LOS ANGELES—The brains of patients with obstructive sleep apnea (OSA) appear to have decreased amounts of cerebral gray matter, researchers have found.[1] Whether the loss of gray matter contributes to OSA or vice versa is not known, but reduced gray matter may perpetuate the dysfunction of upper airway muscles.

Investigators from the Department of Neurobiology at the University of California in Los Angeles used magnetic resonance imaging (MRI) to compare the brain morphologies of 21 men who had documented OSA and 21 healthy controls. The control group also underwent sleep studies to verify the presence or absence of breathing disturbances.

The MRI scans were analyzed using morphometry designed to detect regional gray matter volume changes. Volume measurements were calculated for white matter, gray matter, the two combined, and cerebrospinal fluid.

DIFFERENCES IN GRAY MATTER

The OSA patients had significantly less gray matter than did the control group. The extent of gray matter loss increased with the severity of OSA. Differences in gray matter between the two groups varied from 2% to 18%, depending on which region of the brain was examined.

There were no between-group differences in white matter or cerebrospinal fluid. Because of the decrease in gray matter in the OSA patients, however, the ratio of total gray-to-white matter volume was significantly greater in the control group. In this group, but not in the OSA patients, the ratio of gray-to-white matter volume declined significantly with age.

WHICH CAME FIRST?

According to the researchers, the results of this study suggest two possibilities. The first is that gray matter loss is a consequence of apnea, and the second, that preexisting gray matter abnormalities contribute to the emergence or maintenance of OSA.[1] Damage to certain areas of the brain may play a role in the cognitive deficits that sometimes accompany OSA.

The investigators noted that at least some changes in the brains of patients with OSA may have resulted from the hypoxia that, along with hypercapnia and elevated blood pressure, occurs during episodes of OSA. Peripheral tissue oxygen saturation during apneic episodes in severe OSA can fall to 70% or less. Repeated episodes of hypoxia can damage gray matter, and patients with OSA have reduced cerebral blood flow.[1]

Conversely, reduced regional gray matter may have preceded OSA and contributed to its development. Reductions in gray matter can be congenital or acquired. This study found gray matter loss with age in the control group but not the OSA group, suggesting that damage to gray matter occurs early in OSA patients. Additionally, in a number of medical conditions, cerebellar damage results in a high incidence of sleep-disordered breathing.

We are left with the question of whether brain changes are a result of OSA or whether they precede it. In an editorial, David Gozal, MD, asked, “Is it the chicken or is it the egg?”[2] For now, he wrote, we can only speculate.

—Gale Jurasek

References
1. Macey PM, Henderson LA, Macey KE, et al. Brain morphology associated with obstructive sleep apnea. Am J Respir Crit Care Med. 2002;166:1382-1387.
2. Gozal D. The brain in sleep-disordered breathing: is it the chicken or is it the egg? Am J Respir Crit Care Med. 2002;166:1305-1306.

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