11 Diseases of the Cranial Nerves

The cranial nerves can be affected by disease in isolation, or as a component of a wider disease process. Cranial nerve deficits are caused by lesions of their nuclei or tracts in the brainstem, or of the peripheral course of the nerves themselves and their branches.

The anatomical relationships of the cranial nerves to the base of the skull are shown in Fig. 3.3 (p. 17), while their anatomical course and distribution are summarized in Table 3.3 (p. 17). The causes and clinical manifestations of cranial nerve lesions are discussed in detail in this chapter.

Anatomy. The peripheral olfactory receptors can only be excited by substances dissolved in fluid. The receptors of the olfactory mucosa project their axons through the cribriform plate to the olfactory bulb (Fig. 3.3, p. 17), which lies on the floor of the anterior cranial fossa, beneath the frontal lobe. 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.

Clinical manifestations. Techniques for examining the sense of smell are discussed on p. 16. Only the following types of olfactory disturbances are relevant to neurological diagnosis:

Anosmia. A more or less complete loss of the sense of smell is usually due to disorders of the nose, most commonly rhinitis sicca. The most common neurological

cause of anosmia is traumatic brain injury resulting in a brain contusion and/or traumatic avulsion of the olfactory fibers as they traverse the cribriform plate. The anosmia regresses in one-third of patients, but distortions of olfactory perception, so-called parosmias, often persist, sometimes in the form of unpleasant kakosmia (see below). 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, and diabetes mellitus. 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 anterior, medial portion of the temporal lobe. These hallucinations are sometimes called uncinate fits.

Visual disturbances can be caused by lesions of the retina or of its connections with the visual cortex. Depending on the etiology, the disturbance may consist of an impairment of visual acuity (ranging to total blindness) or a visual field defect, and these

problems may appear either suddenly or gradually. In addition, the site of the lesion determines the type of visual field defect that will be present and whether it will affect only one eye or both. A simple clinical rule is that lesions of the retina and

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Diseases of the Cranial Nerves

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Fig. 11.1 Infarct in the territory of the left posterior cerebral a. in a patient with right homonymous hemianopsia. MR image. a The axial T2-weighted image reveals abnormally bright signal in the cerebral cortex and in the underlying white matter, as well as a

Impairment of Visual Acuity

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. The sudden onset of the defect implies that it has been caused by ischemia. A defect of this type may be permanent, e. g., in occlusion of the central retinal a. due to temporal arteritis or embolization from an atheromatous plaque in the carotid a., 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 disturbance of neural function, such as migraine (retinal migraine). Papilledema, too, can be accompanied by episodes of sudden visual loss (amblyopic attacks). Ocular causes should not be forgotten in the differential diagnosis, e. g., retinal detachment, preretinal hemorrhage, or central vein thrombosis. Correct diagnosis requires a precise clinical history and meticulous examination of the optic disc and fundus.

Sudden bilateral loss of vision may be due to bilateral retinal ischemia, e. g., on standing up in a patient with aortic arch syndrome. Certain types of intoxication can also rapidly produce bilateral optic nerve lesions, e. g., methanol poisoning, which causes blindness within hours. Yet, bilateral visual loss of more or less sudden onset is much more commonly 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 aa. on both sides simultaneously and compressive occlusion of the posterior cerebral aa. by an intracranial mass. Patients often deny that they cannot see (anosognosia). Despite the severe visual loss, the

small hemorrhage within the infarct. b The diffusion-weighted MRI reveals diminished diffusion of protons and water molecules in the first few days after the event. Fresh infarcts are very well seen in this type of study. The hemorrhage, too, is visible in this case.

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 (VEP, p. 56 and Fig. 4.19, p. 59), however, are pathological.

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 (inflammation of the optic n. between the retina and the chiasm) and optic papillitis (inflammation of the optic n. at the level of the optic disc) cause unilateral visual loss within two days or a little longer. Progressive, unilateral visual loss should also always prompt suspicion of a mass: optic glioma, for example, is a primary tumor within the optic n. that is more common in children and an optic sheath meningioma can compress the nerve from outside. Retrobulbar neuritis rarely occurs bilaterally, sometimes in combination with myelitis. Further causes of bilateral loss of visual acuity are Leber hereditary optic atrophy and tobaccoalcohol amblyopia. In the latter condition, the most prominent initial finding is an inability to distinguish the colors red and green. Vitamin B12 deficiency can cause progressive optic atrophy in combination with polyneuropathy.

Pathological Findings of the Optic Disc

This is an area requiring close collaboration between the neurologist and the ophthalmologist.

Papilledema is usually a reflection of intracranial hypertension, but it can also be seen in infectious and inflam-