The hyperadrenergic subgroup of OI is characterized by a clinical spectrum including attenuated plasma renin activity and aldosterone, reduced supine blood volume coupled with dynamic orthostatic hypovolemia, elevated plasma norepinephrine and epinephrine, impaired clearance of norepinephrine from the circulation and evidence of partial dysautonomia. When upright posture is assumed, there is a loss of plasma volume from the blood into the surrounding tissue. In normal subjects, about 14% of the plasma volume may leave the blood within 30 minutes of standing. This loss of plasma volume into interstitial tissues is greatly enhanced in patients with OI; occasional patients will lose more than twice this amount of fluid.
It is little wonder such patients with supine hypovolemia to begin with develop symptoms in a setting of this excessive dynamic orthostatic hypovolemia. Normal subjects reduce urinary sodium excretion on assumption of upright posture, but patients with OI do so ineffectively. This probably contributes to the severity of their hypovolemia. In patients with florid symptoms of orthostatic intolerance in a setting of hypovolemia and increased plasma norepinephrine, several interesting findings emerge. The plasma renin activity and aldosterone are generally slightly reduced in proportion to the degree of the hypovolemia. This suggests that the reduced renin level may be responsible for the hypovolemia. It is possible that impaired sympathetic innervation of the juxtoglomerular apparatus in the kidney may underlie this renin deficit.