Oculomotor (CN) III; Trochlear (CN) IV; Abducent nerves (CN) VI; Eye movement
Orbital conditions interfere with eye movements, as do a congenital squint, and neurological disorders of the brain and cranial nerves. The oculomotor, trochlea and abducent nerves are affected by multiple sclerosis and lesions of the midbrain, producing ptosis, squint and diplopia, The oculomotor nerve may be damaged in tentorial coning, the damage to the parasympathetic fibers producing a dilated pupil. Raised intracranial pressure may also damage the abducent nerve: this is a non-focal sign, due to the long intracranial course of this fine nerve. Nystagmus is particularly associated with disorders of the vestibular apparatus and tumors of the cerebellopontiine angle. Muscle weakness interferes with ocular movement, as in myasthenia gravis.
Figure 16 a-i – Positions of gaze:
When testing eye movements the subject initially is requested to look straight ahead, note being taken of any disconjugate activity (squint), e f and then into the nine positions of gaze.
Normal eye movements are demonstrated by asking the subject to follow your finger, moving up and down and then from side to side, the finger following an H shape (figure 17a-d). Note any nystagmus (most commonly horizontal flicking of the eye medially from the lateral extreme gaze) to each side, whether there is double vision in any direction of gaze, any squint or any defect in eye movement.
In third nerve palsy, the eye is displaced downwards and outwards. With ptosis (drooping of the upper lid), the only movement is further outwards and a little downwards. The defect, however, is often only partial and the diagnosis is supported by normal fourth and sixth nerve function. Associated parasympathetic palsy produces a dilated pupil.
Although the superior oblique muscle acting independently turns the pupil downwards and outwards, its most powerful movement is in downward gaze. The oblique end of the muscle is in line with the optical axis, and produces its maximum force, when the pupil is turned inwards by other optic muscles. The fourth nerve is therefore tested by asking the subject to look downwards and inwards, noting failure of downward gaze. Sufferers usually keep their head tilted, in an attempt to minimize diplopia.
In sixth nerve palsy the eye does not move outwards beyond the midline optical axis.
Detecting an abnormality of nerve function can be complicated if the underlying defect is primarily muscular, such as in thyrotoxicosis or myasthenia gravis, as these defects may affect individual muscles selectively.
A Horner’s syndrome (from damage to the cervical sympathetic chain) produces ptosis, meiosis (small pupil), enophthalmos (sunken eye), failure of sweating on the ipsilateral face and stuffiness of the ipsilateral nasal cavity.