Neuro-ophthalmology and pupils: one-and-a-half syndrome: practice question

A Ophthalmology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Ophthalmology · Neuro-ophthalmology and pupils: one-and-a-half syndrome

Sample item

After a brainstem infarct, a patient cannot move the right eye horizontally in either direction. The left eye cannot adduct but abducts normally, with nystagmus. Where is the lesion?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Right dorsal pons involving the abducens nucleus and MLF

In the one-and-a-half syndrome a pontine lesion involves the ipsilateral abducens nucleus or PPRF, causing gaze palsy to that side, and the adjacent MLF, causing internuclear ophthalmoplegia. Only abduction of the contralateral eye remains.

Option by option

  • A. Right abducens nerve fascicle alone

    An abducens fascicle lesion only paralyses abduction of the ipsilateral eye.

  • B. Right dorsal pons involving the abducens nucleus and MLF (correct)

    Correct. Combined gaze palsy and internuclear ophthalmoplegia point to the pons.

  • C. Left medial longitudinal fasciculus alone

    A left MLF lesion alone causes failed adduction of the left eye but the right eye would move normally.

  • D. Right oculomotor nucleus

    An oculomotor nucleus lesion causes vertical movement, ptosis and pupil involvement.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Ophthalmology question

What is the answer to this Ophthalmology question?

The best answer is B, Right dorsal pons involving the abducens nucleus and MLF. In the one-and-a-half syndrome a pontine lesion involves the ipsilateral abducens nucleus or PPRF, causing gaze palsy to that side, and the adjacent MLF, causing internuclear ophthalmoplegia.

Why is "Right abducens nerve fascicle alone" not the answer?

An abducens fascicle lesion only paralyses abduction of the ipsilateral eye.

Why is "Left medial longitudinal fasciculus alone" not the answer?

A left MLF lesion alone causes failed adduction of the left eye but the right eye would move normally.

Why is "Right oculomotor nucleus" not the answer?

An oculomotor nucleus lesion causes vertical movement, ptosis and pupil involvement.

What concept does this question test?

It tests Neuro-ophthalmology and pupils: one-and-a-half syndrome in Ophthalmology. Only abduction of the contralateral eye remains.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Neuro-ophthalmology and pupils: one-and-a-half syndrome?

Group the red eye by what to examine and what the pupil does. Then use the flashcards and the other Ophthalmology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.