2nd Cranial Nerve – Optice Nerve Tests

Definition:

The use and interpretation of medical examinations to determine the integrity and adequate function the Optic nerve (second cranial nerve). This nerve provides the innervation for one of the special senses: vision.

Test procedure and findings:

The eyes should be examined for any signs of ptosis, nystagmus and any abnormalities in pupil size and symmetry.

When testing the integrity and function of the Optic nerve, the following should be assessed:

  • Visual acuity
  • Colour discrimination
  • Visual fields
  • Optic reflexes
  • Fundoscopic examination

When assessing visual acuity, the patient should remove any visual corrective aids. A Schnellen chart is a convenient and effective means for examining visual acuity.

  • Colour discrimination may be assessed using the Ishihara Colour Test, consisting of thirty-eight different colour plates, and aiming to reveal common colour vision defects as well as rarer types such as protanopia and deuteranopia.
  • Visual fields are assessed by asking the patient to cover one of their eyes while the examiner assesses the opposite eye, and having the patient stare into the practitioner’s eye. The examiner can use the tip of his finger or the end of a pen and move it in each of the four quadrants of the patient’s visual field, mid-way between the patient and the practitioner, checking with the patient whether the object or finger may be seen in the periphery of each of the quadrants tested. This test is done under the assumption that the examiner’s visual fields are intact.
  • The pupillary light reflex is assessed in a relatively dark room, and asking the patient to look at a point in the distance while the examiner shines a light into each pupil. A normal response is for both the pupils to contract when the light is shone in either pupil (consensual and direct light reflexes).
  • A fundoscopic examination is performed in a darkened room. Firstly determine the location of the retina by directing the opthalmoscope at fifteen degrees from the center of the eye and following the red light reflex, until you come across the retina. As the examiner you ought to be looking out for signs of abnormal cup to disc ratio, oedema of the optic disc, arterio-venous nicking, cotton wool spots, emboli and infarcts, Roth spots, and copper wire deformities.

Magnetic resonance imaging may also be warranted, where optic neuritis is suspected. Blood tests are also useful to assess the patient’s level of antibodies in a suspected case of Neuromyelitis optica (NMO) and ESR (erythrocyte sedimentation rate) to help determine whether a patient’s optic neuritis is caused by cranial arteritis.

Considerations:

A thorough case history is invaluable in incorporating incriminating factors such as diabetes cardiovascular pathologies, and hypercholesterolaemia into the general appreciation of the patient.