Review Question - QID 210368

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Visual Pathway Introduction Additional pathways QID: 210368 (M1.OMB.18.4871)

QID 210368 (Type "210368" in App Search)

A 60-year-old African American woman presents to her ophthalmologist with blurry vision. She reports a 2-month history of decreased vision primarily affecting her right eye. Her past medical history is notable for type 1 diabetes and hypertension. She takes insulin and enalapril. She has a 40-pack-year smoking history and drinks a glass of wine at dinner each night. Her family history is notable for glaucoma in her mother and severe diabetes complicated by nephropathy and retinopathy in her father. Her temperature is 99°F (37.2°C), blood pressure is 134/82 mmHg, pulse is 88/min, and respirations are 18/min. On exam, she is well-appearing and in no acute distress. The physician asks the patient to look forward and shines a penlight first in one eye, then the other, alternating quickly to observe the pupillary response to the light. When the light is shined in the right eye, both pupils partially constrict. When the light is shined in the left eye, both pupils constrict further. When the light is moved back to the right eye, both eyes dilate slightly to a partially constricted state. Where is the most likely site of this patient’s lesion?

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Ciliary ganglion

19%

34/176

Lateral geniculate nucleus

11%

19/176

Lens

5/176

Oculomotor nerve

16%

29/176

Optic nerve

47%

82/176

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The patient in this vignette presents with an abnormal swinging flashlight test suggestive of a relative afferent pupillary defect (RAPD). RAPDs arise from unilateral lesions in the visual pathway including the optic nerve.

The swinging flashlight test is a commonly used physical exam maneuver to localize a lesion in the visual pathway. Normally, both pupils constrict quickly and equally when either is exposed to direct light -- the pupils do not change in size when the light is moved between eyes. In a RAPD, the sensory (afferent) stimulus from the affected pathway to the midbrain is reduced due to a lesion in the afferent pathway. As a result, the unaffected pupil will dilate from its prior constricted state when the light is moved from the unaffected to the affected eye. Common causes of an RAPD are unilateral optic nerve lesions and severe unilateral retinal disease.

Incorrect Answers:
Answer 1: The ciliary ganglion has short post-ganglionic ciliary nerves that project from the ciliary ganglion to innervate the sphincter muscle of the iris. A lesion in the ciliary ganglion will therefore lead to an inability to constrict the pupil.

Answer 2: The lateral geniculate nucleus (LGN) receives the major sensory input from the retina and connects the optic nerve input to the occipital lobe via the optic radiation. A lesion in the LGN will produce a homonymous hemianopia.

Answer 3: The lens functions to direct light rays onto the retina. Ocular disease, such as a unilateral lens lesion, will result in a normal swinging flashlight test given that the visual pathway is unaffected. In cases of even severe opaque cataract, a bright enough light will lead to no RAPD.

Answer 4: The oculomotor nerve (cranial nerve III) innervates multiple extraocular muscles and preganglionic parasympathetic nerves to the ciliary ganglion.

Bullet Summary:
An abnormal swinging flashlight test suggests a relative afferent pupillary defect caused by a unilateral lesion to the afferent visual pathway.

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