Preliminary observations on the vasomotor responses to electrical stimulation of the ventrolateral surface of the human medulla

Abstract

Object: Pulsatile arterial compression (AC) of the ventrolateral medulla (VLM) is hypothesized to produce the hypertension in a subset of patients with essential hypertension. In animals, a network of subpial neuronal aggregates in the VLM has been shown to control cardiovascular functions. Although histochemically similar, neurons have been identified in the retro-olivary sulcus (ROS) of the human VLM, but their function is unclear.

Methods: The authors recorded cardiovascular responses to electrical stimulation at various locations along the VLM surface, including the ROS, in patients who were undergoing posterior fossa surgery for trigeminal neuralgia. This vasomotor mapping of the medullary surface was performed using a bipolar electrode, with stimulation parameters ranging from 5- to 30-second trains (20-100 Hz), constant current (1.5-5 mA), and 0.1-msec pulse durations. Heart rate (HR) and blood pressure (BP) were recorded continuously from baseline (10 seconds before the stimulus) up to 1 minute poststimulus. In 6 patients, 17 stimulation responses in BP and HR were recorded.

Results: The frequency threshold for any cardiovascular response was 20 Hz; the stimulation intensity threshold ranged from 1.5 to 3 mA. In the first patient, all stimulation responses were significantly different from sham recordings (which consisted of electrodes placed without stimulations). Repeated stimulations in the lower ROS produced similar responses in 3 other patients. Two additional patients had similar responses to single stimulations in the lower ROS. Olive stimulation produced no response (control). Hypotensive and/or bradycardic responses were consistently followed by a reflex hypertensive response. Slight right/left differences were noted. No patient suffered short- or long-term effects from this stimulation.

Conclusions: This stimulation technique for vasomotor mapping of the human VLM was safe and reproducible. Neuronal aggregates near the surface of the human ROS may be important in cardiovascular regulation. This method of vasomotor mapping with measures of responses in sympathetic tone (microneurography) should yield additional data for understanding the neuronal network that controls cardiovascular functions in the human VLM. Further studies in which a concentric bipolar electrode is used to generate this type of vasomotor map should also increase understanding of the pathophysiological mechanisms of neurogenically mediated hypertension, and assist in the design of studies to prove the hypothesis that it is caused by pulsatile AC of the VLM.