A physiological blind spot exists in all healthy individuals as a result of the lack of photoreceptor cells in the area where the optic nerve passes through the optic disc. In day to day life, the brain does an excellent job of reducing our awareness of the blind spot by using information from other areas of the retina and the other eye to mask the defect.
Blind spot assessment
1. Sit directly opposite the patient, at a distance of around 1 metre.
2. Ask the patient to cover one eye with their hand.
3. If the patient covers their right eye, you should cover your left eye (mirroring the patient).
4. Ask the patient to focus on part of your face (e.g. nose) and not move their head or eyes during the assessment. You should do the same and focus your gaze on the patient’s face.
5. Using a red hatpin (or alternatively, a cotton bud stained with fluorescein/pen with a red base) start by identifying and assessing the patient’s blindspot in comparison to the size of your own. The red hatpin needs to be positioned at an equal distance between you and the patient for this to work.
6. Ask the patient to say when the red part of the hatpin disappears, whilst continuing to focus on the same point on your face.
7. With the red hatpin positioned equidistant between you and the patient, slowly move it laterally until the patient reports the disappearance of the top of the hatpin. The blind spot is normally found just temporal to central vision at eye level. The disappearance of the hatpin should occur at a similar point for you and the patient.
8. After the hatpin has disappeared for the patient, continue to move it laterally and ask the patient to let you know when they can see it again. The point at which the patient reports the hatpin re-appearing should be similar to the point at which it re-appears for you (presuming the patient and you have a normal blind spot).
9. You can further assess the superior and inferior borders of the blind spot using the same process.
Assess the patient's blind spot
The blind spot is located just temporal to central vision at eye level
Map out the patient's blind spot and compare to your own
Causes of an enlarged blind spot
An enlarged blind spot is typically associated with a swollen optic disc (papilloedema) which is most often caused by raised intracranial pressure (e.g. brain tumour, hypertensive crisis, intracranial haemorrhage).
To complete the examination
Explain to the patient that the examination is now finished.
Thank the patient for their time.
If mydriaticdrops were instilled, remind the patient they cannot drive for the next 3-4 hours until their vision has returned to normal.
Wash your hands.
Summarise your findings
Suggest further assessment and investigations
All of the following further assessments and investigations are dependent on the patient’s presenting complaint and in most cases, none of them would need to be performed: