Brain Damage in Alcoholics

Magnetic Resonance and Diffusion Tensor Imaging
by National Institute of Health

Brain imaging using conventional magnetic resonance imaging (MRI) has revealed that several brain structures in people with a history of chronic alcohol dependence are smaller in volume than the same brain structures in nonalcoholic control subjects. Areas that are particularly affected are the frontal lobes, which are involved in reasoning, judgment, and problem solving. Older people are especially vulnerable to the damaging effects of alcohol. It is unclear whether women show consistently more vulnerability to these changes in the brain than men do. In general, alcoholics evaluated before and after a period of abstinence show some recovery of tissue volume, whereas alcoholics evaluated again after continued drinking show further reductions in brain tissue volume. A new MR technique called diffusion tensor imaging (DTI) can aid in detecting the degradation of fibers that carry information between brain cells. With DTI, researchers studying alcoholics have been able to detect abnormalities in white matter not visible with conventional MRI. Ultimately DTI may be useful in elucidating the mechanisms that underlie macrostructural and functional brain changes seen with abstinence and relapse.

Excessive chronic alcohol consumption is associated with significant shrinkage of brain tissue, degradation of fibers that carry information between brain cells, reduced viability of these brain cells, and impairment of associated cognitive and motor functions. Some alcoholism-related tissue damage is partially reversible with abstinence, although residual tissue volume deficits persist even in long-abstinent alcoholics.

Abnormalities are found in both the gray matter and the white matter of the brain. The brain's gray matter consists of nerve cells (neurons), which account for its grayish color, and the surrounding glia cells, which provide mechanical support, guidance, nutrients, and other substances to the neurons. White matter is made up of long, thin extensions of the neurons, called axons, which carry information between neurons. White matter is paler in color than gray matter because the axons are surrounded by myelin - a fatty substance that protects the nerve fibers. The axons form fiber tracts linking nearby and distant neurons across different brain regions.

Imaging in living patients can be used to detect and quantify gray- and white-matter abnormalities on both macrostructural and microstructural levels. Conventional structural magnetic resonance imaging (MRI) reveals the size, shape, and tissue composition (gray vs. white matter) of the brain and its constituent parts. Diffusion tensor imaging (DTI) reveals the integrity of white-matter tracts that link regions of the brain to each other.

MRI is based on the observation that the protons of hydrogen atoms, when placed in a strong magnetic field, can be detected by manipulating the magnetic field. Because the human body is composed primarily of fat and water, it is made up mostly of hydrogen atoms. Variations in behavior of hydrogen atoms in different brain tissue types and structures show up as intensity differences that clinical structural MRI can detect and map to visualize and measure gross brain neuroanatomy.

Diffusion tensor imaging makes use of the fact that water molecules in the brain are always moving - that is, they are in Brownian motion. DTI detects the diffusion, or Brownian movement, of water protons within and between individual cells and yields measures of the magnitude and predominant orientation of this movement. The diffusion properties of water molecules within and between the three-dimensional elements, called voxels, that make up an image reveal the orientation and coherence (extent to which fibers align together) of fibers making up white-matter tracts.

Both MRI and DTI have been applied to the study of alcoholism. Structural MRI has been used for more than a decade to detect gross structural changes, such as tissue shrinkage and its reversal, and has identified brain regions that are particularly vulnerable to the toxic effects of chronic alcohol consumption. DTI, the more recently developed technique, is beginning to reveal microstructural abnormalities in white matter that are consistent with post mortem observations of white-matter damage, such as myelin loss, enlargement of microtubules (small tubular structures found inside nearly all cells), and degradation of membranes, even when that white-matter region appears normal on structural MRI.

Structural MRI

Structural MRI studies of patients with chronic alcoholism are generally consistent with the literature on neuropathology and typically reveal reduced volume of both gray matter and white matter in the cerebral cortex, the folded outer layer of the brain. Older alcoholics show greater volume deficits relative to age-matched control subjects than younger alcoholics, suggesting that the older brain is more vulnerable to the effects of alcohol. MRI usually shows that the greatest loss occurs in the frontal lobes, which are used in reasoning, working memory, and problem solving. Changes also appear in other structures involved in memory, such as the hippocampus, mamillary bodies, thalamus, and cerebellar cortex.

Alcoholic men and women also show thinning of the corpus callosum, a band of white-matter fibers linking the brain's hemispheres. Reduced volume also is reported in the pons, a largely white-matter structure of the brain stem that forms a critical node in multiple circuits linking the cerebellum - which regulates balance, posture, movement, and muscle coordination - to cortical regions of the brain involved in motor and sensory processing, as well as regions where these inputs are integrated. Alcoholics, particularly those with a history of seizures, show reduced white-matter volume in the temporal lobes. Alcoholics also have reduced white-matter volume in a part of the cerebellum known as the cerebellar vermis, where the loss is associated with deficits in postural stability. Lastly, chronic alcohol consumption can lead to specific neurological disorders involving white matter, such as Marchiafava-Bignami disease and central pontine myelinolysis.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.