Relevant Anatomy

Composed of two hemispheres and a midline vermis, the cerebellum sits in the posterior fossa dorsal to the brain stem, in a space constrained by three surfaces: (1) the tentorium superiorly, (2) the skull base formed by the petrous bone and clivus ventrally, and (3) the suboccipital skull convexity surface dorsally and inferiorly (Fig. 70-1A). The posterior fossa communicates with the supratentorial space via the tentorial incisura, through which passes the upper brain stem (midbrain); and it communicates with the spinal canal via the foramen magnum, through which passes the lower brain stem (medulla) and upper cervical spinal cord.

FIGURE 70-1 Posterior fossa. A, Anatomy (midsagittal view). Note the space constrained by three surfaces: (1) the tentorium superiorly, (2) the skull base formed by the petrous bone and clivus ventrally, and (3) the suboccipital skull convexity surface dorsally and inferiorly. Also note the communications with the supratentorial space via the tentorial incisura and with the spinal canal via the foramen magnum. B, Consequences of mass: cerebellar hemorrhage, or infarction with swelling, may cause effacement of the fourth ventricle, compression of the brainstem against the clivus, obstruction of CSF flow through the aqueduct, and herniation upward through the tentorial incisura and/or downward through the foramen magnum.

The cerebellum modulates motor function, and corrects for differences between intended and actual movements. Because pathways to and from the hemispheres are mostly uncrossed or doubly crossed, hemispheric lesions cause coordination deficits in the ipsilateral limbs. Lesions of the vermis result in truncal ataxia and dysarthria, and lesions of the inferior cerebellum tend to cause vestibular dysfunction. When axial muscle groups are affected, deficits may not have an obvious laterality.

Blood is supplied to the cerebellum via three pairs of arteries from the vertebrobasilar system, each of which originates ventrally in the posterior fossa, and must encircle (and supply) the appropriate brain-stem region to reach the cerebellum (Fig. 70-2). These three trunks include: (1) the posterior inferior cerebellar artery (PICA), which originates from the vertebral artery 1 to 3 cm proximal to the vertebrobasilar junction and supplies the lateral medulla, inferior vermis, and posterior-inferior cerebellar surface, (2) the anterior inferior cerebellar artery (AICA), which usually originates from the inferior third of the basilar artery, and supplies the caudal pons and the petrosal surface of the cerebellum, and (3) the superior cerebellar artery (SCA), which supplies the caudal midbrain/rostral pons, the superior cerebellar peduncle and dentate nucleus, the superior vermis and the tentorial surface of the cerebellum.

FIGURE 70-2 Blood supply of the posterior fossa. Three pairs of arteries originate from the vertebrobasilar system ventral to the brainstem, and then encircle and supply the appropriate brainstem region to reach the cerebellum. These trunks include the posterior inferior cerebellar artery (PICA), the anterior inferior cerebellar artery (AICA), and the superior cerebellar artery (SCA), which are seen on this lateral view. Most infarctions arise in the inferior-lateral cerebellum (PICA territory), while most hemorrhages arise near the deep cerebellar nuclei.

Pathogenesis of Cerebellar Hemorrhage and Infarction

The infratentorial compartment, or posterior fossa, is approximately one-eighth of the entire intracranial space.¹ Based on MRI volumetric analysis, the volume of the posterior fossa is approximately 200 cc in men and slightly less in women.^2,3 The three components of that volume are brain parenchyma (80%), circulating blood (10%), and CSF (10%).⁴ An expanded cerebellum associated with a clot or edema can directly compress the brain stem against the clivus, obliterating the subarachnoid cisterns. A mass lesion in the cerebellum, such as a hematoma or edematous infarct, can also cause effacement of either the fourth ventricle or the aqueduct, and lead to obstructive hydrocephalus.

If we estimate 18-20 cc of CSF in the posterior fossa, an infarct or hematoma adding 18 cc of mass to the posterior fossa is sufficient to displace the entire volume of CSF in this compartment. After the CSF reserve space is obliterated, any further enlargement of the hematoma or edema of the surrounding brain and/or infarction reduces the volume of circulating blood, and begins to force brain parenchyma out of the posterior fossa through the tentorial incisura or the foramen magnum (Fig. 70-1B). Given that the volume of a sphere is 4/3×πr³ (where r is the radius), an 18-cc spherical hematoma with a diameter of 3.2 cm (radius 1.6 cm) correlates with the popular assertion that a 3-cm cerebellar hematoma usually warrants urgent surgical evacuation. The causes of cerebellar infarction and hemorrhage are outlined in Table 70-1.

Table 70-1 Causes of Cerebellar Hemorrhage and Infarction

Clinical Manifestations

While cerebellar stroke is the most worrisome cause of dizziness, nausea, and gait instability, the same constellation of symptoms may result from a peripheral disorder, such as vestibular neuritis or labyrinthitis. Furthermore, the clinical presentation of cerebellar hemorrhage is not reliably distinct from that of cerebellar infarction. In Raco’s series comparing 70 patients with cerebellar hemorrhage and 52 patients with cerebellar infarction, both entities commonly presented with the acute onset of nausea, vomiting, dizziness, and unsteady gait. Headache was more common as an initial symptom in cerebellar hemorrhage.⁶ Dizziness may occur with or without vertigo.¹⁶ On physical exam, dysarthria, ataxia, and nystagmus are common. Ataxia may be appendicular, axial or both, depending on the relative involvement of the cerebellar hemispheres and vermis. Nystagmus and/or hearing loss may be a sign of cerebellar stroke or of a peripheral vestibular disorder.¹⁶

Posterior circulation strokes, particularly infarction, often involve both the cerebellum and the brain stem. In such situations, various brain-stem stroke syndromes are evident in combination with the manifestations of cerebellar dysfunction. Depending on the level affected, these syndromes may include ipsilateral hemifacial analgesia, contralateral hemibody analgesia, Horner’s syndrome, motor deficits, and cranial neuropathies (Table 70-2). The neurologic deficit often indicates the arterial territory affected by the infarct, but anatomic variations are common between AICA and PICA, and the territory affected can vary considerably.

Table 70-2 Posterior Circulation Stroke Syndromes

Distinguishing the signs of primary brain-stem stroke from those of secondary brain-stem compression is extremely important. Direct brain-stem compression can manifest either as gaze restrictions, lower cranial nerve dysfunction, or altered sensorium from suppression of the reticular activating system. After the early symptoms and signs of the infarction are defined, delayed neurologic deterioration 1 to 7 days following symptom onset is often indicative of cerebellar swelling and impending upward (transtentorial) or downward (tonsilar) herniation.^18–23 Hydrocephalus may further contribute to the decline. Certainly, any change in sensorium warrants urgent surgical intervention, as the patient can quickly proceed through the later stages of brain-stem compression, which include coma, abnormal respirations, posturing, and death.

Radiologic Evaluation

CT scanning is the definitive diagnostic procedure of choice for cerebellar hemorrhage, and is also useful in defining subacute or chronic infarction. The initial CT may be normal for several hours following a cerebellar infarct, during which time a diffusion-weighted MRI can be diagnostic. Both types of studies (CT or MRI) provide information about any mass effect on surrounding structures, and shows hydrocephalus when present. Posterior fossa mass effect can be visualized as distortion or effacement of the fourth ventricle, compression of the brain stem, effacement of the basal cisterns, and cerebellar herniation through the tentorial incisura (vermis) or foramen magnum (tonsils). They can also identify primary brain-stem involvement in the stroke, which carries a worse prognosis.

Several CT criteria have been correlated with neurologic deterioration and the need for surgical intervention. The concept of a “tight posterior fossa” was described by Weisberg in a series of 14 patients with cerebellar hemorrhage, all of whom had effacement of the basal cisterns and obstructive hydrocephalus. Nonsurgical management resulted in 100% mortality; of the eight patients who underwent craniectomy and hematoma evacuation, all survived and six became ambulatory.²⁴ Taneda and colleagues correlated the degree of quadrigeminal cistern effacement with patient outcomes.²⁵ Many surgeons consider a hematoma diameter of greater than 3 cm to be an indication for surgery.^26–28 Another treatment algorithm stresses the importance of fourth ventricular compression over hematoma size.⁵

MRI scanning is preferable in defining an underlying tumor or vascular malformation. Vascular imaging (MRA or CTA) can often effectively define a parent vessel thrombosis, peripheral branch occlusion, or dissection of the vertebral artery. In questionable circumstances, four-vessel transfemoral arteriography is required to establish a final diagnosis and dictate future management. Echocardiography and laboratory tests can help identify cardiac and systemic risk factors.

Initial Management

An acute cerebellar hemorrhage or infarct requires neurosurgical evaluation. A patient with an acute cerebellar stroke without hydrocephalus or mass effect should be admitted for observation and continued evaluation, generally in a neurologic ICU. Serial neurologic exams and brain imaging are complementary. For ischemic stroke, antiplatelet and thrombolytic therapy may be given in accordance with American Heart Association guidelines, but should be avoided if surgical intervention is likely.³⁰

In cerebellar hemorrhage, follow-up CT scans should be done daily (or more often in unstable patients) for the first several days following the ictal event, to evaluate for edema progression and/or rehemorrhage. Cerebellar infarction can be expected to increase in volume, with the first radiographic signs of mass effect developing 1 to 6 days after onset. Patients with progressive neurologic deterioration typically have characteristic findings over time on CT scanning, including fourth ventricular shift, basal cistern compression, brain-stem deformity, and hydrocephalus.²⁹

Indications and Timing for Surgical Intervention

Surgical Intervention

Acute cerebellar stroke, whether ischemic or hemorrhagic, requires an initial clinical assessment to identify signs of hydrocephalus or brain-stem compression, and surgical decompression via a suboccipital craniectomy and a ventriculostomy should be considered an urgent decision in such patients. The neurosurgeon should be involved early in cases likely to need surgery.

Some patients have a very poor prognosis that may limit the treatment offered, particularly those in prolonged deep coma or those whose stroke primarily involves the brain stem. When a cerebellar hemorrhage or infarction does not initially warrant surgery, the patient should be admitted to an ICU or similar setting for serial neurologic exams and brain imaging.

After a decision is made to proceed with surgery, preparations for the procedure should be rapid, to optimize the patient’s chances of a good recovery. Mannitol, head elevation, hyperventilation, and ventriculostomy can be temporizing measures to control increased intracranial pressure. The ventriculostomy is often placed frontally, and can be done while waiting for the operative theater to become available. Once the patient is in the operating room and asleep, the patient is positioned prone in three-point pin fixation with the head elevated above the heart. The neck and upper back are slightly flexed to open the craniocervical junction and allow a comfortable working angle to the posterior fossa, carefully avoiding excesses that can cause jugular venous obstruction or kinking of the endotracheal tube. Securing the patient with straps allows for axial rotation which makes the operative position more comfortable for the operating surgeon. The occipital and suboccipital regions are prepped and draped in the traditional fashion, with room to place an occipital burr hole and ventricular drain if needed.

A midline incision is preferable for the craniectomy, although occasionally a paramedian incision may be utilized. Bone removal is wide enough to decompress cerebellar swelling as well as to provide access to remove the offending pathology. The bone removal must extend inferiorly to access the cisterna magna and widely decompress the foramen magnum, and generally includes removal of the posterior arch of C1. The dura is opened to expose the midline and lateral cerebellar surface affected by the hematoma or infarct, and the incision is carried inferiorly below the foramen magnum so that the tips of the cerebellar tonsils are visualized and that adequate decompression of the brain stem has been accomplished.

Once the dura is opened, the operating microscope is used for the intradural portion of the procedure. With cerebellar hemorrhage, ultrasound can be helpful in localizing the hematoma and planning the incision in the cerebellar cortex. The entire clot does not need to be removed, but the evacuation should be aggressive enough to remove most of the mass effect caused by the hematoma; surrounding edematous tissue should be preserved. For cerebellar infarction, the clearly dead and necrotic tissue, which is more soft and suctionable than the surrounding normal cerebellum, should be removed, especially those portions in the lateral hemisphere and near the foramen magnum. No attempt should be made to define the ischemic penumbra; injury to the PICA, deep cerebellar nuclei, and brain stem must be avoided. After adequate decompression, the cerebellum and foramen magnum appear much more relaxed within the dural opening.

The dura is closed loosely, but in a watertight fashion, invariably using a patch graft. The suboccipital bone is not replaced; the muscle, fascia, and dermis are closed securely to minimize the chance of pseudomeningocele formation, CSF leak, and wound breakdown. Postoperatively, the ventriculostomy is opened to measure and control ICP and to keep CSF build-up from the recently closed wound. After several days of stability, the catheter is weaned as tolerated, and a ventriculoperitoneal shunt is later placed as necessary. Prior to feeding, the patient’s risk of aspiration is evaluated, and early tracheostomy and feeding tubes are placed in high-risk patients.

Cases

Case 1

A 51-year-old woman with a history of hypertension presented to the emergency room of another facility after the acute onset of headache, nausea, vomiting, and gait instability. She became comatose in the ER and was intubated. After a CT showed a large right cerebellar hemorrhage with hydrocephalus, a right frontal ventriculostomy was placed, and the patient was transferred to our institution. Her CT showed a 5×3-cm hematoma in the right cerebellar hemisphere and vermis, causing deformity of the brain stem, effacement of the basal cisterns, compression of the fourth ventricle, and hydrocephalus (Fig. 70-3). She underwent an emergent suboccipital craniectomy and evacuation of the hematoma, and made a dramatic recovery postoperatively. At follow-up 4 months later, she was neurologically intact except for mild gait instability.

FIGURE 70-3 Cerebellar hemorrhage. A to D, Large right cerebellar hemorrhage, causing compression of the brainstem and basal cisterns. The upper fourth ventricle is effaced and there is obstructive hydrocephalus. Prior to the patient’s transfer to our institution, a right frontal ventriculostomy catheter had been placed, with associated frontal pneumocephalus. E and F, CT 1 month after suboccipital craniectomy and evacuation of the hematoma. The brain stem is decompressed, the cisterns are open, and there is no hydrocephalus.

Case 2

An 81-year-old man with a history of hypertension and atrial fibrillation presented after the acute onset of nausea and gait instability. His neurologic examination showed left dysmetria and gaze-paretic nystagmus. His initial CT was normal, and MRI showed diffusion restriction in the left PICA distribution (Fig. 70-4A and B). He was admitted to the neurologic intensive care unit, and found to have dysarthria 40 hours after the onset of symptoms. A CT showed the left cerebellar infarction with mass effect, causing transtentorial compression of the brain stem (Fig. 70-4C and D). He underwent an emergent suboccipital decompression, and recovered well after the surgery.

FIGURE 70-4 Cerebellar infarction. A, Normal initial CT in a patient with acute cerebellar symptoms. B, MRI shows diffusion restriction in the left PICA territory. C and D, Forty hours after the onset of symptoms, CT shows edematous left cerebellar infarction with transtentorial compression of the brainstem. E and F, CT 10 days after suboccipital craniectomy shows decompression of brain stem.

Conclusion

While the pathophysiology of cerebellar hemorrhage is distinct from that of cerebellar infarction, it is appropriate to consider these two entities together, because they have similar neurologic sequelae. Both conditions typically present acutely with symptoms and signs of cerebellar dysfunction, and may progress rapidly to coma from mass effect in the posterior fossa. When brain-stem compression and obstructive hydrocephalus occurs, urgent suboccipital decompression is warranted. Timely surgery can be life-saving, and patients often have a good functional outcome.