Supracerebellar arachnoid cyst and reversible tonsillar herniation: magnetic resonance imaging and pathophysiological considerations

An anatomical model for studying cerebellar tonsillar herniation related to raised intracranial pressure

Brain herniation is one of the most feared complications of many neurological pathologies. However, current understanding of the mechanisms behind brain herniation syndromes is poor. By investigating the correlations between raised intracranial pressure (ICP) and herniation of the cerebellar tonsils, we hope to develop a model that can be used to study intracranial fluid dynamics and its effects on brain tissue. This will facilitate evaluation of patients with elevated ICP and development of novel treatments including surgical approaches for decompressing the posterior cranial fossa and upper cervical spine. A standard suboccipital surgical approach was used to expose the foramen magnum and observe movements of the cerebellar tonsils in fresh cadavers. A urinary Foley catheter balloon in the parietal extradural space was used to simulate a mass effect while ICP was monitored. The baseline anatomy differed widely among the cadaver specimens. However, and overall, we found that as ICP rises, the cerebellar tonsils descend through the foramen magnum at a rate of 0.3 mm per 1 mmHg increase in ICP. A mean descent of 6.2 mm was observed for a mean ICP increase of 17 mmHg. In this cadaveric study, we present a method and model for exploring brain herniation syndromes in the context of ICP changes. This could allow for further models to study the effects of other neuropathologies on the cerebellar tonsils, including posterior cranial fossa mass lesions and cerebellar hemorrhage.

Orthostatic Enlargement of a Supracerebellar Arachnoid Cyst With Cerebellar Descent Visualized by Upright Computed Tomography

BACKGROUND

Posterior fossa arachnoid cysts are often asymptomatic, but can rarely cause postural headache, the mechanism of which remains unknown.

CASE DESCRIPTION

We present a 40-year-old woman with an asymptomatic supracerebellar arachnoid cyst. Upright computed tomography (CT) showed enlargement of the arachnoid cyst and caudal descent of the cerebellum compared with supine CT with narrowing of the craniocervical junction cerebrospinal fluid space.

CONCLUSIONS

This finding aids in understanding the association of posterior fossa arachnoid cysts with orthostatic headache. Clinicians should be aware of possible posture-related changes in intracranial structures.

Improvement of visual acuity caused by opticochiasmal astrocytoma after treatment for associated hydrocephalus--case report

A 33-year-old male presented with syncopal attacks. He had a history of slowly progressive deterioration of visual acuity in both eyes. His visual deterioration began in the left eye at age 12 years and in the right eye at age 20 years. His left eye was completely blind by age 33 years. He had received no prior treatment for these visual disturbances. Magnetic resonance (MR) imaging on admission showed a large tumor with inhomogeneous intensity occupying the bilateral optic canals and orbital cavities, with extension to parasellar region. Arachnoid cysts were found in the left middle fossa and supracerebellar space, which had caused herniation of the cerebellar tonsils to the foramen magnum. Open biopsy and histology verified that the tumor was pilocytic astrocytoma. Arachnoid cysts associated with the hydrocephalus were treated with a ventriculoperitoneal shunt. Seven years after surgery, visual acuity of the right eye had improved although the left eye remained blind. Follow-up MR imaging demonstrated marked reduction in size of the ventricles and arachnoid cysts, but the tumor size did not change. This case illustrates the benign nature of low-grade glioma in this region in spite of the large size and long history. Deterioration of visual acuity may be reversible in some cases of opticochiasmal astrocytoma. In our case, the improvement in visual acuity probably resulted from growth arrest of the tumor and improvement in the dynamics of cerebrospinal fluid flow.

Posterior fossa decompression for life-threatening tonsillar herniation in patients with gliomatosis cerebri: report of three cases

OBJECTIVE AND IMPORTANCE

Gliomatosis cerebri (GC) is a rare type of primary brain tumor that diffusely infiltrates more than two lobes of the brain while the normal cerebral architecture is maintained. To the best of our knowledge, the association between an acquired tonsillar herniation and GC has never been reported. In this article, we describe three patients with progressive gliomatosis of the cerebellar hemispheres who subsequently showed signs and symptoms secondary to tonsillar herniation. Early recognition of this potentially life-threatening complication allowed us to recommend prompt surgical intervention.

CLINICAL PRESENTATION

One patient with primary, or Type I, GC presented with suboccipital headaches, and two patients with secondary, or Type II, GC presented with the signs and symptoms of progressive myelopathy. Serial imaging studies demonstrated progressive involvement of the cerebellum, descent of the cerebellar tonsils through the foramen magnum, and cervicomedullary spinal cord compression.

INTERVENTION

Once the tonsillar herniation was recognized, all three patients underwent posterior fossa decompression, a cervical laminectomy to the lowest level of the tonsillar herniation, and duraplasty. All three patients experienced immediate improvement in their conditions.

CONCLUSION

Early recognition of tonsillar herniation, a possibly overlooked cause of death in patients with GC, allows for early surgical intervention as a potentially lifesaving procedure and significant improvement in the patient's condition.

Hindbrain herniation: an unusual occurrence after shunting an intracranial arachnoid cyst. Case report

The case is presented of a prepontine arachnoid cyst in a 28-year-old woman. She was treated by craniotomy, stereotactic aspiration, internal shunting, and finally by insertion of a cystoperitoneal shunt. Eight months later she developed an asymptomatic Arnold-Chiari type I malformation. The pathogenesis of this unusual hindbrain herniation is explained on the basis of the differential pressure between two compartments and cephalocranial disproportion.