Setting the Record Straight: New Article Takes on Misconceptions About Intracranial Compliance

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Noam Alperin, Ph.D., professor of radiology and biomedical engineering at the University of Miami Miller School of Medicine, would like to correct the record on intracranial compliance (ICC). Compliance is a mechanical property of a compartment that determines the relationship between changes in volume and inside pressure. Compartments with a fixed volume have no compliance.

In a commentary published in the journal Magnetic Resonance Materials in Physics, Biology and Medicine — entitled “Does the brain have mechanical compliance?” — Dr. Alperin addresses the misconception that ICC is actually a property of the brain. In other words, it’s the brain that determines the interplay between intracranial volume and pressure. Alperin would like his colleagues to know that brain compliance does not actually exist.

“Many people keep referring to brain compliance where there is no such thing,” Dr. Alperin said. “Compliance is not a property of the brain, and the volume of the intracranial compartment is not constant. Unfortunately, these misconceptions are slowing down progress in the field of cerebral spinal fluid (CSF) physiology.”

More than an intellectual discussion

This is far more than an intellectual discussion among scholars. Clinicians often measure ICC by injecting fluid into the cranium, but these infusion methods do not provide reliable results and have limited diagnostic value.

Dr. Alperin and colleagues have developed a way to measure ICC noninvasively using MRI, which enables them to diagnose CSF-related disorders, such as hydrocephalus and Chiari malformations.

“We are the only place in the U.S. with access to this technology,” Dr. Alperin said. “Currently, several UM clinicians are taking advantage of this and are referring their patients for this protocol to measure ICC and intracranial pressure noninvasively.”

In the article, Dr. Alperin notes that, while the brain seems soft, that impression should not be mistaken for compliance. The brain is pliable like water. While its shape conforms to applied pressure, its volume does not change.

The brain’s vessel is the cranial vault and the surrounding dura. These structures do not expand much, but they still can accommodate a small increase in volume without a large increase in pressure. However, ICC is not constant, and this ability rapidly vanishes as intracranial volume increases, such as when the brain is bleeding.

Without fully understanding what ICC is and where it comes from, scientists have trouble understanding how it can be measured by MRI. Correcting these misconceptions will drive expanded MRI use to measure ICC. In addition to dispelling the misconceptions associated with ICC, the article provides valuable tips on this MRI technique to advance its adoption.

“The idea that the intracranial volume is fixed has existed since the 1800s and is known as the Monro–Kellie doctrine,” Dr. Alperin said, “so this is a milestone. The Earth is not flat; the intracranial volume is not constant, and the brain has no compliance.”