neurologic disorders
How to Assess the Cranial Nerves
8th Cranial nerve
Because the 8th (vestibulocochlear, acoustic, auditory) cranial nerve carries auditory and vestibular input, evaluation involves Evaluation None Vestibular function tests is first tested in each ear by whispering something while occluding the opposite ear. Any suspected loss should prompt formal Testing to confirm findings and help differentiate conductive hearing loss from sensorineural hearing loss. The can be evaluated by testing for Nystagmus . The presence and characteristics (eg, direction, duration, triggers) of nystagmus help identify vestibular disorders and sometimes differentiate central from peripheral vertigo. Vestibular nystagmus has 2 components: A slow component caused by vestibular input A quick, corrective component that causes movement in the opposite direction (called beating) The direction of the nystagmus is defined by the direction of the quick component because it is easier to see. Nystagmus may be rotary, vertical, or horizontal and may occur spontaneously, with gaze, or with head motion. When trying to differentiate central from peripheral causes of vertigo, the following guidelines are reliable and should be considered at the onset: There are no central causes of unilateral hearing loss because peripheral sensory input from the 2 ears is combined virtually instantaneously as the peripheral nerves enter the pons. There are no peripheral causes of CNS signs. If a CNS sign (eg, cerebellar ataxia) appears at the same time as the vertigo, the localization is virtually certain to be central. Evaluation of vertigo using nystagmus testing is particularly useful in the following situations: When patients are having vertigo during the examination When patients have acute vestibular syndrome When patients have episodic, positional vertigo If patients have acute vertigo during the examination,nystagmus is usually apparent during inspection. However, visual fixation can suppress nystagmus. In such cases, the patient is asked to wear +30 diopter or Frenzel lenses to prevent visual fixation so that nystagmus, if present, can be observed. Clues that help differentiate central from peripheral vertigo in these patients include the following: If nystagmus is absent with visual fixation but present with Frenzel lenses, it is probably peripheral. If nystagmus changes direction (eg, from one side to the other when, for example, when the direction of gaze changes), it is probably central. However, absence of this finding does not exclude central causes. If nystagmus is peripheral, the eyes beat away from the dysfunctional side. When evaluating patients with acute vestibular syndrome(rapid onset of severe vertigo, nausea and vomiting, spontaneous nystagmus, and postural instability), the most important maneuver to help differentiate central vertigo from peripheral vertigo is the head thrust maneuver. With the patient sitting, the examiner holds the patient's head and asks the patient to focus on an object, such as the examiner's nose. The examiner then suddenly and rapidly turns the patient's head about 20° to the right or left. Normally, the eyes stay focused on the object (via the vestibular ocular reflex). Other findings are interpreted as follows: If the eyes temporarily move away from the object and then a frontal corrective saccade returns the eyes to the object, nystagmus is probably peripheral (eg, Vestibular Neuronitis ). The vestibular apparatus on one side is dysfunctional. The faster the head is turned, the more obvious is the corrective saccade. If the eyes stay focused on the object and there is no need for a corrective saccade, nystagmus is probably central (eg, cerebellar stroke). the Nystagmus is done to test for obstruction of the posterior semicircular canal with displaced otoconial crystals (ie, for A latency period of 5 to 10 seconds Usually, vertical (upward-beating) nystagmus when the eyes are turned away from the affected ear and rotary nystagmus when the eyes are turned toward the affected ear Nystagmus that fatigues when the Dix-Hallpike maneuver is repeated In contrast, positional vertigo and nystagmus related to CNS dysfunction have no latency period and do not fatigue. The Epley canalith repositioning maneuver (see figure) can be done for both sides to help confirm the diagnosis of BPPV. If the patient has BPPV, there is a high probability (up to 90%) that the symptoms will disappear after the Epley maneuver, and results of a repeat Dix-Hallpike maneuver will then be negative. 