The corneal reflex consists of a bilateral blink response elicited by touching the cornea lightly, for example, with a piece of cotton wool. As well as observing whether the patient blinks, the examiner should also ask whether the stimulus was felt: a difference in corneal sensitivity may be the earliest abnormality in this reflex. Synkinetic jaw movement may also be observed (see Corneomandibular Reflex).
The afferent limb of the corneal reflex is via the trigeminal (V) nerve, the efferent limb via the facial (VII) nerve to orbicularis oculi. The fibers subserving the corneal reflex seem to be the most sensitive to trigeminal nerve compression or distortion: an intact corneal reflex with a complaint of facial numbness leads to suspicion of a nonorganic cause. Reflex impairment may be an early sign of a cerebello-pontine angle lesion, which may also cause ipsilateral lower motor neurone type facial (VII) weakness and ipsilateral sensorineural hearing impairment (VIII). Trigeminal nerve lesions cause both ipsilateral and contralateral corneal reflex loss.
Cerebral hemisphere (but not thalamic) lesions causing hemiparesis and hemisensory loss may also be associated with a decreased corneal reflex.
The corneal reflex has a high threshold in comatose patients, and is usually preserved until late (unless coma is due to drug overdose), in which case its loss is a poor prognostic sign.