• Afferent pathway: Optic (II) nerve.
• Efferent pathway: Parasympathetic component of the Oculomotor nerve (III) bilaterally (responsible for a consensual reflex)
• Loop
  • Light stimulates optic nerve which synapses to the Pretectal nucleus situated in the brainstem.
  • Pretectal area on one side innervates Edinger-Westphal nuclei (parasympathetic pre-ganglionic nuclei of the Oculomotor nerve) bilaterally.
  • Edinger-Westphal nuclei then sends axons to the ciliary body on unilaterally via the parasympathetic branch of the oculomotor nerve resulting in constriction of pupils.

• Afferent pathway: Frontal lobe
• Efferent pathway: Parasympathetic component of the Oculomotor nerve (III)

• Argyll-Roberston pupil is a small irregular pupil which is able to accommodate but does not react to light.
  • Argyll-Robertson Pupil = ARP = Accomodation Reflex Preserved
  • Also referred to in less politically correct texts as the “prostitute’s pupil”: accommodates but doesn’t react.
  • Suggestive of a dorsal midbrain lesion, commonly associated with Neurosyphillis in the past. More common causes now are diabetes, multiple sclerosis and midbrain lesions

• Holmes-Adie pupil is a myotonic pupil that usually presents in young or middle aged women. It can be associated with autonomic dysfunction and slowed deep tendon reflexes.
• A myotonic pupil is caused by damage to the post-ganglionic parasympathetic fibres in the ciliary ganglion.
• Findings
  • Pupil is irregular, unilaterally dilated and reacts very slowly to light stimulation and
  • With persistent light stimulus the pupil eventually constricts but excessively
• Check for slow deep tendon reflexes (Achilles) and look/ask for evidence of autonomic dysfunction (postural hypotension/excessive sweating/chronic diarrhoea).

Video on the Holmes-Adie Pupil

• An afferent pupillary defect is caused by a complete lesion to the optic nerve, anterior to the optic chiasm.
• In the right eye it will result in:
  • A dilated right pupil
  • A right pupil that does not constrict to light
  • A lack of consensual reflex in the patient’s left eye
• The right eye is still able to constrict to light stimulus in the left eye as the efferent pathway is intact.

• A relative afferent pupillary defect is due to a difference in the afferent pathway functioning between both eyes due to retinal or optic nerve.
• Importantly, a RAPD occurs if there is a relative difference in functioning between both eyes. If both eyes are equally affected there will not be a RAPD; therefore it is not possible to have a bilateral RAPD.
• Despite both eyes having intact direct and consensual pupillary light reflexes, when light is quickly moved from one eye to the other the affected eye with the RAPD will demonstrate abnormal dilatation when light is shone on it.

Videos on Relative Afferent Pupillary Defect (RAPD)

• Click here for an excellent RAPD explanation

Video on how to examine for a RAPD

Relative Afferent Pupillary Defect

• Regarding the swinging light test:
  • Normally, swinging a bright light between both eyes would elicit equal, sustained pupillary constriction due to the direct and consensual reflexes working together.
  • If, however, the patient has a partial lesion of their right optic nerve (right RAPD) their right pupil would abnormally dilate in response to light as the direct reflex of the affected (right) eye is weaker than the consensual reflex of the unaffected (left) eye.
  • The unaffected left eye, having lost the light stimulus, dilates causing the affected eye to dilate through the consensual reflex as it direct reflex is impaired by the RAPD.
• Using the swinging light test is a sensitive and specific test for identifying RAPD, even if the patient has significant corneal damage/cataracts or glaucoma
• To complete, offer to perform fundoscopy to search for retinal disease and signs of optic atrophy.

• Can be split into Retinal causes and Optic nerve disorders:
• Optic Nerve disorders:
  • Optic ischaemic neuropathy
  • Optic neuritis
  • Optic nerve compression
  • Unilateral severe glaucoma
• Retinal disorders:
  • Central retinal artery/vein occlusion
  • Severe ischaemic diabetic retinopathy
  • Retinal detachment
  • Infections: CMV/HSV
  • Tumours: Retinoblastoma/melanoma

• Diabetic Retinopathy can be split into Non-proliferative and Proliferative Retinopathy:
  • Non-Proliferative Retinopathy:
    • Background Retinopathy: Microaneurysms (dots) → Haemorrhages of blood into retinal layers (blots) → Hard exudates (lipid exduates)
    • Pre-proliferative Retinopathy: Cotton wool spots (areas of nerve fibre layer infarct and associated build up of axonoplasmic flow) → Venous changes (dilatation/beading/loops) → Multiple haemorrhages
  • Proliferative Retinopathy:
    • New vessel formation at the optic disc and elsewhere → prone to bleeding further
  • Diabetic Maculopathy – can occur in non/proliferative diabetic retinopathy
    • Macular oedema
    • Reduced acuity
  • Retinopathy progresses: ischaemia → proliferation → bleeding → retinal fibrosis → retinal detachment

• Based on the Keith-Wagner-Barker Classification:

• Signs of hypertensive retinopathy are not sequential and can present differently based on the severity and duration of hypertension.
• Exudative signs (Microaneurysms, dot/blot/flame haemorrhages, cotton wool spots + exudates) are caused by disruption in blood brain barrier and are non-specific signs, present in both diabetic and hypertensive retinopathy.
• Sclerotic signs (arteriole thickening, narrowing and arteriovenous nipping) indicate chronic hypertension

Hypertensive retinopathy signs on fundoscopy

• Suggests a lesion at the Optic chiasm
• Pituitary tumours compress optic chiasm from below à affect the upper visual fields first and progress further down.
  • Look for signs of pituitary disease i.e. Cushings / Acromegaly / Hypopituitarism / Gynaecomastia.
• Craniopharyngeal tumours compress optic chiasm from above à affect the lower visual fields first and progress upwards.
• Differentials of causes of bitemporal hemianopia include:
  • Pituitary adenoma
  • Sarcoidosis/Tuberculosis
  • Metastatic disease
  • Internal carotid artery aneurysm

• Homonymous: visual field defect affecting the same side in both eyes
• Congruous: visual field defect is exactly the same in both eyes
• Lesions posterior to the optic chiasm are homonymous as the nerve fibres from contralateral eye have crossed such that any lesion would affect both eyes.
  • Visual defects arising from lesions posterior to, but not including, the Lateral Geniculate Nucleus (LGN) are congruous, affecting the optic radiations and visual cortex.