Neuroanatomy: As will all reflexes it has an afferent (sensory) and efferent (motor) arm. The reflex is mediated by the nasociliary branch of the ophthalmic branch (Vi) of the trigeminal or 5th cranial nerve that senses the stimulus on the cornea, lid, or conjunctiva. The temporal and zygomatic branches of the facial or 7th cranial nerve initiates the motor response. The reflex is driven via interneurones in the medulla.
Interpretation: An absent corneal reflex can be due to sensory loss in Vi (e.g. neuropathy or ganglionpathy), weakness or paralysis of the facial muscles (myopathy) or facial nerve (facial palsy, for example Bell's palsy) or brain stem disease. For a myopathy to cause a loss of the blink reflex the weakness has to be very severe, for example a chronic progressive external ophthalmoplegia (CPEO)
Contact lenses may diminish or abolish the testing of this reflex; therefore an absent corneal reflex is not necessarily abnormal. The examination of the corneal reflex is useful in unconscious patients and if present indicates that the lower brain stem is functioning. It is used as part of the assessment for determining if someone is brain dead; if the corneal reflex is present the person can't be diagnosed with brain death.
Clinical demonstration: The following YouTube video shows you how to do a corneal reflex:
Neurophysiology: The blink reflex can be tested electrophysiologically by stimulating the supra-orbital nerve and measuring the blink in both eyes. The ipsilateral blink occurs quicker (R1 component) compared to the contralateral blink that occurs a few milliseconds later with the R2 component. In the figure below you will notice that the R2 component affects both eyes, i.e. the ipsilateral eye has a double input. The figure below demonstrates the hypothesized wiring diagram of the blink reflex.