Do you know what causes "red eye" when you take a flash photograph?
The choroid is a layer of tissue at the back of the eye that contains a large number of blood vessels. Red eye usually happens when a flash photograph is taken in dim light. In dim light, the pupil of the eye is dilated and allows plenty of light to enter the eye. Red eye is caused when the choroid reflects the light of the flash. The pupil does not constrict fast enough to reduce the amount of light that enters the eye. Therefore, the flash of light reflects back out of the eye and is recorded on film. Some cameras use red eye reduction methods that send out a short burst of light before the film is exposed. The brief burst of light allows the pupil to constrict and thus reduces red eye.
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But you also get white eye "leukocoria". In paediatrics this is an important sign of retinoblastoma.
You can use an opthalmoscope to view the red reflex. In a darkened room simply direct the light of the ophthalmoscope into the eye and while viewing the patients eye through the view finder and focus. Move closer to get a larger field of view. You should see a nice bright red reflex.
You can also use the ophthalmoscope as a simple screening test for strabismus. Sit about 1 metre from the patient, dim the room lights, ask the patient to fixate the ophthalmoscope light. The examiner sits still and floods each eye equally with the beam of light. Large strabismus will be easily seen. Smaller angle strabismus will show itself as a difference in the red reflex - the deviating eye red reflex will appear duller. This is called Bruckner test.
Patients with convergent deviations (esotropia) of about 15 degrees may show leukocoria in the deviating eye on photos. A red reflex will be seen in the fixing eye. However, light entering the deviating eye will hit the optic nerve head and this will be seen as a bright white reflex. Remember all patients show show a white reflex on photos need a careful fundus examination.
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