Computational Investigation of Cerebrospinal Fluid Dynamics in the Posterior Cranial Fossa and Cervical Subarachnoid Space in Patients with Chiari I Malformation.

PURPOSE: Previous computational fluid dynamics (CFD) studies have demonstrated that the Chiari malformation is associated with abnormal cerebrospinal fluid (CSF) flow in the cervical part of the subarachnoid space (SAS), but the flow in the SAS of the posterior cranial fossa has received little attention. This study extends previous modelling efforts by including the cerebellomedullary cistern, pontine cistern, and 4th ventricle in addition to the cervical subarachnoid space. METHODS: The study included one healthy control, Con1, and two patients with Chiari I malformation, P1 and P2. Meshes were constructed by segmenting images obtained from T2-weighted turbo spin-echo sequences. CFD simulations were performed with a previously verified and validated code. Patient-specific flow conditions in the aqueduct and the cervical SAS were used. Two patients with the Chiari malformation and one control were modelled. RESULTS: The results demonstrated increased maximal flow velocities in the Chiari patients, ranging from factor 5 in P1 to 14.8 in P2, when compared to Con1 at the level of Foramen Magnum (FM). Maximal velocities in the cervical SAS varied by a factor 2.3, while the maximal flow in the aqueduct varied by a factor 3.5. The pressure drop from the pontine cistern to the cervical SAS was similar in Con1 and P1, but a factor two higher in P2. The pressure drop between the aqueduct and the cervical SAS varied by a factor 9.4 where P1 was the one with the lowest pressure jump and P2 and Con1 differed only by a factor 1.6. CONCLUSION: This pilot study demonstrates that including the posterior cranial fossa is feasible and suggests that previously found flow differences between Chiari I patients and healthy individuals in the cervical SAS may be present also in the SAS of the posterior cranial fossa.

Brain tissue adaptability to slow-growing tumors: case report of clivus meningioma.

A 46-year-old female patient with a large slow-growing craniocervical junction tumor is presented. Her complaints began 6 months before with sensory and painful sensations, sphincter impairment, and motor events (spastic tetraparesis, more pronounced on the left extremities). Magnetic resonance of the head revealed a rounded tumor of 2.5 cm in diameter, by its characteristics corresponding to meningioma, at the level of C1 vertebra and craniocervical junction, with the base at spinal canal anterior wall, occupying most of the craniocervical junction, compressing spinal cord and medulla oblongata. Intracerebral computed tomography angiography showed spared lumen and a satisfactory image of vertebral arteries bypassing the expansive growth at the occipital foramen, confirming slow tumor growth. Antiedematous therapy led to transient improvement in extremity strength and partial recovery of neurologic deficit, which resolved completely upon neurosurgical operation and rehabilitation. This case report exemplifies brain adaptability to slowly growing expansive neoplasms, based on its volume reduction up to the moment when further adaptation is not possible anymore, i.e. breaking of the mechanism of adaptation. Because of brain adaptability, such slowly growing tumors may stay asymptomatic for a long time. Brain plasticity also includes adaptation and autoregulation of the circulation, thus ensuring stable blood flow.

Biomechanical evaluation of the craniovertebral junction after inferior-third clivectomy and intradural exposure of the foramen magnum: implications for endoscopic endonasal approaches to the cranial base.

OBJECT: Endoscopic endonasal approaches to the craniovertebral junction (CVJ) and clivus are increasingly performed for ventral skull-base pathology, but the biomechanical implications of these approaches have not been studied. The aim of this study was to investigate the spinal biomechanics of the CVJ after an inferior-third clivectomy and anterior intradural exposure of the foramen magnum as would be performed in an endonasal endoscopic surgical strategy. METHODS: Seven upper-cervical human cadaveric specimens (occiput [Oc]-C2) underwent nondestructive biomechanical flexibility testing during flexion-extension, axial rotation, and lateral bending at Oc-C1 and C1-2. Each specimen was tested intact, after an inferior-third clivectomy, and after ligamentous complex dissection simulating a wide intradural exposure using an anterior approach. Angular range of motion (ROM), lax zone, and stiff zone were determined and compared with the intact state. RESULTS: Modest, but statistically significant, hypermobility was observed after inferior-third clivectomy and intradural exposure during flexion-extension and axial rotation at Oc-C1. Angular ROM increased incrementally between 6% and 12% in flexion-extension and axial rotation. These increases were primarily the result of changes in the lax zone. No significant changes were noted at C1-2. CONCLUSIONS: Inferior-third clivectomy and an intradural exposure to the ventral CVJ and foramen magnum resulted in hypermobility at Oc-C1 during flexion-extension and axial rotation. Although the results were statistically significant, the modest degree of hypermobility observed compared with other well-characterized CVJ injuries suggests that occipitocervical stabilization may be unnecessary for most patients.

Corticospinal activation confounds cerebellar effects of posterior fossa stimuli.

OBJECTIVE: To investigate the efficacy of magnetic stimulation over the posterior fossa (PF) as a non-invasive assessment of cerebellar function in man. METHODS: We replicated a previously reported conditioning-test paradigm in 11 healthy subjects. Transcranial magnetic stimulation (TMS) at varying intensities was applied to the PF and motor cortex with a 3, 5 or 7 ms interstimulus interval (ISI), chosen randomly for each trial. Surface electromyogram (EMG) activity was recorded from two intrinsic hand muscles and two forearm muscles. Responses were averaged and rectified, and MEP amplitudes were compared to assess whether suppression of the motor output occurred as a result of the PF conditioning pulse. RESULTS: Cortical MEPs were suppressed following conditioning-test ISIs of 5 or 7 ms. No suppression occurred with an ISI of 3 ms. PF stimuli alone also produced EMG responses, suggesting direct activation of the corticospinal tract (CST). CONCLUSIONS: CST collaterals are known to contact cortical inhibitory interneurones; antidromic CST activation could therefore contribute to the observed suppression of cortical MEPs. SIGNIFICANCE: PF stimulation probably activates multiple pathways; even at low intensities it should not be regarded as a selective assessment of cerebellar function unless stringent controls can confirm the absence of confounding activity in other pathways.

Persistent primitive hypoglossal artery associated with lower basilar artery aneurysm: diagnosis and clinical implications.

An elderly male was evaluated for subarachnoid hemorrhage and found to have a persistent primitive hypoglossal artery with its associated abnormal posterior fossa circulation and a lower basilar artery aneurysm. Angiography findings are described and are corroborated with computed tomography and magnetic resonance imaging. Its surgical and clinical relevance are discussed.

Endoscopic anatomy of the pterygopalatine fossa.

BACKGROUND: The pterygopalatine fossa can be involved with a variety of infectious and neoplastic processes. This region can be entered endoscopically, but endoscopic landmarks to localize the neurovascular structures in the pterygopalatine fossa have not yet been reported. OBJECTIVES: The purpose of this study is to describe the location of the neurovascular structures in the pterygopalatine fossa in relation to consistent intranasal landmarks. METHODS: Endoscopic dissections of cadaveric heads were performed. The locations of neurovascular structures in the region were defined. RESULTS: The sphenopalatine foramen (SPF) served as the primary intranasal landmark to the pterygopalatine fossa (PPF). Mean distances from the SPF were measured with the following results: SPF to sphenopalatine ganglion (SPG), 4 mm medially and 6 mm laterally; SPF to foramen rotundum (FR), 7 mm; and SPF to vidian canal (VC), 2 mm. The internal maxillary artery followed an irregular and inconsistent course, making it difficult to define a reliable landmark for its location in the fossa. CONCLUSION: Entering the PPF inferior to the horizontal plane of the SPF along a vertical line drawn inferiorly from the infraorbital canal will avoid injury to the major neural structures in the fossa. Because of the inconsistent course and location of the internal maxillary artery, this structure may be at risk no matter where the fossa is entered. These landmarks will allow the surgeon to enter the PPF with more accuracy and less patient morbidity.

Subdural pressure measurement during posterior fossa surgery. Correlation studies of brain swelling/herniation after dural incision with measurement of subdural pressure and tactile estimation of dural tension.

Thirty-two patients with posterior fossa tumours or arteriovenous malformations were subjected to elective craniotomy in the prone position. The intracranial pressure (ICP) was measured by a subdural approach in the open area of the exposed dura. Estimation of dural tension before dural incision and the degree of brain swelling/herniation after opening the dura were correlated with the subdural pressure measured with intact dura. The results indicate that at ICP < 10 mmHg, brain swelling/herniation rarely occurred, while at ICP > or = 10 mmHg some degree of brain swelling/herniation was always present. The neurosurgeon's tactile estimation of dural tension correlated poorly with any tendency to brain swelling/herniation. It is concluded that measurement of subdural pressure is a better predictor of the risk of brain swelling/herniation than the tactile estimation of dural tension during posterior fossa surgery.