Cerebral Microhemorrhages Detected by Susceptibility-Weighted Imaging in Amateur Boxers

Discussion

The neuropathologic findings in TBI are tearing of the septum pellucidum with thalamic gliosis, atrophy and gliosis of the inferior cerebellar folia, and loss of pigmented neurons in the substantia nigra.⁶ Cerebral and cerebellar atrophy, ventricular dilation, posttraumatic cysts, cavum septum pellucidum, cerebral contusion, intraparenchymal and extra-axial hematomas, axonal tearing, and white matter microhemorrhages have been seen in boxing trauma.^1,8–11 TBI in boxing has been presented as either focal or DAI.¹² DAI associated with punctuate hemorrhages in the deep subcortical white matter and brain stem is not routinely visible on CT or conventional MR imaging sequences. Detection of the number, size, and location of microhemorrhages may help to explain the clinical findings and severity of brain damage due to repeated head blows. Additionally, the involvement of the brain-stem in DAI is a very important predictor of long-term outcome.⁴

The largest study of CT scans in boxers was that of Jordan et al,¹¹ who evaluated 388 active professional boxers. CT findings were normal in 93% of the boxers and showed “borderline” atrophy in 6%. Boxers with CSP (14%) were more likely to have cerebral atrophy. One of the earliest studies using MR imaging in boxers was performed by Cabanis et al.¹³ Of the 12 active and 40 retired boxers (13 amateur, 39 professional) examined, only 2 exhibited CSP. Cerebral atrophy was noted in 8, and the mamillary bodies and optic chiasm were described as small in 30. No correlative clinical data were presented, and given that this study was performed by using a 0.15T scanner with 9-mm sections in the early days of MR imaging, technical limitations are likely to have restricted the conclusions drawn from their findings.

Levin et al¹⁴ and Jordan and Zimmerman¹⁵ each studied 9 amateur boxers and reported no abnormal MR imaging findings. Jordan and Zimmerman⁸ subsequently examined 21 active boxers (16 professional) and 1 retired professional boxer. Most of the CT and MR imaging findings were normal, and no clear relationship between boxing exposure and radiologic findings was noted. The authors emphasized the superiority of MR imaging over CT with respect to its usefulness in delineating equivocal CT abnormalities. The most recent systematic study on structural brain abnormalities in amateur boxers as detected by MR imaging was performed in 1992 by Holzgraefe et al,¹⁶ who found no brain abnormalities and a 0% prevalence of cerebral microhemorrhages in 13 boxers. However, this study was performed on a 0.5T MR imaging scanner, and images of only 10-mm thickness were acquired. In postmortem studies of retired boxers, superficial siderosis of the cerebellum has been discovered.¹⁷ Although this should be detectable by using MR imaging, no case reports exist in which this condition has been noted.

Several recent studies have used newly developed MR imaging techniques to elucidate the pathology in the boxing population. Zhang et al¹ performed a study on 24 professional boxers and observed no statistically significant increase in the detected diffusion value, compared with the value in the control group. In another study by Zhang et al,² normal conventional MR imaging findings were detected in 42 of 49 professional boxers. Nonspecific white matter lesions were detected in the remaining 7. However, a significant difference in diffusion anisotropy measurements was detected between boxers and the control group. In boxers, an increased brain average diffusion constant and decreased diffusion anisotropy values were found in the corpus callosum and internal capsule, and it was emphasized that these findings may be preclinical signs of brain injury in professional boxers. The presence of microhemorrhage was not sought in either study.

A 3T MR imaging study by Hahnel et al³ showed that microhemorrhages were present in 7% of the subjects. Although the prevalence was higher in 42 amateur boxers than in the control group, the prevalence was not significantly different between boxers (3 cases) and controls (0 cases). In the present study, microhemorrhages were detected by using 3T MR imaging and GE sequencing, with 1-mm section thickness. In 1 boxer, 2 separate microhemorrhages were detected, whereas 2 other boxers had only 1 lesion each. The microhemorrhages were located exclusively in the corticomedullary junction of the frontal or temporal lobe.

SWI is a full-velocity-compensated high-resolution 3D GE sequence. SWI provides additional clinically useful information that is often complementary to conventional MR imaging sequences and is used in the evaluation of various neurologic disorders, including TBI and coagulopathic or other hemorrhagic disorders. SWI is particularly helpful for the evaluation of DAI, often associated with punctuate hemorrhages in the deep subcortical white matter, which are not routinely visible on CT or conventional MR imaging sequences. It exploits the magnetic susceptibility differences between tissues, resulting in phase differences between regions containing paramagnetic deoxygenated blood products (deoxyhemoglobin, intracellular methemoglobin, or hemosiderin) and surrounding tissue.⁴ Studies by Tong et al^18,19 and Babikian et al²⁰ have shown that SWI has 3–6 times the sensitivity of conventional T2*GE sequences for detecting the size, number, volume, and distribution of hemorrhagic lesions in DAI. Furthermore, Babikian et al demonstrated that the number and volume of SWI hemorrhagic lesions appeared to correlate with specific neuropsychological deficits.

In the present study, the prevalence of cerebral microhemorrhage was higher in the boxers (2/21; 9.5%) compared with the nonboxer controls (0/21; 0%), but the difference was not statistically significant (χ² = 0.525, df = 1, P = .4688). The 2 foci of microhemorrhages detected in the boxers could be seen only on SWI images and were not detectable by using T2 FSE or T2*GE. Most interesting, the 2 microhemorrhages were in the brain stem, which is a typical location for DAI and results in axonal stretching. Thus, the usefulness of SWI in the detection of brain stem lesions was recognized. In either SWI or T2*GE, susceptibility artifacts, particularly those close to bone, may hide peripheral lesions.

Our study has some limitations. First, the study was performed on an unsatisfactory number of boxers. Second, although most boxers had trained and fought for a long time, some had not fought recently because they had retired (n = 13, 61.9%). Although SWI is a sensitive imaging technique for the detection of acute-to-subacute hemorrhaging, foci of chronic hemorrhaging, as might have been present in the retired boxers, may not be seen in SWI due to hemorrhage resorption. Third, the prevalence of microhemorrhages in the boxing population, especially in boxers with knockouts, may be higher than that found in our study because most of the boxers (n = 11, 52.38%) in our study had never been knocked out. In other studies, the number who had been knocked out ranged from 1 to 8.