various kinds. They serve to fix a newly selected object in the center of vision. Small microsaccades have an angular velocity of 20 degree/s, larger ones up to 700 degree/s. Saccades are the elementary type of rapid eye movement.
Once the gaze has been fixated on a given object, slow pursuit movements serve to keep it in view if it is moving. The pursuit system is responsible for executing these conjugate eye movements: from the visual cortex in the occipital lobe, impulses travel to the eye fields of the temporal lobe (“medial superior temporal visual area”) and the neighboring parietal cortex. These areas are interconnected with the PPRF and 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 result in saccadic (jumpy) pursuit movements. If the saccade system is damaged as well, gaze palsy results (see later).
Convergence enables fixation on a near object and is accomplished by simultaneous adduction of both eyes.
In purely descriptive terms, nystagmus is an involuntary, repetitive, rhythmic movement of the eyes. Nystagmus is often, but not always, pathologic.
Examples of physiologic nystagmus include optokinetic nystagmus (see “ ▶ 18.104.22.168”) and the type of vestibular nystagmus that is induced by rotation in a swivel chair. End-gaze nystagmus (see “ ▶ Vestibular Function” in section ▶ 3.3.2) is also physiologic, as long as it is symmetric in both directions. Pathologic nystagmus, on the other hand, indicates the presence of a lesion in the anatomic structures that subserve eye movements. Many components in this system can be damaged, and thus nystagmus has a wide spectrum of possible causes (see later).
Phenomenologic Classification of Nystagmus
As already discussed to some extent in section ▶ 3.3.2, nystagmus can be classified by various criteria:
Saltatory versus pendular nystagmus: most types of nystagmus are either of the salutatory (jerking) type, that is, 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 pathologic component. The rapid phase is a physiologic correction that returns the eyes to their original position.
Nystagmus can be spontaneous (see “ ▶ Vestibular Function” in section ▶ 3.3.2) 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 [see discussion on gaze-evoked nystagmus later]).
The examiner must also determine whether nystagmus is equally severe in both eyes, or whether it is weaker or perhaps absent in one eye. Nystagmus that is unequal in the two eyes is also called dissociated nystagmus.
Type of nystagmus
Must be symmetrically present
If asymmetric, dissociated, slowed, or absent
Must be symmetrically present
If asymmetric, dissociated, or absent
Up to 5 degrees/s is normal in the dark
If present in the light
If asymmetric or dissociated
Always pathologic, but need not indicate active disease
Nystagmus induced by the vestibulo-ocular reflex suppression test
(cf. VOR suppression test; nystagmus suppression test, ▶ Fig. 12.6)
Source: Henn V. Nystagmus: Klinische Prüfung und Pathophysiologie. Akt Neurologie 1978;5:237–244.
There are a few rarer types of nystagmus whose phenomenology is quite complex and not easily described by the criteria listed earlier. These types of nystagmus are summarized in ▶ Table 12.3.
Oral brainstem and diencephalon
Caudal medullary lesion, floccular lesion, vitamin B12 deficiency
(Rostral) midbrain tegmentum
Nystagmus with eyelid retraction
Pons and periaqueductal region
Medial longitudinal fasciculus
(gaze myoclonus, dancing eyes)
Brainstem and cerebellum
Pons, compression by cerebellar hemorrhage (lesion of central tegmental tract)
Ocular flutter (ocular myoclonus)
As for opsoclonus and gaze dysmetria
Fig. 12.5 The main types of nystagmus. For each type of nystagmus, the figure shows the intensity and direction of beating, depending on the direction of gaze. Positioning nystagmus is rotatory when the patient looks toward the lower ear (an exceptional case).
Topical Classification of Pathologic Nystagmus
Often, the type of nystagmus that is present provides a clue to the site of the lesion:
Gaze-paretic nystagmus may be due to disease of the eye muscles themselves or a lesion of the cranial nerves innervating them or the corresponding brainstem nuclei. It 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 nerve or its brainstem nuclei. It is typically a spontaneous nystagmus that beats away from the side of the lesion, regardless of the direction of gaze (nystagmus in a fixed direction, cf. ▶ Table 12.2). It is typically inhibited by fixation; often, it can be observed only when the patient wears Frenzel goggles or shakes 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 asymmetric or even beat only to the side of the lesion. If so, it may be hard to distinguish from vestibular nystagmus.
Nystagmus due to brainstem lesions. Vestibular spontaneous nystagmus, gaze-evoked nystagmus, upbeat or downbeat vertical nystagmus, and positional 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 [INO]).
Positioning nystagmus is a mainly rotatory nystagmus that lasts several seconds after changes of position of a particular type; it is found in , a disorder of the peripheral portion of the vestibular system (see section ▶ 12.6.2).
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, pathologic structural lesion.
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 (VOR).
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, asymmetric, or dissociated optokinetic nystagmus is pathologic.
The vestibulo-ocular reflex is a function of the labyrinth that serves to stabilize gaze fixation during rapid movements of the head: it produces a compensatory eye movement in the direction opposite to 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 section ▶ 12.3.1, ▶ 22.214.171.124). Vestibular nystagmus can be suppressed by fixation on an object moving in tandem with the head (nystagmus or VOR suppression test, see later). An inability to suppress the VOR by fixation is pathologic.
Nystagmus suppression test (= VOR suppression test). In this test, the person stretches both arms forward, holds his or her thumbs up, and fixates gaze on them. When the person is rapidly rotated around the long axis of the body, there is normally no nystagmus, because vestibular nystagmus can be suppressed by visual fixation ( ▶ Fig. 12.6). If nystagmus does appear, this indicates a lesion in the cerebellum or its connections with the vestibular apparatus of the brainstem.
Fig. 12.6 Nystagmus suppression test. The patient extends the arms, fixates gaze on his or her own thumbs, and is then rapidly rotated “en bloc” by the examiner. In a normal individual, gaze fixation on the thumbs prevents the appearance of nystagmus. Failure to suppress nystagmus indicates a central lesion, usually in the cerebellum.