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A 63 year old woman presents to the Emergency Department with the complaint of diplopia. She is keeping her right eye closed.
She awoke this morning and was seeing double. Her health is otherwise normal and her only past history is of a melanoma excised many years ago.

Her Neurological examination in general is normal. Her eye examination shows that eye movements are affected but eyelids and pupils are spared.

The video of the movements and a video lecture is available on resustv.com. The use of video was extremely helpful in this patient. I found it difficult to put together what was happening whilst examining her, however after taping it on my phone camera, I could sit and observe the eye movements in detail and decide where the deficits were.

Images of the eye movements appear below.

Look to the right
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When asked to look to her right both eyes are able to do so and there is no diplopia.
Look to the left
When asked to look to her left, the left eye abducts, but the right eye cannot look past the midline.
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Eyelids and pupils are normal.

So in summary:
She is unable to adduct the right eye and the left eye has an abduction nystagmus. Diagnosis?

This is a classic right sided internuclear ophthalmoplegia, localising the lesion to the medial longitudinal fasciculus (MLF) of the brainstem. This lies at the midline under the 4th ventricle. The side of the ophthalmoplegia is named after the side where the adduction deficit exists.

Interestingly, two days after this patient, another gentleman presented with what looked like a intracranial bleed and had aphasia and similar issues with eye movements. It was straightforward to be able to say that there was probably a bleed in the brainstem, which there was (not always as easy as this).

CLINICAL
The patient will have impaired adduction in one eye and nystagmus (reason unknown) in the contralateral eye which can abduct. The patient usually complains of diplopia on horizontal gaze, although not normally present on primary gaze. The patient may also complain of loss of stereopsis (depth perception) and vertigo. Although most of the deficit is in horizontal gaze, the vertical gaze may also be affected and can result in inability to maintain a vertical gaze as well as vertical gaze nystagmus. Could this be mistaken for a partial third nerve palsy? Absolutely. However in its classical form above, it is very much pathognomonic, however there are ways to tell. Perhaps the best clinical way is that although adduction of the eye is affected, convergence is intact in internuclear ophthalmoplegia.

DIFFERENTIALS
CNIII palsy is the most common.
Guillain-Barre Syndrome may also present in this way.

AETIOLOGY
The most common causes of this condition is multiple sclerosis or cerebrovascular disease. The only other cause to consider here is tumour given the patient’s history of melanoma.

INVESTIGATIONS
This patient required an MRI to determine cause.

PROGNOSIS
The prognosis is quite good as many of the deficits resolve. If the diplopia is significantly affecting the patient patch the eye (they don’t have depth perception anyway).