The Eye and Orbit

The extracocular muscles are responsible for movement of the eyes. The eyes move in a coordinated fashion, and failure to do so results in double vision, diplopia. The conjugate movement of the eyes means that for any one movement, different muscles and nerves might be involved in the two eyes. The eyes are set in the orbits in such a way that they naturally tend to point laterally. Looking straight ahead means that both eyes have to be adducted. Testing of the extraocular muscles and their nerves is somewhat complicated due to the position of the eyes, offset from the sagittal plane.

There are six extraocular muscles:

1. Superior rectus - oculomotor nerve

2. Inferior rectus

3. Medial rectus

4. Inferior oblique

5. Lateral rectus - abducent nerve

6. Superior oblique - trochlear nerve

Eye movements are usually due to the action of more than one muscle. However each muscle can be tested with the eye in the optimal position for its action. The superior and inferior recti are tested with the eye abducted. The obliques are tested with the eye adducted.

Pupil

The iris controls the size of the pupil. The sphincter pupillae muscle is a circular muscle which constricts the pupil, under the control of parasympathetic fibres which arise in the brain and travel with the oculomotor nerve to the orbit. In the orbit the fibres synapse in the ciliary ganglion then pass into the eye to innervate the muscle. The dilator pupillae muscle is a radial muscle which dilates the pupil, controlled by sympathetic nerve fibres which arise in the superior cervical ganglion and travel to the eye around the internal carotid and ophthalmic arteries. Dilation and constriction of the pupils occurs in a conjugate fashion. Shining a bright light in one eye will send impulses through that optic nerve to the CNS. Efferent impulses will travel out through the oculomotor nerves to both eyes to cause simultaneous constriction of the pupils. This light reflex tests the integrity of the components, i.e. the optic nerve, central connections, oculomotor nerve and peripheral connections.

Failure of the pupil to constrict when light is shone into either eye indicates an oculomotor nerve lesion. This will usually be accompanied by ptosis on the affected side, since raising the eyelid by the levator palpebrae superioris muscle is mainly through the striated muscle component supplied by the oculomotor nerve. If one pupil is smaller than the other and fails to dilate even when shaded then it is likely that the sympathetic supply to the eye is damaged. The skin on the face may be warm and dry if the sympathetic lesion is close to the C8, T1 roots, and there may be some ptosis which can be corrected to some extent voluntarily. This is Horner's syndrome. The sympathetic fibres are commonly involved in tumours of the apex of the lung (pancoast tumour) which involve the sympathetic fibres as they pass around the subclavian artery into the cervical sympathetic trunk.

Cornea

The cornea is sensitive to touch. Lightly touching the cornea with a piece of cotton wool results in reflex blinking. This reflex tests the afferent fibres from the cornea through the ophthalmic division of the trigeminal, the central connections, the temporal and zygomatic fibres of the facial nerve which cause the orbicularis oculi muscles to close the eye. A similar response can be elicited by touching the eyelashes, or by bringing something rapidly towards the eye.

Tears and dry eye

The eye is kept moist by secretion from the lacrimal gland being wiped across cornea by blinking. The lacrimal gland is innervated by fibres from the facial nerve which travel through the pterygopalatine ganglion.