Patients with familial dysautonomia cannot regulate their blood pressure: on being tilted upright, their blood pressure and heart rate fall, but both can rise to high levels during emotional arousal.
It is believed that their inability to regulate blood pressure is due to a lack of sensory endings that monitor blood pressure (baroreceptors).
We recorded muscle sympathetic nerve activity in these patients for the first time, and showed that the neurones do not fire with a characteristic bursting pattern, rather tending to fire continuously during periods of emotional arousal.
This suggests that the sympathetic neurones that control blood pressure are intact, but are deprived of the normal baroreceptor inputs that constrain them to fire in the intervals between heart beats.
Abstract Familial dysautonomia (Riley–Day syndrome) is an hereditary sensory and autonomic neuropathy (HSAN type III), expressed at birth, that is associated with reduced pain and temperature sensibilities and absent baroreflexes, causing orthostatic hypotension as well as labile blood pressure that increases markedly during emotional excitement. Given the apparent absence of functional baroreceptor afferents, we tested the hypothesis that the normal cardiac-locked bursts of muscle sympathetic nerve activity (MSNA) are absent in patients with familial dysautonomia. Tungsten microelectrodes were inserted percutaneously into muscle or cutaneous fascicles of the common peroneal nerve in 12 patients with familial dysautonomia. Spontaneous bursts of MSNA were absent in all patients, but in five patients we found evidence of tonically firing sympathetic neurones, with no cardiac rhythmicity, that increased their spontaneous discharge during emotional arousal but not during a manoeuvre that unloads the baroreceptors. Conversely, skin sympathetic nerve activity (SSNA), recorded in four patients, appeared normal. We conclude that the loss of phasic bursts of MSNA and the loss of baroreflex modulation of muscle vasoconstrictor drive contributes to the poor control of blood pressure in familial dysautonomia, and that the increase in tonic firing of muscle vasoconstrictor neurones contributes to the increase in blood pressure during emotional excitement.