Thursday, April 22, 2010

Paging the Page Kidney: Biomedical Musings

I recently saw a medical condition I had never encountered before. A teenager had been diagnosed several months ago with hypertension (high blood pressure) by his primary care physician. After dietary modifications failed to lower his readings to acceptable levels, an ultrasound showed an abnormality in one kidney. MRI further defined a mass within the kidney consistent with a resolving hematoma, or subcapsular bruise.

Irvine H. Page demonstrated that tightly wrapping a kidney with cellophane would induce hypertension in 1939; in his honor, this type of kidney problem is called a Page kidney. More information, including an MRI and pathologic specimen, can be seen in this feature in New England Journal of Medicine.

So how does wrapping a kidney in cellophane or adding a subcapsular mass cause high blood pressure?

Page Kidney

The left image shows a diagram of a normal kidney with its artery entering and branching. Repeated branches occur until each vessel ends in a filtering unit or glomerulus. The capsule forms the large border around the pink “meat” of the diagrammed kidney; this firmer scar-like structure holds the organ in place. If bleeding occurs within the kidney, from a blow to the flank or a kidney biopsy, a bruise may form under the capsule as in the image on the right. This bruise or hematoma cannot push the firmly made capsule out, so it squishes the working part of the kidney, including blood vessels in the region.

When the kidney sees a reduction in blood flow, it cannot tell if low blood pressure in the body or compression by a hematoma (or some wacky scientist wrapping it in cellophane) has caused the problem. The blood vessels near the glomeruli respond by pumping out a hormone called renin that turns on the angiotensin-aldosterone system, raising blood pressure throughout the body).

Thus, a squished kidney causes high blood pressure. This form of hypertension can often be cured by draining the hematoma or removing the offending kidney.

And 25 years after completing medical school, I still see new things!