Can be tested by the swinging torch test to compare the afferent pathway of the pupillary reflex. This can be used as a marker of the integrity of the afferent limb of the reflex and tests both retinal and nerve pathways.
Imagine a defect with the right eye - the swinging torch experiment
- Step 1 :A torch is shone in the right eye - due to a defect there is no pupillary constriction either direct or consensual
- Step 2 : Torch is shone in the left eye. The left pupil constricts as a direct response and the right pupil constricts too as a consensual response
- Step 3 : Torch then shone back in the right eye which had just been constricting and so is seen to dilate when the light shines
- This is called a Marcus Gunn pupil and is a sign of a relative afferent pupillary defect
- Brief oscillations of the pupillary size called hippus occur normally in response to light which should not be confused with an afferent pupillary defect.
Causes
- Optic Neuritis: Inflammation of the optic nerve e.g. MS, is a common cause of RAPD.
- Ischemic Optic Neuropathy: ischaemia of optic nerve can lead to RAPD, often seen in conditions like giant cell arteritis or anterior ischaemic optic neuropathy (AION).
- Retinal Detachment: can cause a significant reduction in afferent input, leading to RAPD.
- Optic Nerve Compression: Tumours/masses that compress the optic nerve can cause RAPD.
- Glaucoma: Advanced glaucoma can damage the optic nerve, potentially leading to RAPD.
- Severe Retinal Disease: e.g. central retinal artery occlusion (CRAO) or large retinal infarctions can lead to RAPD.
Diagnosis
- Swinging Flashlight Test: main clinical test for detecting RAPD. The examiner shines a light in one eye and then quickly moves it to the other eye. In the presence of RAPD, the affected eye will show less pupillary constriction compared to the unaffected eye.
- Visual Acuity Testing: visual acuity can help determine the extent of visual impairment associated with RAPD.
- Color Vision Testing: Ishihara test can help identify color vision deficiencies that may be associated with optic nerve damage.
- Ophthalmoscopy: Direct examination of the optic nerve head and retina can reveal signs of optic neuritis, retinal detachment, or other underlying conditions.
- Imaging Studies: MRI or CT scans may be used to identify optic nerve lesions, tumours, or other structural abnormalities.
Summary
A relative Afferent Pupillary Defect (RAPD) is a significant clinical sign indicating asymmetry in the sensory input from the optic nerves or retinas. It is commonly detected using the swinging flashlight test and is associated with various conditions such as optic neuritis, ischaemic optic neuropathy, and retinal detachment. Early diagnosis and treatment of the underlying cause are essential for preserving vision and improving outcomes.