Eye Veterinary Clinic, Leominster, Herefordshire, United Kingdom
Purpose: To illustrate the value of the tapetal reflex in ophthalmic photography of the dog and cat.
Methods: Most dog and cat eyes contain a tapetum, a layer of cells situated in the inner choroid immediately outside the choriocapillaris. The tapetum is brightly coloured in shades of green, yellow or orange and its function is to reflect light back onto the photoreceptors to maximise the stimulus in dim light. The “red reflex” is important in clinical examination and ophthalmic photography in man. In most dogs and cats the equivalent of the red reflex is therefore the tapetal reflex which is not red but the colour of the tapetum i.e shades of yellow etc.
Results: The tapetal reflex is particularly valuable and effective in retro-illuminating corneal lesions, cataracts and post-cataract surgery changes. The brightness of the tapetum allows the veterinary eye photographer to take effective photos of intraocular structures even at arm’s length mimicking the view seen by distant direct ophthalmoscopy. There are particular challenges in photographing eyes with a tapetal reflex. The tapetum is extremely bright and the photographer must have control of the aperture (typically requiring a high f-stop) and the exposure to avoid over-exposure of the pupillary area. If the pupil is correctly exposed then the rest of the patient’s face may well be under-exposed but this only adds to the effectiveness of the image. The camera should be angled upwards to obtain the tapetal reflex as that is where it is situated.
Conclusion: Tapetal retro-illumination is a creative opportunity for the veterinary eye photographer and can produce striking images of animal eyes.