Operations on or near the brain stem in the posterior fossa can produce abrupt, profound, cardiovascular responses that may signal potential damage to the brainstem. Surgeries that stimulate the floor of the fourth ventricle (which affects the pons and medulla) or the cerebellopontine angle (ie acoustic neuromas or microvascular decompression of cranial nerve V) can result in hypertension, usually in association with bradycardia. These hemodynamic responses are typically abrupt and transient. Other cardiovascular responses may include tachycardia and hypertension, bradycardia and hypotension (particularly after stimulation of the vagus nerve or glossopharyngeal nerve), and ventricular dysrhythmias.
It is necessary to pay attention to the EKG and arterial pressure during surgery in these regions. If such changes occur, the surgeon should be alerted immediately in order to avoid the manipulation that provokes the response and to provide warning of the risk of damage to the adjacent cranial nerve nuclei and respiratory centers. Because the responses are transient, using pharmacologic treatments to mask the changes is undesirable unless the changes are recurrent and severe.