Brainstem dysfunction

The brainstem consists of the midbrain, pons, and medulla oblongata. Some people separate the clinical signs associated with dysfunction of the midbrain, pons, and medulla oblongata, but it’s often difficult and, quite frankly not very common, to cleanly localize the lesion to just the midbrain or pons. It’s certainly possible for a patient to show only dysfunction of cranial nerves III or IV suggesting midbrain disease, but patients more commonly have clinical signs referable to multiple areas of the brainstem. Important structures in the brainstem include the nuclei giving rise to most of the cranial nerves (III-XII), the Ascending Reticular Activating System (ARAS) controlling level of consciousness, central vestibular nuclei, the chemoreceptor trigger zone, and the heart rate and respiratory centers. Additionally, the primary gait generators for dogs and cats are located in the brainstem (likely midbrain), involving the extrapyramidal tracts (e.g., rubrospinal tract). The proprioceptive, corticospinal, and corticonuclear motor tracts cross in the midbrain. As a result, lesions cranial to the midbrain (i.e., prosencephalon) will cause contralateral postural reaction deficits and/or weakness, while lesions caudal to the midbrain (pons, medulla, cerebellum, spinal cord) will cause ipsilateral deficits.

Common clinical signs of brainstem disease

  • Altered mental status (dull, stupor, coma, disoriented)
  • Weakness and ataxia (tetraparesis, ipsilateral hemiparesis)
  • Cranial nerve deficits (III-XII possible)
  • Postural reaction deficits (ipsilateral unless lesion is cranial midbrain where they’re contralateral)
  • Vestibular dysfunction
  • Irregular respiration


Case example of a dog with right brainstem localization involving cranial nerves VII, VIII, IX, X, (possibly XI) and XII.