Abducens nerve /Lateral Rectus palsy How to diagnose 6th nerve palsy ?

Abducens nerve -6th nerve palsy : Lateral Rectus palsy

THE ABDUCENS NERVE
The nucleus of the abducens, or sixth cranial nerve (CN VI), lies in the mid to lower pons, in the gray matter of the dorsal pontine tegmentum in the fl oor of the fourth ventricle, encircled by the looping fi bers of the facial nerve.

It supplies the lateral rectus muscle which causes abduction of eyes.

Sixth nerve palsies are common, and many resolve with no explanation.
With a complete CN VI palsy, the eye cannot be abducted and often rests in a position of adduction.
Patients present with horizontal diplopia worse at distance.


Clinical examination:

Assessment of ocular movements should include an assessment of visual acuity. When acuity is impaired, the patient may not be able to adequately fixate.

Head position should also be observed as head tilt is a compensatory mechanism to minimise diplopia.

Assess for visual fixation.

A normal patient can fixate steadily on an object of regard, whether near or distant. Ideally,a linear target should be held perpendicular to the direction of gaze.

The examination of extra-ocular movements is done by assessing versional pursuit movements in the six cardinal positions of gaze.

The target should slowly trace a large letter “H” for the patient to follow.


Causes:

CNS Neoplasms & trauma
Demyelinating diseases
Microvascular neuropathy

Elevated ICP often produces CN VI dysfunction due to stretching
of the nerve over the petrous tip as the increased pressure forces the brainstem attachments inferiorly.

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