Mesenrhombencephalitis: MR findings in nine patients

Mesenrhombencephalitis is a serious form of brainstem inflammation predominantly involving the deep and vital portions of the brain, that is, the mesencephalon (midbrain) and rhombencephalon (pons, medulla). Mesenrhombencephalitis is difficult to diagnose on the basis of clinical and laboratory findings alone, and access to this portion of the brain for surgical biopsy carries high morbidity. We describe the MR appearance of mesenrhombencephalitis and correlate the imaging findings with clinical information.

Unenhanced and contrast-enhanced MR images of nine patients with mesenrhombencephalitis were reviewed retrospectively and correlated with clinical, laboratory, and pathologic data. The patients were categorized according to the cause of the disease: three had herpes simplex, one had Listeria monocytogenes, and five had mesenrhombencephalitis of undetermined cause. The three patients with clinical and MR evidence of herpes simplex mesenrhombencephalitis (one confirmed by brain biopsy) were comatose at presentation, with cranial nerve abnormalities in two and seizures in one. One patient with L. monocytogenes (established by blood culture) had cranial nerve palsies, fever, and pain in the ear. Five additional patients had headache (three), fever (three), nausea and vomiting (four), cranial nerve palsies (three), coma (two), and hyporeflexia (one) or hyperreflexia (four). Brain biopsy performed in two patients revealed chronic inflammation of unspecified cause; in one, it was compatible with viral encephalitis.

MR images in three patients with herpes simplex mesenrhombencephalitis showed T2 signal hyperintensity in the midbrain (two), pons (one), medulla (one), and temporal lobes (three). Parenchymal foci of hemorrhage (methemoglobin, one patient) and leptomeningeal enhancement (one patient) were identified in the temporal lobes. T2-weighted MR images in one patient with L. monocytogenes showed signal hyperintensity in the brainstem, vermis, midbrain, and internal capsules. On T1-weighted images, low signal was present in these areas, which enhanced with paramagnetic contrast agents. In the remaining five patients, T2-weighted MR images showed patchy signal hyperintensity in the pons, medulla, and thalamus in three each and in the midbrain and temporal lobes in one each. T1-weighted MR images showed normal findings (two) or signal hypointensity in the thalamus and pons in one patient each. Areas of leptomeningeal and parenchymal enhancement were identified in one patient each. Brainstem swelling was seen in three patients, one of whom had petechial hemorrhage in the pons and hydrocephalus.

Mesenrhombencephalitis is a serious illness that is diagnosed by a combination of imaging, clinical, laboratory, and pathologic studies. MR imaging may be crucial to the early diagnosis of this illness, and radiologists must be familiar with this uncommon entity and its MR findings in order to make timely diagnoses and facilitate treatment.