Final answer:
An MLF lesion in internuclear ophthalmoplegia leads to a deficit in conjugate lateral gaze and accounts for the failed abduction in one eye, resulting in diplopia. This condition is often due to damage to the abducens nerve or the medial longitudinal fasciculus itself.
Step-by-step explanation:
An MLF lesion seen in internuclear ophthalmoplegia (INO) involves damage to the medial longitudinal fasciculus, which disrupts the coordination between the eyes by impairing conjugate lateral gaze. This leads to the inability of the lateral rectus muscle of one eye to contract properly, while the other eye moves normally. In INO, the affected eye fails to abduct, causing symptoms such as diplopia or double vision. During the testing of eye movements, clinicians look for command following of both eyes.
In cases of INO, there is a noticeable discrepancy, as the eye on the affected side does not abduct, resulting in an inability to maintain conjugate lateral gaze. This condition typically arises from damage to the abducens nerve or the MLF, which connects various brainstem nuclei involved in eye and head position control.