A patient has a lesion at the level of the right pontine tegmentum affecting the medial longitudinal fasciculus (MLF). On testing horizontal gaze to the left, which finding is expected?
- A Failure of right eye adduction on leftward gaze; nystagmus in the left abducting eye ✓
- B Failure of left eye abduction on leftward gaze; normal right eye
- C Failure of both eyes to deviate left; preserved convergence
- D Upbeat nystagmus in primary gaze with normal horizontal movement
Correct answer: A. Failure of right eye adduction on leftward gaze; nystagmus in the left abducting eye
Explanation
A right MLF lesion produces right internuclear ophthalmoplegia (INO): the right eye fails to adduct on leftward conjugate gaze because the signal from the left abducens nucleus cannot reach the right medial rectus subnucleus via the right MLF. The abducting left eye shows nystagmus. Convergence is typically preserved because the medial rectus pathway for convergence bypasses the MLF. Option B describes a sixth nerve palsy, and option C suggests a gaze palsy from paramedian pontine reticular formation damage.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.