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Cranial nerve examination
Eye dilation
One component of the examination (III) uses the pupillary light reflex to assess the status of the oculomotor nerve (CNIII).
Purposepart of the neurological examination

The cranial nerve exam is a type of neurological examination. It is used to identify problems with the cranial nerves by physical examination. It has nine components. Each test is designed to assess the status of one or more of the twelve cranial nerves (I-XII). These components correspond to testing the sense of smell (I), visual fields and acuity (II), eye movements (III, IV, VI) and pupils (III, sympathetic and parasympathetic), sensory function of face (V), strength of facial (VII) and shoulder girdle muscles (XI), hearing and balance (VII, VIII), taste (VII, IX, X), pharyngeal movement and reflex (IX, X), tongue movements (XII).

Components

CN I

The first test is for the olfactory nerve. Smell is tested in each nostril separately by placing stimuli under one nostril and occluding the opposing nostril. The stimuli used should be non-irritating and identifiable. Some example stimuli include cinnamon, cloves, and toothpaste. Loss of the sense of smell is called anosmia and can be either unilateral or bilateral. Bilateral loss can occur with rhinitis, smoking, or aging. Unilateral loss indicates a possible nerve lesion or deviated septum. This test is usually skipped on a cranial nerve exam.[1]

The short axons of the first cranial nerve regenerate on a regular basis. The neurons in the olfactory epithelium have a limited life span, and new cells grow to replace the ones that die off. The axons from these neurons grow back into the CNS by following the existing axons—representing one of the few examples of such growth in the mature nervous system. If all of the fibers are sheared when the brain moves within the cranium, such as in a motor vehicle accident, then no axons can find their way back to the olfactory bulb to re-establish connections. If the nerve is not completely severed, the anosmia may be temporary as new neurons can eventually reconnect.[2]

CN II

Vision via the optic nerve is examined both in fields of vision, and in clarity of vision.

CN III, CN IV, CN VI

The three nerves that control the extraocular muscles are the oculomotor nerve (CN III), the trochlear nerve (CN IV), and the abducens nerve (CN VI). As the name suggests, the abducens nerve is responsible for abducting the eye, which it controls through contraction of the lateral rectus muscle. The trochlear nerve controls the superior oblique muscle to rotate the eye along its axis in the orbit medially, which is called intorsion, and is a component of focusing the eyes on an object close to the face. The oculomotor nerve controls all the other extraocular muscles, as well as a muscle of the upper eyelid. Movements of the two eyes need to be coordinated to locate and track visual stimuli accurately. When moving the eyes to locate an object in the horizontal plane, or to track movement horizontally in the visual field, the lateral rectus muscle of one eye and medial rectus muscle of the other eye are both active. The lateral rectus is controlled by neurons of the abducens nucleus in the superior medulla, whereas the medial rectus is controlled by neurons in the oculomotor nucleus of the midbrain.[2]

CN V

Testing the trigeminal nerve involves testing its three branches.

CN VII

The facial nerve is tested by inspecting for facial asymmetry and involuntary movements. The individual is asked to:

  1. Raise both eyebrows
  2. Frown
  3. Close both eyes tightly so that you can not open them. Test muscular strength by trying to open them
  4. Show both upper and lower teeth
  5. Smile
  6. Puff out both cheeks

The sensory component is tested for taste. Testing this is as simple as introducing salty, sour, bitter, or sweet stimuli to either side of the tongue. The patient should respond to the taste stimulus before retracting the tongue into the mouth. Stimuli applied to specific locations on the tongue will dissolve into the saliva and may stimulate taste buds connected to either the left or right of the nerves, masking any lateral deficits.[2]

CN VIII

The vestibulocochlear nerve is tested for hearing and balance.

More sensitive hearing tests are Rinne test and Weber test. The Rinne test involves using a tuning fork to distinguish between conductive hearing and sensorineural hearing. Conductive hearing relies on vibrations being conducted through the ossicles of the middle ear. Sensorineural hearing is the transmission of sound stimuli through the neural components of the inner ear and cranial nerve. A vibrating tuning fork is placed on the mastoid process and the patient indicates when the sound produced from this is no longer present. Then the fork is immediately moved to just next to the ear canal so the sound travels through the air. If the sound is not heard through the ear, meaning the sound is conducted better through the temporal bone than through the ossicles, a conductive hearing deficit is present.[2]

The Weber test also uses a tuning fork to differentiate between conductive versus sensorineural hearing loss. In this test, the tuning fork is placed at the top of the skull, and the sound of the tuning fork reaches both inner ears by travelling through bone. In a healthy patient, the sound would appear equally loud in both ears. With unilateral conductive hearing loss, however, the tuning fork sounds louder in the ear with hearing loss. This is because the sound of the tuning fork has to compete with background noise coming from the outer ear, but in conductive hearing loss, the background noise is blocked in the damaged ear, allowing the tuning fork to sound relatively louder in that ear. With unilateral sensorineural hearing loss, however, damage to the cochlea or associated nervous tissue means that the tuning fork sounds quieter in that ear.[2]

CN IX, CN X

The glossopharyngeal nerve (CN IX) and vagus nerve (CN X) are tested for:

CN XI

The accessory nerve is tested for:

CN XII

The hypoglossal nerve has a sole motor function for most of the muscles of the tongue:

See also

References

  1. ^ Jon Brillman; Scott Kahan (1 March 2005). In A Page Neurology. Lippincott Williams & Wilkins. pp. 4–. ISBN 978-1-4051-0432-6. Retrieved 27 June 2011.
  2. ^ a b c d e f g  This article incorporates text available under the CC BY 4.0 license. Ju, William (November 11, 2023). Neuroscience. Toronto: University of Toronto. 1.6 The Neurological Exam.
  3. ^ "Examination of the Cranial Nerves". 22 April 2016.
  4. ^ "Examination of the Cranial Nerves". Archived from the original on 2021-03-09. Retrieved 2014-06-26.