Introduction

   Hypertensive encephalopathy is a well-known disease entity which is caused by the failure of autoregulation, break-down of the blood-brain barrier of the involved brain area, and extravasation of fluid and protein into the brain parenchyma.^6)8)9)10)11) Brain computed tomography (CT) or magnetic resonance imaging (MRI) usually reveals diffuse brain edema and sometimes shows enlarged ventricle without the evidence of an obvious mass lesion or other lesions such as cerebral hemorrhage or infarction. Patients usually present with headache, nausea, vomiting, and visual disturbances. A seizure is one of the most common presenting symptoms.^3)10)12)13) Changes in consciousness and very high blood pressure are often observed upon initial examination. As the encephalopathy usually involves the subcortical white matter of the parieto-occipital lobe,^{1)2)3)6)8)12)13)} the pathological process is often termed parieto-occipital encephalopathy or posterior leukoencephalopathy synonymously.³⁾⁴⁾⁶⁾ Although, a supratentorial lesion is predominant, rare cases of cerebellar encephalopathy have been reported.
   We report a patient with hypertensive encephalopathy that was characterized by isolated cerebellar edema and tonsillar herniation requiring urgent posterior fossa decompression. The inspection of pathological tissue samples was also performed.

Case Report

   A 51-year-old white male patient presented with drowsiness and confused mental state. The patient's pupils were isocoric with sluggish light reflexes. His optic disc margins were blurred. Blood pressure was 230/150 mmHg and pulse rate was 52 per minute. He revealed oliguria (less than 10 ml per hour). The initial Glasgow Coma Scale was assessed as 9 of 15. Past medical history revealed that the patient has been suffering from hypertension. Brain CT scans showed diffuse swelling and hypodensity of the brain stem and cerebellar hemispheres with collapsed fourth ventricle and slightly dilated lateral and third ventricles (Fig. 1). Diffusion weighted MRI showed a small area of acute infarction on the left thalamus. T1 and T2 images showed abnormal signal and swelling of the cerebellar hemispheres (Fig. 2).
   During the initial aggressive medical management, blood pressure was controlled well with parenteral hydration and the use of diuretics. The patient's vital signs became stable and he improved from the oliguric state, however his alertness progressively deteriorated. As we assumed that the thalamic infarction might be caused by upward herniation through the tentorium, we performed suboccipital decompressive craniectomy including wide opening of the foramen magnum. After general anesthesia with isoflurane, opening of the cisterna magna made the cerebellar hemispheres slackened down. Cerebellar tonsils were herniated down to the level of the atlas. Because tonsillar herniation was more prominent than on the right side, we performed partial resection of the right cerebellar hemisphere and tonsillectomy.
   On the following day of the surgery, the patient could open his eyes spontaneously. A follow-up CT and MR image showed resolving posterior fossa pressure with appearance of the fourth ventricle (Fig. 3). Microscopic examination of the specimen revealed hyaline thickening of the media of the arachnoidal vessel, consistent with benign hypertensive vascular change (Fig. 4). He showed no neurological deficits 3 weeks after the surgery .

Discussion

   The constancy of cerebral blood flow is controlled by the autoregulation of cerebral arteries.¹²⁾ In chronic persistent hypertension, an upward shifting of the autoregulatory zone is recognized.¹¹⁾ High blood pressure is transmitted to the small cerebral arterioles. If the upper limit of the regulation range is overcome, this leads to a failure of the blood-brain barrier and extravasation of fluid and protein into the brain parenchyma.^3)4)10)12) A resultant brain edema is manifested in a CT or MR scan.
   Hypertensive encephalopathy is usually noticed in supratentorial areas, especially in the parieto-occipital region and infrequently in the basal ganglia, brain stem, and the cerebellum. The causes of the discrete distribution of this disease entity to these areas have been discussed, but not clearly explained. Differential distributions of sympathetic innervations have been mentioned often, but these studies do not explain the rarity of cerebellar and brain stem lesions due to the dearth of innervations in the vertebrobasilar circulation.¹⁾¹³⁾ Hypertensive encephalopathy principally involving the posterior fossa has been described but only on rare occasions. 
   Verrees et al. has reported two cases of primary hypertension-induced cerebellar encephalopathy causing obstructive hydrocephalus.¹²⁾ Schwartz et al. also described two patients with concomitant cerebellar involvement.⁹⁾ Even if the resultant hydrocephalus secondary to hypertensive encephalopathy is revealed in CT or MRI scans, medical management is initially attempted except for the cases of a seriously deteriorating neurological status.^3)9)12)13) Wang et al. have described a case of posterior fossa swelling due to hypertensive encephalopathy that presented with ventriculomegaly. In this patient, the ventriculomegaly decreased after the symptoms diminished.¹³⁾ Reviewed literature supports that this disease is usually cared for with medical treatment. Even if it is treated by surgery, shunt or external drainage of the cerebrospinal fluid was the only surgical procedure.¹³⁾ But in our case, neurologic deterioration was progressed rapidly regardless of the initial medical therapy. The patient's consciousness was not recovered althought arterial blood pressure was controlled with antihypertensive medication effectively. Consequently, emergent decompressive surgery was indispensable. In addition upward herniation resulting from cerebellar edema occurred. Anterior thalamic infarction is probably due to the thalamoperforator injury during upward herniation. Nagaa et al have reported a data of auditory brainstem responses after expansion of an infratentorial epidural mass in anesthetized cats.⁷⁾ Upward transtentorial herniation of the midbrain and foraminal herniation of the cerebellum in acute expanding lesions of the posterior fossa were noted in their experiment. In the operation of the presenting case, isoflurane was used for inhalation anesthetics. Isoflurane is used in most part of the brain surgery till quite recently. Isoflurane decrease the cerebral metabolism but, the decrease of the cerebral blood flow is less pronounced.⁵⁾ Barbiturates are used for cerebral protection because they can reduce CMRO2. They also cause vasoconstriction. Besides isoflurane many newer anesthetics are studied and challenged in the field. Desflurane, sevoflurane and propofol also have effect in decreasing the intracranial pressure and thought to be useful in the restoration of the cerebral autoregulation. These anesthetics have described that they preserve the cerebral autoregulation during the anesthesia. But their effect for recovery of cerebral autoregulation is controvertial.⁵⁾ Because most cases described above were effectively treated with initial medical management, the pathologic examination of microscopic findings of the involved brain tissue was not performed. In vitro studies and several autopsy or biopsy cases have been previously reported. In some cases, amyloid angiopathy, vasculitis, and other connective tissue pathologies were inspected.³⁾⁸⁾

Conclusion

   Uncontrolled hypertensive encephalopathy, refractory to medical treatment, can be successfully treated with early decompressive surgery. We described a patient with hypertensive cerebellar encephalopathy accompanying hydrocephalus that was successfully treated with surgery and confirmed pathologically. If tissue confirmation can be obtained like the present case, this can be helpful for differential diagnosis, the treatment of the patient, and research on the causes of regional vascular heterogeneity.

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