"The main problem with identifying alcohol-related brain damage has been to identify those lesions caused by alcohol itself versus those caused by other common alcohol-related factors, such as thiamin deficiency," said Elisabeth Stephanie Smith, a member of the Ear, Nose and Throat Clinic at the University of Ulm, Germany, and first author of the study.
For example, she said, some alcoholics lose white matter, which can lead to overall brain shrinkage, and may be partially reversible. Alcohol-related damage has also been noted in specific regions of the cerebral cortex hypothalamus and cerebellum, and possibly in the hippocampus, amygdala and locus ceruleus. "Many of the regions that are normal in alcoholics are damaged in those who have developed the Wernicke-Korsakoff syndrome," she added. "While documented dendritic and synaptic changes in alcoholics, together with receptor and transmitter changes, may explain the functional changes and cognitive deficits that precede more severe structural neuronal changes."
For this study, researchers recruited two groups of males from the University of Ulm: 19 head- and neck-tumour patients, representative of heavy drinkers according to previous research; and 19 age- and nicotine-matched plastic-surgery patients, representative of social drinkers according to previous research. All participants were given a standardized questionnaire regarding alcohol use, blood tests, and a hearing examination. Recordings and evaluation of brainstem auditory evoked potentials (BAEPs) were used to measure brain damage in the subjects.
Deviations from standard BAEP latencies and amplitudes generally reflect various diseases affecting the auditory nerve and central auditory pathways. "BAEPs are diffuse currents circulating in the brain," explained Smith. "When an acoustic stimulus on the brain is presented, a particular current response is activated. This response can be detected by electrodes and displayed as a kind of time-dependent voltage. Diseases like cerebellar angle tumours or multiple sclerosis cause defects in the transmission of brain currents, affecting the amplitude and/or latency of the current response, which can in turn be detected by means of BAEP."
The results indicate that cumulative life-long alcohol consumption affects BAEP latencies, which reflect damage to central auditory pathways, which means hearing loss.
"Not only does chronic alcohol consumption cause the already mentioned brain shrinkage," said Smith, "but it also leads to defects of the central auditory tracks, which causes delays in neurotransmission time. These defects cannot be detected by commonly used listening tests for hearing performance."
Interestingly, the study also found that the BAEP latencies of social drinkers appear to be significantly more sensitive to the increase of cumulative life-long alcohol consumption than do those of moderate and heavy drinkers.
"The finding of this article does not necessarily imply that social drinkers' hearing is at more risk of damage than that of moderate and heavy drinkers," said Tilman Keck, assistant clinical professor in the department of Otorhinolaryngology at the University of Ulm. "In fact, drinkers with lower life-long alcohol consumption still have a normal amount of healthy nerves in the brain whereas drinkers with high life-long alcohol consumption have a much larger amount of defective nerves." It is an issue of "saturation," he noted.
"For each unit of further alcohol consumption, the absolute amount of nerves damaged for both kinds of drinkers is the same," he said. "However, the relative change of brain damage and subsequent further degradation of the hearing performance in the brainstem due to alcohol consumption will be significantly higher for drinkers with lower life-long alcohol consumption than for those with high life-long alcohol consumption."
"In other words," said Smith, "if nearly all the nerves are damaged, no further nerve damage and thus no more degradation of the hearing performance can be possible." The bottom line, she added, is that "even an amount of alcohol consumption which is normally accepted by society can have a damaging effect on the auditory system."
Smith said that future studies may examine the relationship between chronic alcohol consumption and distorted speech.