A 68-year-old male presents with sudden onset of coma. On examination, he is obtunded and has midsized pupils. His reflexes are difficult to elicit but present, and he has clearly extensor plantar responses bilaterally. He is intubated, and initial CT brain scan without contrast is normal.

a)  Excluding a toxidrome, what is the most likely dagnosis?(25% marks)

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College answer

Midbrain infarct


What we know:

  • The coma is of sudden onset
  • Pupils are mid-sized (presumably both)
  • Intact reflexes
  • Extensor (abnormal) plantars
  • Normal CT brain

So: the main focal finding is bilateral mid-dilated pupils. There is actually quite a large list of possibilities as to what this could be. Here's a nice diagram from Critical Care and Resuscitation (case report by  P.D. Thomas, 2000).

Clasisfication of fixed dilated puils from Thomas (2000)

However, with all the toxic stuff excluded, the intracranial lesions causing this finding usually are:

  • Severe diffuse hypoxic brain injury
  • Bilateral midbrain lesion
  • Bilateral 3rd nerve injury (eg. traumatic)
  • Bilateral  eye trauma
  • Terminal stages of brainstem herniation

So the CT is normal, so we know the patient is not coning immediately, and presumably there is no intracranial explanation as to why their third nerves would suddenly be severed (nor would that give you upgoing plantars anyway). Diffuse hypoxic brain injury might also give you a relatively normal scan result, and presumably the examiners would have given you at least partial marks for this, but overall the midbrain lesion becomes more likely.



Plum, Fred, and Jerome B. Posner. The diagnosis of stupor and comaVol. 19. Oxford University Press, 1982. - warning! This link takes you to a download of the whole 9.0Mb file, which represents the entire volume of the 1980 second edition.

Bateman, David E. "Neurological assessment of coma." Journal of Neurology, Neurosurgery & Psychiatry 71.suppl 1 (2001): i13-i17.

Sato, Hiromasa, Kosuke Naito, and Takao Hashimoto. "Acute isolated bilateral mydriasis: case reports and review of the literature." Case reports in neurology 6.1 (2014): 74-77.

Thomas, P. D. "The differential diagnosis of fixed dilated pupils: a case report and review." Critical Care and Resuscitation 2.1 (2000): 34.