The brain stem is a critical part of the central nervous system (CNS); it houses the cranial nerve nuclei and cardiopulmonary centers, and serves as a conduit for descending tracts into the spinal cord. The brain stem is located in the posterior fossa, and anatomically is composed of the midbrain (most rostral part of the brain stem), pons, and medulla. Gliomas are the most common cause of brain stem enlargement. However, a variety of other pathologic processes such as infection, metastasis, infl ammation or demyelination, hemorrhagic lesions, posterior reversible encephalopathy syndrome, and hypertrophic olivary degeneration also may involve the brain stem, and occasionally cause diagnostic dilemmas. In some instances, combining the clinical presentation with the radiologic fi ndings may even obviate the need for biopsy of this critical region of the central nervous system.
Imaging FindingsBrain Stem Glioma. The low-grade brain stem glioma is the most common primary brain stem neoplasm. These lesions most commonly occur in the pons, and typically exhibit a diffusely infi ltrating pattern of growth. In some cases, brain stem gliomas will show an exophytic growth pattern, which is quite specifi c for this lesion. 1 Another characteristic imaging feature of brain stem gliomas is the tendency to engulf the basilar artery as the lesion grows. Brain stem gliomas are usually slow-growing lesions, especially in the tectal region; however, gradual compression of the aqueduct of Sylvius or fourth ventricle with resultant hydrocephalus may be encountered. Occasionally, cysts, necrosis, or hemorrhage may occur within the gliomas. Calcifi cation, however, is quite rare and should prompt consideration of an alternative diagnosis. On MRI, brain stem gliomas are usually T1-hypointense with corresponding T2-weighted/fl uid-attenuated inversion recovery (FLAIR) hyperintensity, with the area of abnormal T2-weighted/FLAIR hyperintensity more extensive than the area of abnormal T1-hypointense signal (Figure 1). On contrast-enhanced MRI, low-grade gliomas show minimal to no enhancement with the exception of an exophytic component, which can enhance prominently. Higher grade brain stem gliomas will show more extensive enhancement. A pertinent clinical feature of brain stem gliomas is that they occur primarily in the pediatric population.On contrast-enhanced MRI, low-grade brain stem gliomas show minimal to no enhancement except for an exophytic component, which can enhance prominently.
