Examination of Ocular Nerves and Function

The ocular nerves are the oculomotor nerve, abducens nerve and the trochlear nerve. Examination of these nerves may include: testing for ocular function, the light reflex, the accommodation reflex, Horner’s syndrome, The eyelids examination, ocular movements etc. These are very important examination skills in neurologic investigations.

Testing for ocular function: The pupils are examined for size, shape, equality and position on both sides.

The light reflex: When light is focussed on one eye, there is constriction of both the homolateral (direct light reflex) and the contralateral pupil (indirect or consensual light reflex). The afferent pathway of this reflex is through the optic nerve, and the efferent is through the oculomotor nerve. Each eye should be tested separately.

The accommodation reflex: This is elicited by asking the patient to shift his gaze to some near object after he has relaxed his accommodation by gazing into the distance. The response to be noted is pupillary constriction and convergence of the eyes.

Horner’s sydrome: This is seen in paralysis of sympathetic innervation to the eye. The affected pupil is smaller than its fellow (miosis), but it reacts normally to light and accommodation. There is partial ptosis as a result of paralysis of the tonic elevators of the tarsus (Muller’s muscles). There is absence of sweating (anhydrosis) on the affected side of the face. A mild degree of enophthalmos (sunken eye) may also be noticeable.

The eyelids: The patient is asked to open and close his eyes, without and against resistance. Note the size of the palpebral fissure. Paralysis of the levator palpebrae superioris causes drooping of the upper eyelid (ptosis). Paralysis of upper tarsal muscles causes pseudoptosis, in which there is parital ptosis but the patient can voluntarily raise the eyelid. Partial ptosis is often compensated by the patient by the over-action of the frontalis muscle of the forehead on that side. A narrow palpebral fissure indicates paralysis either of third nerve or sympathetic, whereas a wide palpebral fissure indicates seventh nerve paralysis or sympathetic over-activity.

The ocular movements: This is tested by having the patient move his eyeballs in the six cardinal directions-laterally, medially, upward and laterally, upward and medially, downward and laterally and downward and medially. The lateral rectus moves the eye outwards and the medial rectus inwards. When the eye is turned outwards, the elevator is superior rectus and the depressor is inferior rectus. When the eye is turned inwards, the elevator and depressor are the inferior and superior oblique muscles respectively. In third nerve palsy, there is ptosis, with inability to move the eyeball upward, downward and medially. On lifting the lid passively, the eye is found deviated outwards and slightly downward due to unopposed actions of lateral rectus and superior oblique muscles. In addition, the pupil is dilated and non-reacting (iridoplegia). There is also paralysis of accommodation (cycloplegia) due to parasympathetic involvement. In paralysis of the fourth nerve, the eye is rotated outwards (extorted) with weakness on downward movement, most marked when the eye is of special difficulty in reading or going down stairs, and the head if tilted towards the opposite shoulder. In sixth nerve lesion, the eyeball is deviated inwards due to the unopposed action of the medial rectus.