Relative Afferent Pupillary Defect (Marcus-Gunn Pupil)

Relevant physical signs

• Testing for an RAPD
  • RAPD demonstrates an ipsilateral partially-injured optic nerve
  • For a right-sided RAPD:
    • At rest, both pupils are of equal size in dim light
    • When light is shone into the left eye, both pupils constrict normally
    • When the light is swung over to the right side one second later, both pupils dilate, although they remain smaller than at rest.
    • When the light is swung back to the left side, both pupils constrict again
    • Pupils always remain equal
  • RAPD is shown by weaker bilateral pupil constriction in the affected eye compared with the other
• Red desaturation
  • Test visual acuity to finger counting
  • Hold up a red hat pin an ask the patient what colour it is
• Eye movements
  • Internuclear ophthalmoplegia
  • Complex ophthalmoplegias
  • Nystagmus
• Other cranial nerves
  • Jaw jerk (pseudobulbar palsy)
  • Look for facial weakness
  • Look for pseudobulbar palsy
    • Increased gag reflex
    • Absent palatal movement
    • Spastic tongue (cannot be protruded)
• Cerebellar
  • Assess for cerebellar / bulbar / pseudobulbar speech
  • Intention tremor / dysmetria
  • Dysdiadochokinesis
  • Assess gait if possible
• Others
  • Pronator drift

Differential diagnosis

• Optic neuropathies
  • Optic neuritis
    • Multiple sclerosis
    • Neuromyelitis optica
  • Giant cell arteritis
  • Glaucoma
  • Traumatic optic nerve injury
  • Optic nerve glioma
  • Sarcoidosis
  • Systemic lupus erythematosus
  • Sjögren’s syndrome
  • Wegner’s granulomatosis
• Orbital disease
  • Thyroid eye disease
  • Orbital cellulitis
  • Orbital tumour
• Infection
  • Cryptococcus
  • Lyme disease
  • West Nile virus
  • Cytomegalovirus
  • Toxoplasmosis
  • Herpes simplex virus
  • Syphilis
• Iatrogenic
  • Ethambutol
  • Infliximab
  • Radiation-induced optic nerve damage
• Leber’s optic neuropathy
• Retinal disease
  • Retinal detachment
  • Central retinal vein occlusion
  • Central retinal artery occlusion
  • Severe macular degeneration

Investigations

• Visual evoked potentials
• Consider aquaporin-4 antibodies (neuromyelitis optica)
• Lumbar puncture looking for unpaired oligoclonal bands
• Magnetic resonance imaging of the brain and spinal cord looking for lesions disseminated in space
• Consider temporal artery biopsy
• Consider auto-antibody panel
• Thyroid function tests if clinically-indicated

Management

• See section on Multiple Sclerosis
• For giant cell arteritis: prednisolone 1mg/kg for two weeks, then tail