The human brain weighs less than 2% of our total body weight, yet consumes 20% of the body’s energy. The brain is fueled by oxygen and glucose rich blood, which flows through a network of tiny arteries supplying each brain cell. These blood vessels are constantly working to regulate flow and keep it stable. They act as the brain’s own defense line, making sure it can protect itself against systemic blood pressure changes. Each time we stand up, they dilate to re-route more blood to reach areas with higher metabolic demands.
In patients with familial dysautonomia (FD), blood pressure swings wildly from low to high based on gravity and emotions. Patients lack the ability to coordinate the autonomic nervous system, so they cannot control the pressure of blood reaching the brain. After just moments of standing, blood pressure can plummet to extraordinarily low values; at which any healthy person would lose consciousness and faint.
But quite remarkably, patients with FD seldom faint. To investigate why, the NYU Dysautonomia team set out to measure how the brain responds to hypotension. Using a small ultrasound probe they were able to monitor blood flow in the brains of patients with FD and healthy volunteers while sitting and standing. The measurements showed that despite very low blood pressure at the level of the brain when standing, the pattern of blood flow changes in patients was not different to controls. Patients were still able to keep their blood flow constant and ensure that the brain was receiving oxygenated blood to remain conscious in the face of hypotension.
Despite being born with a catastrophic lesion that prevents the control of blood pressure, these patients have extraordinary abilities to protect the brain. “The tiny arteries that regulate blood flow in the brain work remarkably well” explained Dr. Lucy Kaufmann, “this probably explains why patients rarely faint when the blood pressure is low and they have a low incidence of hemorrhagic stroke despite their blood pressure surging to high levels.”
This finding teaches us a lot about the intrinsic ability of the cerebral vessels to quickly respond to ensure the brain is receiving enough blood to remain upright and conscious. Because patients have the ability to compensate for hypotension, when they do faint, it should raise suspicions. “We need to rule out whether something that else may be going on like dehydration, bleeding, anemia, or hypoxia due to a lung infection” added Dr. Kaufmann. Patients with FD are born with an inherent skill to regulate the brain’s circulatory system and adapt to support consciousness. This is something they do better than you and I.
This work was supported by the Dysautonomia Foundation, Inc., a non-for-profit patient advocacy group that has supported research and treatment in FD for over 50 years.
Fuente Mora C, Palma JA, Kaufmann H, Norcliffe-Kaufmann L. Cerebral autoregulation and symptoms of orthostatic hypotension in familial dysautonomia. J Cereb Blood Flow Metab. 2016 Sep 9. pii: 0271678X16667524. [Epub ahead of print] PMID: 27613312