Relative afferent pupillary defect

Relative Afferent Pupillary Defect (RAPD or Marcus Gunn pupil) is a condition of the eye characterized by an abnormal pupillary light reflex. It is an important clinical sign, indicating a discrepancy in the afferent pathway of the eye, which may be due to various ocular and neurological disorders. The condition is named after Scottish ophthalmologist Robert Marcus Gunn, who first described it in the late 19th century.

Etiology

RAPD occurs when there is an asymmetry in the input from the optic nerves to the brain, leading to a relative decrease in the pupillary constriction in response to light in one eye compared to the other. This can be due to a variety of causes, including:

• Optic neuritis
• Retinal detachment
• Severe glaucoma
• Advanced cataract
• Optic nerve or retinal damage due to trauma
• Ischemic optic neuropathy
• Tumors affecting the optic nerve

Pathophysiology

The pupillary light reflex involves the transmission of light signals from the retina through the optic nerve to the brain, and then back to the eye through the pupillary constrictor muscles. In RAPD, the defect in the afferent pathway (from the eye to the brain) results in a reduced or absent pupillary constriction in the affected eye when it is directly illuminated, compared to when the other eye is illuminated.

Clinical Presentation

The hallmark of RAPD is an asymmetric pupillary response to light. This is typically observed using the swinging flashlight test, where a light is moved back and forth between the two eyes. In a normal response, both pupils constrict equally, regardless of which eye is illuminated. In RAPD, the affected pupil shows a lesser degree of constriction or even dilates when the light is shone directly into it, compared to the unaffected eye.

Diagnosis

RAPD is primarily diagnosed through clinical examination, with the swinging flashlight test being the most commonly used method. Other diagnostic tests may include:

• Visual acuity tests
• Visual field testing
• Optical coherence tomography (OCT)
• Magnetic resonance imaging (MRI) of the brain and orbits

Treatment

Treatment of RAPD focuses on addressing the underlying cause. This may involve:

• Medical therapy for conditions like optic neuritis or glaucoma
• Surgical intervention for issues such as retinal detachment or tumors
• Neurological evaluation and treatment for central nervous system causes

Prognosis

The prognosis for RAPD depends on the underlying condition causing the defect. Early detection and treatment of the causative disorder can improve visual outcomes and, in some cases, may resolve the pupillary defect.

See Also

• Pupillary light reflex
• Optic neuritis
• Glaucoma
• Retinal detachment