Marcus Gunn pupil

Marcus Gunn Pupil

The Marcus Gunn pupil, also known as a relative afferent pupillary defect (RAPD), is a medical sign observed during the swinging flashlight test, which is used to assess the function of the optic nerve and the retina. This condition is named after the Scottish ophthalmologist Robert Marcus Gunn, who first described it in the late 19th century.

Pathophysiology

The Marcus Gunn pupil occurs when there is a defect in the afferent pathway of the eye, typically due to damage to the optic nerve or severe retinal disease. In a healthy visual system, when light is shone into one eye, both pupils constrict equally due to the consensual light reflex. However, in the presence of an RAPD, the affected eye will demonstrate less constriction compared to the unaffected eye when light is shone into it.

The underlying mechanism involves a discrepancy in the sensory input from the two eyes. When light is directed into the affected eye, the reduced sensory input results in less stimulation of the Edinger-Westphal nucleus, leading to less constriction of both pupils. Conversely, when light is directed into the unaffected eye, the pupils constrict normally.

Clinical Significance

The presence of a Marcus Gunn pupil is an important clinical sign that can indicate:

• Optic neuritis
• Retinal detachment
• Severe glaucoma
• Ischemic optic neuropathy
• Compressive lesions affecting the optic nerve

It is crucial for clinicians to identify an RAPD as it can provide valuable information about the integrity of the visual pathway and guide further diagnostic testing.

Diagnosis

The swinging flashlight test is the primary method for detecting a Marcus Gunn pupil. During this test, a light is alternately shone into each eye while observing the pupillary response. A positive test is indicated by a paradoxical dilation of the pupils when the light is moved from the unaffected eye to the affected eye.

Management

Management of a Marcus Gunn pupil involves addressing the underlying cause of the afferent defect. This may include:

• Treatment of optic neuritis with corticosteroids
• Surgical intervention for retinal detachment
• Management of glaucoma with medications or surgery

Also see

• Pupillary light reflex
• Optic nerve
• Ophthalmology
• Neurology

Classification
External resources