Oculomotor Nerve | Cranial Nerve III / CN III Assessment

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Oculomotor Nerve | Cranial Nerve III / CN III Assessment

Eye movements are controlled by three cranial nerves: the oculomotor, trochlear, and abducens nerve. Weakness of one or more of the extraocular muscles impairs movement of the affected eye and restricts its ability to gaze in a particular direction.

The ability to move the eye in all other directions is controlled by the oculomotor nerve. It innervates the medial rectus, superior rectus, inferior rectus and inferior oblique muscles to cause orbital rotation. It can be distinguished from the trochlear nerve (CN IV), which controls downward eye movement toward the nose, and the abducens nerve (CN VI) which controls horizontal eye movement.

Pupil Assessment

Examination of the cranial nerves III, IV & VI is usually carried out together and starts with the pupils. Inspect the size shape and symmetry of the pupils. The pupillary light reflex is elicited by shining a light into the eyes and assesses the oculomotor nerve which innervates the sphincter pupillae muscle constricting the pupil.

Shining a light into one eye should result in constricting of that eye’s pupil, called the direct response, as well as constriction of the other pupil called the indirect response. Damage to the oculomotor nerve results in absence of the light reflex.

Next assess the accommodation reflex. The patient is asked to first look into the distance and then focus on the tip of their nose in the second step. Here the pupil should also constrict.
Eye Movement Assessment

Eye movements in each direction can be assessed in six steps. Without moving the head, the patient is asked to gaze:

Upward and to the right
Upward and to the left
Horizontally to the right
Horizontally to the left
Downward and to the right
And downward and to the left

Alternatively, ask the patient to follow a hatpin or pen in an H-Pattern.Ask the patient if they experience double vision and check for failure of movement.