Cranial Nerves

Generally speaking, if cranial nerves II to VIII are intact, it is less likely that the lower cranial nerves are involved in isolation. Cranial nerve I abnormality can be found in isolation, but when it occurs is often related to injury or tumor (e.g., olfactory groove meningioma).

CRANIAL NERVE I There are occasions when testing olfactory perception is of value. In testing olfaction, the patient needs to be able to identify an odor or to distinguish between two odors, neither pungent (which may test cranial nerve (cranial nerve) V more than cranial nerve I). One cause for loss of olfactory perception is distraction of the olfactory nerves from the cribriform plate, as may occur with fracture through the cribriform plate or from falls onto the occiput.

CRANIAL NERVE II Examination of the visual fields by confrontation is important whenever a CNS hemispheral lesion is suspected or whenever the patient complains of a visual deficit. Visual field testing can be done at the bedside by comparing the patient's visual perception of moving fingers, or a moving small, white object, to your own perception in all four quadrants of the visual fields. Rapid screening of visual field deficit can be done with both eyes, but, customarily, the visual field of each eye should be tested alone.

The physiologic blind spot, increased in papilledema and inflammation of the optic nerve, can be measured using a small, white object and bisecting the distance between you and the patient. When the patient looks with one eye at your eye, find your own physiologic blind spot in the temporal field lateral to the point of fixation, by determining where the white object disappears. Measure the size of the patient's physiologic blind spot to yours, with the assumption that yours is normal.

Lesions posterior to the optic chiasm produce a homonymous visual field defect that may be complete, e.g., hemianopsia, or partial, e.g., quadrantanopsia. Lesions affecting the crossing fibers of the optic chiasm produce a bitemporal defect, which requires testing the visual field of each eye alone for recognition. Visual fields may constrict as a consequence of glaucoma, papilledema, or retinitis pigmentosa.

In nondominant parietal lobe dysfunction, bedside testing may also demonstrate visual inattention to double simultaneous stimulation. The patient perceives the visual stimulus when tested singly in the abnormal visual field but is inattentive to the stimulus when both visual fields are stimulated simultaneously. The consultant generally does more complex visual field testing.

Fundoscopic examination is an important part of the examination in conditions such as hypertensive urgencies, visual loss or visual complaints, ocular pathology, and suspected increased intracranial pressure (ICP).

The use of mydriatics for fundoscopic examination in altered mental status is problematic, because another examiner may mistake the pupillary dilatation for evidence of herniation. Furthermore, pupillary size as a finding of value in assessing clinical changes is lost.

Papilledema develops rapidly in the child (unless the fontanelles are still open), but it is not likely to be found in the adult within the first 24 h following an acute CNS lesion that results in ICP. Instead of papilledema, the early fundoscopic findings of increased ICP are:

1. The ratio of the diameter of retinal venules to arterioles increases from the normal 3:2 ratio to as high as 4:1;

2. Capillary engorgement occurs that makes the optic disc pinker;

3. The physiologic cup fills in from the bottom and all sides, obscuring the white lamina cribrosa at its depth;

4. Venous pulsations disappear, which, when seen in the upright patient, are normally at the edge of the physiologic cup. Venous pulsations are difficult for many examiners to identify; but, when seen, they may be abated by pressure on the globe. This phenomenon can be used to confirm that venous pulsations are indeed present;

5. The optic disc begins to elevate. Initially the elevation of the disc is nasal, superior, and inferior, and lastly, temporal. To determine if slight elevation of the optic disc exists, one focuses on a vessel that traverses the disc and then focuses on the same vessel on the retina. If the vessel is seen 1 diopter more positive on the disc, the disc is elevated.

Increased ICP occurring acutely many times results in an alteration of the sensorium due to involvement at a midbrain level. However, one cannot use this association to prove the presence or absence of increased ICP. Patients with pseudotumor cerebri may have increased ICP with papilledema but without demonstrable alteration of mental status.

CRANIAL NERVES III, IV, AND VI Extraocular muscle function and pupillary function are a basic part of the examination with all CNS complaints. First, one observes the pupils, their size, shape, and equality, then their reaction to light. Is there a consensual reaction to light? Is there a reaction to convergence? Table...218:1 describes common pupillary abnormalities.

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