Orthostatic challenge reveals impaired vascular resistance control, but normal venous pooling and capillary filtration in familial dysautonomia

Patients with familial dysautonomia (FD) frequently have profound orthostatic hypotension without compensatory tachycardia. Although the aetiology is presumed to be sympathetic impairment, peripheral vascular responses to orthostasis have not been assessed. The aim of this study was to evaluate the control of vascular responses to postural stress in FD patients. Measurements of heart rate, blood pressure, cardiac stroke volume and cardiac output (CO), by impedance cardiography, and calf-volume changes, by impedance plethysmography, were taken from nine FD patients and 11 control subjects while supine and during head-up tilt. During leg lowering, we also assessed the venoarteriolar reflex by measuring skin red-cell flux. Head-up tilting for 10min induced sustained decreases in mean arterial pressure in the FD patients, but not in the controls. Total peripheral resistance (TPR, i.e. mean arterial pressure/CO) increased significantly in the controls (39.8±6.8%), but not in the FD patients. Calf-volume changes during tilting, when normalized for the initial calf volume, did not differ significantly between the patients (4.62±1.99ml·100ml^-1) and the controls (3.18±0.74ml·100ml^-1). The vasoconstrictor response to limb lowering was present in the patients (47.7±9.0% decrease in skin red-cell flux), but was impaired as compared with the controls (80.7±3.4%) (P<0.05). The impaired vasoconstriction during limb lowering and absent increase of TPR during tilting confirm that orthostatic hypotension in FD is due primarily to a lack of sympathetically mediated vasoconstriction without evidence of abnormally large shifts in blood volume towards the legs during orthostasis. This may be due, in part, to a preserved myogenic response to increased vascular pressure in the dependent vascular beds.