Posterior Fossa

Suboccipital craniotomy (or craniectomy) exposes the posterior fossa beneath the transverse sinuses. A midline exposure, often with removal of the posterior arch of C1, reaches the vermis, both cerebellar hemispheres, the fourth ventricle and the cerebellomedullary fissure; paramedian and lateral exposures reach the cerebellar hemispheres and the cerebellopontine angle. The superior extension (supracerebellar infratentorial) reaches the pineal region and the dorsal midbrain^[1]. Rhoton's anatomical work on the cerebellum and fourth ventricle provides the anatomical framework for these approaches^[2].

• Cerebellar tumours (metastases, haemangioblastomas, astrocytomas) and fourth-ventricular tumours (medulloblastoma, ependymoma).
• Cerebellar haemorrhage and infarction with mass effect.
• Chiari malformation decompression.
• Cavernous malformations of the dorsal brainstem through the floor of the fourth ventricle.
• Pineal-region tumours by the supracerebellar route.

The fourth ventricle can be entered either by splitting the inferior vermis or through the cerebellomedullary fissure. Mussi and Rhoton showed that opening the tela choroidea and inferior medullary velum, which form the lower roof of the ventricle, gives access to the whole ventricle up to the aqueduct without dividing the vermis (the telovelar approach)^[3]. This avoids vermian splitting, which has been associated with cerebellar mutism and truncal ataxia. The floor of the fourth ventricle contains the facial colliculus, the hypoglossal and vagal triangles and the striae medullares; safe entry zones into the brainstem from the floor lie above and below the facial colliculus^[4].

• Prone (Concorde) position with the head flexed to open the craniocervical junction, or sitting for pineal-region and some vermian lesions.
• Consider a frontal or occipital burr hole for an external ventricular drain in patients with hydrocephalus.
• Midline incision from the inion to C2, dissecting through the avascular midline raphe.
• Craniotomy from just below the transverse sinus to the foramen magnum; remove the posterior arch of C1 if the tonsils or the lower fourth ventricle must be exposed.
• Y-shaped dural opening, controlling the occipital sinus and the marginal sinus.
• Open the cisterna magna to release CSF; separate the tonsils and open the tela choroidea for the telovelar route.

• Venous air embolism, especially in the sitting position: screen for a patent foramen ovale and monitor with precordial Doppler or transoesophageal echocardiography.
• Hydrocephalus and CSF leak or pseudomeningocele: watertight closure and management of CSF dynamics.
• Cerebellar mutism syndrome after midline fourth-ventricular surgery, particularly in children.
• Cranial-nerve and brainstem injury from manipulation of the ventricular floor.