The illustrations below illustrate a test being carried out in an unconscious patient.

The patient’s head is being moved from side to side.

This image is "borrowed" from Life In The Fast Lane, who appropriated it from Poser JB, et al. Plum and Posner’s Diagnosis of Stupor and Coma (4th edition), Oxford university Press, 2007. It has been lightly modified.

a) Name the neurological structures involved in the reflex being tested

b)  What  does  the  result  shown  in  the  illustrations  suggest  about  the  cause  of unconsciousness?

[Click here to toggle visibility of the answers]

College Answer

a) Name the neurological structures involved in the reflex being tested

•    VIII nerve and vestibular nucleus
•    III and VI nerves and nuclei
•    Median longitudinal fasciculus

b)  What  does  the  result  shown  in  the  illustrations  suggest  about  the  cause  of unconsciousness?

The cause is unlikely to be due to an anatomical  lesion affecting  the reticular  activating system (Brain stem function is still present)

Discussion

The reflex arc for this physical sign can be found in Fetter's 2007 review article.

a) The vestibulocochlear nerve, the brainstem nuclei of the vestibulocochlear nerve, the fibers to the cerebellum, the fibers from the cerebellum, the medial longitudinal fasciculus (MLF) and the 3rd and 6th cranial nerves.

b) The cause of the unconsciousness in a patient with a negative  oculocephalic reflex is some sort of destructive brainstem pathology or brain death.

A positive oculocephalic reflex is a good sign. In an intubated patient with comically huge eyeballs, it should look like this:

Normal oculocephalic and oculovestibular reflexes

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.

A more detailed overview of this reflex can be found at LITFL's page on the examination of the unconscious patient. 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, and the image from the college question has been ..borrowed ... from there.

References

Nathanson, Morton, Philip S. Bergman, and Paul J. Anderson. "Significance of oculocephalic and caloric responses in the unconscious patient." Neurology7.12 (1957): 829-829.

Fetter, Michael. "Vestibulo-ocular reflex." (2007): in: Developments in Ophthalmology, Vol.40 (ed: A.Straube and U.Buttner), Karger 35-51.

Clinical Examination of the Critically Ill Patient, 3rd edition by L.I.G. Worthley - which can be ordered from our college here.

Clinical Examination: whatever edition, by Talley and O'Connor. Can be acquired any damn where.