Right Lateral Rectus (Answer: B)
The right lateral rectus muscle is primarily affected in this patient. The lateral rectus is the sole muscle responsible for abduction (outward movement) of the eye, and it is innervated by the abducens nerve (cranial nerve VI) 1, 2.
Clinical Reasoning
When a patient presents with:
- Impaired abduction of the right eye
- Deficit greatest when looking toward the right (ipsilateral gaze)
- Diplopia (binocular double vision)
This clinical triad points directly to right lateral rectus dysfunction. The lateral rectus muscle is singularly responsible for moving the eye laterally (abduction), and when it fails, the eye cannot move outward properly 1.
Why Not the Other Options?
Right medial rectus (Option A): This muscle performs adduction (inward movement), not abduction. Dysfunction would cause inability to look nasally, not laterally.
Left medial rectus (Option C): While the left medial rectus works synergistically with the right lateral rectus during rightward gaze (conjugate eye movements), the primary deficit described is abduction of the right eye specifically. The problem is localized to the right eye's ability to move outward.
Right superior oblique (Option D): This muscle primarily causes depression (downward movement) when the eye is adducted, plus intorsion. It does not control horizontal abduction 3.
Anatomical Considerations
The abducens nerve (CN VI) has a long intracranial course, making it vulnerable to various pathologies including microvascular ischemia, increased intracranial pressure, trauma, infections (such as sphenoid sinusitis), and compressive lesions 4, 1, 5.
Key diagnostic point: Binocular diplopia that resolves with covering either eye, combined with restricted abduction and preserved adduction, confirms an abduction deficit rather than a convergence or divergence problem 2, 6.
Clinical Pitfalls to Avoid
When evaluating impaired abduction, consider:
- Sixth nerve palsy versus lateral rectus restriction (e.g., from orbital trauma, thyroid eye disease, or muscle entrapment in blowout fractures) 5
- In sagging eye syndrome or myopic strabismus fixus, the lateral rectus may sag inferiorly, mechanically impairing abduction 4
- Neuroimaging with MRI/MRA or CT/CTA is indicated if there are signs of increased intracranial pressure, progressive symptoms, or atypical features suggesting compressive lesions 4
The straightforward presentation of isolated abduction deficit in the right eye when looking right makes the right lateral rectus the primary affected muscle.