The oculocephalic reflex and the cold caloric (oculovestibular) reflex are used to test brainstem function in profoundly comatose patients. An intact reflex basically confirms that the brainstem is intact. Involved are all the oculomotor nerves (provided you move the head up and down, not only left and right) as well as the 8th nerve. Of course, one cannot perform this test if one has a ruptured eardrum, or if the ear canal is completely clogged with macerated brain tissue (as is often the case in these base of skull fracture situations). A demonstration of a normal oculocephalic reflex was held for Question 25.4 from the first paper of 2011 - the candidates were expected to explain how this helps in the diagnosis of coma.
The ideal reference for this topic would have to be the LITFL page, which borrows heavily from Plum and Posner's Diagnosis of Stupor and Coma.
For a complete test and a maximal stimulus, the following manoeuvres must be performed:
So, a normal response is the conjugate deviation of gaze, and an abnormal response is a persistent fixed gaze, with no eye movement. Weirdly, if you grab the head of a conscious person and give it a good twist, their eyes will remain facing midline because the voluntary control of eye movement overcomes this reflex. Thus, the oculocephalic reflex is only testable in the unconscious patient.
So, what if it's normal?
The pathways which command this reflex involve vestibular nuclei, lower pontine tegmentum, the upper pontine tegmentum, the midbrain paramedian tegmentum, and the medial longitudinal fasciculus. These are large, central brainstem regions, which overlap with the ascending arousal system. Thus, it would be highly unlikely that a structural lesion of some sort (like a stroke) has taken out the rest of the brainstem, leaving these regions intact.
In other words, if the oculocephalic reflex is intact, the coma is unlikely due to a structural brainstem lesion.
The oculocephalic reflex represents a submaximal stimulus, and if one were taking their cranial nerve examination seriously, one would also insist on performing a cold caloric test.
The recommended testing method is as follows:
The effect of the cold water in the ear is to fool the hair cells of the ampulla into thinking that the head is being turned to the opposite side. If this test is performed with cold water in an awake patient, the eyes will still tend to drift to the tested ear. The awake patient will try to compensate for this with fast sacchades back to midline, or some other point of fixation. This has given rise to the COWS mnemonic: Cold - Opposite, Warm - Same. COWS refers to the direction of fast nystagmus in the awake patient. In the comatose patient, there will be little or no nystagmus, and the deviation of the eyes will be completely opposite (Warm- Opposite, Cold - Same). In fact, the presence of nystagmus on cold caloric testing in the apparently unconscious patient suggests that they are in fact conscious, and only pretending to be comatose.
Plum and Posner, on page 66 of their famous textbook (4th edition), include a well known diagram of lesions at different levels and their associated findings on oculocephalic and caloric testing. One major flaw of this diagram is the artists' otherwise laudable attachment to the realistic portrayal of the human head. Unfortunately, that does mean that one really needs to squint to see which way the eyes are going. In order to rectify this minor problem, as well as to avoid any copyright complications, a series of diagrams with comically exaggerated features is offered.
A lateral pontine lesion has taken out the vestibular nuclei on the right side. Neither eye will respond to stimulus on that side (as they will not recognise it as a stimulus).
As you can see, only the abducens responses are present. CN VI still abducts the gaze towards the ice water. However, oculomotor and trochlear nerve nuclei (CN III and IV) are not talking to each other. The result resembles a complete lesion in the midbrain (i.e. destruction of the CN III and IV nuclei)- but in a bilateral MLF lesion, the pupillary light reflex would be spookily preserved, and that is how you would discriminate between them.
This is a syndrome where both the MLFs are taken out, and one of the abducens nuclei. The result is a situation where only the unaffected side will behave normally in response to cold caloric testing (that's the "half").