7.5 Optic Nerve Function

Optic nerve function testing is a critical component of many examinations. Candidates should be able to rapidly determine whether there is optic nerve dysfunction; this is important both for localisation of pathology, and for determining the severity of disease.

Examination Outline

Examination Structure

1. Visual Acuity (Best Corrected)

2. Visual Fields to Confrontation

See Chapter 7.6 Visual Fields to Confrontation

  • It is important to check visual fields prior to brightness saturation

3. Redness Saturation

  • Show the patient with a red top (atropine) bottle, red top blood collection tube or a red card, and cover the eye with suspected abnormal vision first. Covering the patient’s eye with your own hand is often faster and more reliable than asking them to do it themselves. Ask “Is this 100% red?”. If it is, ask “if this is 100% (or $100 worth of) redness, how much is this?” (as you alternate to cover the other eye). It is important that the patient be told to describe redness, not brightness. Redness and brightness desaturation correlate well with the presence of a RAPD. Red saturation of less than 90% compared with the other eye is significant.[vii]

4. Brightness Saturation

  • Shine light from a direct or indirect ophthalmoscope into one of the patient’s eyes at a time, covering the eye with suspected abnormal vision first. It is important to have the light the same intensity in both eyes - hold it straight on and avoid using a pen torch (light from these are often uneven). Ask “Is this 100% bright?”. If it is, ask “If this is 100% (or $100 worth of) brightness, how much is this?” (as you alternate to cover the other eye)

5. Pupils / RAPD

6. Additional Examinations

  1. IOP. Ask to check the pressure and view the optic disc (proactive candidates may proceed to this without prompting but may be stopped by the examiner if it is not necessary)
  2. Colour vision. The most accurate tests to use are the City University and Farnsworth-Munsell 100 hue or D15. Ishihara plates are intended as a screening test for red-green congenital colour vision defects, but are widely used nevertheless to screen for acquired colour vision problems

Investigations

A) Functional studies

  1. Goldmann perimetry
  2. Automated perimetry (e.g. Humphrey Visual Fields 30-2)
  3. Pattern VEP to look for latency delay

B) Structural Studies

  1. OCT retinal nerve fibre layer, optic disc, macular ganglion cell layer analysis
  2. CT orbits / brain if urgent or compressive meningioma is suspected
  3. MRI optic nerve and brain with gadolinium for visual pathway lesions (e.g. demyelinating lesions)

          

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