The corneal reflex is usually tested after the pupils, but the cranial nerves involved are out of order. The afferent arc is mediated by the nasociliary branch of the ophthalmic branch (Vi) of the trigeminal or 5th cranial nerve, and the efferent arc is the seventh (facial) nerve.
Naturally, on their way to examine the eye, the intensivist typically needs to open the eyelids.
Weird eyelid signs
Though generally neglected as a tedious inconvenience, the eyelids are actually a source of interesting clinical signs. For instance, Plum and Posner report that "the eyelids of a comatose patient close smoothly and gradually, a movement that cannot be duplicated by an awake individual simulating unconsciousness." Eyelid tone is remarkable: in the comatose individual, the resting tone of the orbicularis oculi muscle keeps the eyes shut. Thus, failure to keep the eyelids closed in coma, or failure to re-closed them after forced opening, may suggest facial nerve weakness. Plum and Posner list a whole series of eyelid signs and their pathophysiological correlations:
- Ptosis: braistem aor hemispheric injury, or Horner's syndrome
- Absence of tone, or failure to close the forcibly opened eye: facial nerve palsy (CN VII)
- Smooth gradual closure of the eye: true coma, rather than "pseudocoma"
- Strong resistance to eye opening, "blepharospasm" and rapid forceful closing: - often voluntary; the patient is not truly unconscious
- Tonic opening: tonically retracted eyelids; also known as Collier's sign, sually associated with midbrain and pontine lesions
The afferent arm (the sensory component) is served by the trigeminal nerve (CN V), and the efferent arm is served by the facial nerve (CN VII).
Generally, in ICUs araound Australia this seems to be done by scratching the cornea with the corner of a gauze piece; however an alternative and completely atraumatic method is to use a drop of saline. The cornea, rather than the sclera, must be tested.
When one eye is tested, both should blink. When the opposite eye doesn't blink, a contralateral facial nerve palsy may be the cause. When only the untested eye blinks, the seventh nerve palsy is ipsilateral. A sensory lesion obviously results in a negative corneal reflex (i.e. neither eye blinks when the affected eye is tested).
Bell's phenomenon or the "oculogyric reflex" is mentioned in Diagnosis of Stupor and Coma. It is the closure of both eyelids and elevation of both eyes of the profoundly comatose patient, and it indicates that the whole reﬂex pathway is intact. Specifically, it involves the fifth, seventh and third nerve nuclei. Apparently, this reflex has some sort of defensive role, moving the pupil under the lid in response to noxious corneal stimuli. Apparently, the mesencephalic reticular nucleus is reponsible for integrating the eyelid and eye movements. Ergo, a midbrain lesion may result in blinking without upward eye movement. Conversely, a lower motor neuron lesion of the seventh nerve (or damage to the nerve itself) may result in upward eye movement without blinking.