Neuroscience Clerkship

 

 

MONRO - KELLIE DOCTRINE

 

More than two centuries ago, Alexander Monro applied some of the principles of physics to the cranial cavity and for the first time hypothesized that the blood circulating in the cranium was of constant volume at all times. This hypothesis was supported by experiments by Kellie. What finally came to be known as the Monro–Kellie doctrine, or hypothesis, is that the sum of volumes of brain, CSF, and intracranial blood is constant. An increase in one should cause a decrease in one or both of the remaining two. Specifically, a bleed into the brain which occupies space and increases intracranial pressure (ICP), must cause a displacement of some of the substance in the brain such as CSF or blood. Conversely, a loss of brain volume results in an increase in blood and CSF to maintain a constant intracranial pressure. The ramifications of this theory have dramatic implications in clinical practice today: small changes in brain volume (bleed or tumor, etc.) can be compensated for, despite a solid encasing skull. However, larger more rapid changes in volume need to be evacuated in order to keep ICP down since perfusion pressure of the brain = systemic pressure – ICP.

Above: The Monro-Kellie doctrine states that the sum of the contents of the intracranial cavity is a constant. (Top) in the normal state, 80% is filled by brain; 10% by CSF and 10% by blood. (2nd Row) when a mass in present, initially the volume of CSF decreases to compensate. (Third Row) if the mass enlarges, not only must the volume of CSF decrease, but also the blood volume (venous blood first). (Bottom Row) if the mass continues to expand, the blood volume will be reduced further resulting in brain ischemia and brain death; or the contents of the brain must move out the intracranial cavity (i.e., herniation of brain tissue through the foramen magnum; an event that leads to brainstem compression and death as well).

For the interested reader, below is a brief historic outline:

Portrait of Alexander Monro

Alexander Monro (1733–1817), a Scottish anatomist from a well-known medical family, like his father and his son, was a professor of anatomy at the medical school of Edinburgh. He is credited not only for his contribution to the hypothesis in discussion, but also for his description of the interventricular foramina bearing his name.

 

In a monograph published in 1783, Monro made the following points:

The brain was enclosed in a nonexpandable case of bone

The substance of the brain was nearly incompressible

The volume of the blood in the cranial cavity was therefore constant or nearly constant

A continuous outflow of venous blood from the cranial cavity was required to make room for the continuous incoming arterial blood

Thus, appeared in the literature for the first time the hypothesis that the blood circulating in the cranium was of constant volume at all times.

George Kellie of Leith, another Scot and himself a former pupil of Monro, studied the amount of venous blood in the brain of humans and animals that had died of various causes, including drowning, hanging, and exsanguination. He noted that there was no significant difference in the amount of the brain venous blood in these various circumstances, a finding that supported Monro's observations. In his experiments on animals that had been bled to death, Kellie noted that the brain had maintained its usual appearance and was not pale or drained of blood. He also noted that the amount of blood in the cerebral vessels of dogs given lethal doses of prussic acid was not affected by gravitation. Kellie was fascinated that congestion of cerebral vessels was not noted in some instances in which it might be most suspected. He remarked on postmortem observations on the bodies of two pirates who were hanged at Leith and were dissected by Monro and himself. Although extracranial tissues were congested and engorged, the intracranial vessels were not. Kellie similarly noted that:

• The brain was contained in, and exactly filled, an unyielding case of bone

• The brain itself was only minutely compressible

• It was unlikely that any circulating fluid could be withdrawn from or any overflow could be introduced within the cranial cavity without simultaneous equivalent replacement or displacement.