Left RAPD (Answer A)
In a patient with dense left optic neuropathy, normal ocular motility, and no anisocoria in darkness, a left relative afferent pupillary defect (RAPD) is the expected finding.
Clinical Reasoning
The clinical scenario describes unilateral optic nerve dysfunction affecting the left eye. The key to answering this question lies in understanding the pupillary pathway and what each finding represents:
Why Left RAPD is Correct
A relative afferent pupillary defect occurs when there is asymmetric afferent input from the two optic nerves 1. The presence of a large RAPD should warrant a search for compressive optic neuropathy or other etiologies of visual impairment 1. In dense optic neuropathy, the affected eye has impaired ability to transmit light signals to the brain, resulting in:
- When light is shined on the affected (left) eye: both pupils constrict poorly or dilate paradoxically
- When light is shined on the normal (right) eye: both pupils constrict briskly
- This asymmetry creates the RAPD on the affected side
The guideline explicitly states that a RAPD of 0.3 or more log units (i.e., easily visible) is not typically seen in amblyopia and that the presence of a large RAPD should warrant a search for compressive optic neuropathy or other etiologies of visual impairment 1. Signs of optic nerve dysfunction include decreased visual acuity, color vision impairment, a relative afferent pupillary defect in the affected eye, and visual field deficits 2.
Why Other Options Are Incorrect
Option B (Left fixed dilated pupil) would indicate a third cranial nerve palsy affecting the parasympathetic fibers, not an optic neuropathy. The question specifically states there is no anisocoria in darkness, which rules this out. A pupil-involving third nerve palsy would show anisocoria and typically presents with ptosis and ophthalmoplegia 3, 4.
Option C (Right RAPD) is anatomically incorrect. The RAPD occurs on the side of the optic nerve damage, not the contralateral side. The afferent defect is ipsilateral to the neuropathy.
Option D (Bilateral miosis) is not a characteristic finding of unilateral optic neuropathy. Both pupils typically remain equal in size in darkness when there is isolated optic nerve pathology without involvement of the efferent pupillary pathway.
Clinical Pearls
- Normal ocular motility in this case excludes third nerve palsy, which would affect extraocular muscles
- No anisocoria in darkness confirms that the efferent pupillary pathway (third nerve parasympathetic fibers) is intact
- The RAPD is tested using the swinging flashlight test, where paradoxical dilation occurs when light swings to the affected eye 1
- Dense optic neuropathy requires urgent evaluation with neuroimaging to identify the underlying cause 5, 2, 6