Optic Nerve | Cranial Nerve II / CN II Assessment

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Optic Nerve | Cranial Nerve II / CN II Assessment

The optic nerve is the second of the 12 cranial nerves and is part of the visual pathway. It can be damaged due to intracranial hypertension or a variety of metabolic disorders, by vasculitis, and diseases such as MS. The optic nerve has a sensory and reflex function, namely visual acuity and the pupillary light reflex respectively.

Many ways to assess visual acuity have been described and Kerr et al. (2010) evaluated the diagnostic accuracy of 7 of them. Sensitivity ranged between 25 to 74% and specificity from 27 to 100%. Due to their low sensitivity, they are a poor screening test yet it’s the best tool we have so the clinical value is unknown.

Visual Acuity

You may test visual acuity with the help of a Snellen or LogMAR chart. To start the patient stands 3 or 6m away from the chart depending on which one is used. Each eye is tested individually so the patient covers the other eye with their hand. If they use glasses or contact lenses, these should be worn during testing. They then read the letters on the chart out aloud. In case uncorrected acuity is less than 20/20 or 6/6 vision you can use a pinhole test where the patient reads through a 2mm pinhole in a piece of cardboard or a special device.

Visual Quadrants

Other ways include testing the visual quadrants. For example, with the patient sitting in front of you ask them to look you in the eyes. Then place the fingers in all quadrants at around 60° from the meridian. Then ask the patient to indicate which fingers are wiggling. Furthermore, you can move from the periphery diagonally to the midline and ask the patient to respond once the fingers appear in their visual field.

Reflex Function

To test the reflex function, ask the patient to make a shield between their eyes with one hand. Then use a flashlight to shine light into the pupil and observe for the narrowing of the patient’s pupils in both eyes to check for intact direct and consensual reflexes. Compare this with the other eye. Optic nerve damage would result in no reflex contraction of the pupils upon shining the light in the affected eye. Illuminating the other eye results in a normal response. Of course, the lights in the room should be dim.