1.Pathological mechanism Brainstem cavernous hemangioma is a congenital vascular anomaly with an average lesion diameter of about 1.5-2.0 cm. It is largely lobulated or “mulberry-shaped”. On light microscopy, it consists of many sinusoidal dilated vascular lumens with only endothelial cells in the walls, lacking elastic fibers and smooth muscle, resulting in high vascular fragility. In general, the dilated vessels are not interspersed with normal brain parenchyma. Electron microscopic and immunohistochemical studies revealed that the cavernous hemangioma lumen lacks an envelope of extravascular cells and astrocytic protrusions, and the basement membrane is embedded in a dense collagenous fibrous matrix. In addition, the tight junctions between endothelial cells were incomplete and the intercellular spaces were large. It is hypothesized that it is the absence of these important structures of the normal vasculature that causes the imperfection of the blood-brain barrier in cavernous hemangiomas, leading to chronic exudation of blood cells, iron-containing yellow deposits around the lesion, and thrombosis in the lumen of the vessel. BCM often coexists with capillary dilation and venous malformations, and Porter et al. reported 86 cases of surgically resected BCM in which malformed veins were present around the lesion; some preoperative MR showed “jellyfish head-like” changes. Mathiesen et al. concluded that malformed veins are responsible for the drainage of important brainstem structures, and therefore the main trunk of the malformed veins should be avoided during resection of the malformed vascular mass to avoid irreversible venous brainstem infarction. Porter et al. concluded that venous malformations play an important role in the occurrence and recurrence of BCM. 2.Clinical manifestations BCM is mostly characterized by sudden aggravation of the original headache, dizziness, nausea and vomiting or the reappearance of the disappeared symptoms, and only a few of them are gradually aggravated. Due to this pattern of relapse-remission, some patients are misdiagnosed as multiple sclerosis or cerebral infarction]. Unlike episodic cavernous hemangiomas that often present as seizures, sexual seizures have rarely been reported in BCM. Hemiparesis and hemianesthesia, cerebral nerve damage and ataxia are the three most common signs. If the lesion is located on the cephalocaudal or ventral-dorsal side of the brainstem, relatively specific clinical manifestations will occur. If the lesion is located in the midbrain, there will be a significant increase in intracranial pressure, and manifestations specific to the midbrain such as red nucleus tremors, involuntary laughter, and episodic disorders of consciousness will also occur; if the lesion is located in the pons, some patients will have isotropic gaze disturbance; and in the medulla oblongata lesion, intractable eructation and dysphagia are relatively specific.