3.28: Posterior skull base (CP angle lesions and seventh/eighth cranial nerves)

R.S. Aarathhi Dhevi Vikramavel, Subarekha TB, S. Rajakokila

Abbreviations

AICA – Anterior inferior cerebellar artery
CPA – Cerebellopontine angle
ECA – External carotid artery
ELST – Endolymphatic sac tumour
EP – Ecchordosis physaliphora
FD – Fibrous dysplasia
GJP – Glomus jugulare paraganglioma
IAC – Internal auditory canal
ICA – Internal carotid artery
LCH – Langerhans cell histiocytosis
PICA – Posterior inferior cerebellar artery
PSB – Posterior skull base
vHL – von Hippel Lindau

Introduction

• Posterior skull base comprises of the posterior margin of temporal bone and the occipital bone,
• Unique areas in the posterior skull base include the cerebellopontine angle, foramen magnum, jugular fossa and hypoglossal canal.

Relevant anatomy

Cerebellopontine angle – Internal auditory canal:

Boundaries – pons medially, posterior surface of the temporal bone laterally, cerebellum posteriorly, prepontine cistern anteriorly, tentorial leaf superiorly and IX- XI cranial nerves inferiorly.

(Please see Chapters 3.26 and 3.27 for further details and images)

Contents of the cerebellopontine angle cistern:

Facial nerve

Vestibulocochlear nerve

AICA loop

Choroid plexus of 4th ventricle at the foramen of Luschka

Flocculus of cerebellum

1. Facial nerve
- There are four components of nerve fibres within the facial nerve
  - • General somatic efferent – motor supply to muscles of facial expression, stapedius, posterior belly of digastric and stylohyoid
  - • Special visceral efferent – parasympathetic secretomotor supply to lacrimal gland, submandibular and sublingual salivary glands.
  - • General somatic afferent – sensory input from the pinna and external auditory canal.
  - • Special visceral afferent – taste sensation from the anterior two-thirds of the tongue, soft palate and floor of the mouth.
- Main motor nucleus of the facial nerve is located in the inferior pons at the level of the inferior cerebellar peduncle. The parasympathetic nuclei of the facial nerve – the superior salivatory and lacrimal nuclei are located just posterolateral to motor nucleus. The sensory nucleus is located just posterolateral to the parasympathetic nuclei.
- Motor root of the facial nerve curves posteriorly around the abducent nucleus and forms the facial colliculus at the floor of the fourth ventricle and leaves the brainstem at the pontomedullary junction.
- The sensory and parasympathetic efferent supply is via the nervus intermedius which leaves the brainstem between the motor root of facial nerve and vestibulocochlear nerve. It traverses along with the facial nerve in the cerebellopontine angle and internal auditory canal.
- The facial nerve is divided into intracranial, intratemporal and extracranial segments; the intracranial segment has intraaxial, cisternal and meatal segments. Of these the cisternal and meatal segments lie in the CPA -IAC region.
- Further details about facial nerve anatomy are discussed in the chapter on cranial nerves.
2. Vestibulocochlear nerve
- Cochlear nerve
- The spiral ganglia, which are bipolar – having two axonal projections from a nerve cell, are located in the cochlear modiolus. From the spiral ganglia, the peripherally projecting axons project on to the hair cells of organ of corti in the scala media of cochlea. The centrally projecting axons of the spiral ganglia coalesce to form the cochlear nerve and pass through anteroinferior part of the fundus of internal auditory canal.
- Superior vestibular nerve arises from the vestibule, passes through the superior part of macula cribrosa to enter the internal auditory canal.
- Inferior vestibular nerve – multiple branches pass through the inferior part of macula cribrosa and join to form the inferior vestibular nerve. Singular nerve from the ampulla of the posterior semicircular canal also passes through the macula cribrosa inferior and joins the inferior vestibular nerve.
- At the region of porus acousticus, the cochlear and vestibular nerves join together and form the vestibulocochlear nerve.
- Up to the level of the porus acousticus the VIII nerve is covered by Schwann cells. Just medial to the porus acousticus, it acquires the oligodendritic cover of central nervous system. [It has the longest oligodendritic cover among cranial nerves – ∼15 mm, from the porus acousticus to its root entry zone]. As a result most schwannomas originate lateral to the porus acousticus, that is, from the canalicular portion of the nerve.
- The vestibulocochlear nerve enters the brainstem at the level of the inferior cerebellar peduncle.
3. Loop of anterior inferior cerebellar artery in the IAC
- The AICA arises from the basilar artery in most cases. It traverses posteriorly abutting the anterior surface of the pons. In mid–course, it loops laterally to reach the porus acousticus. In 20% of individuals, it enters the IAC before turning back on itself to course along the anterior surface of cerebellum. It is divided into the pre-meatal, meatal and post-meatal segments accordingly. In the meatal segment, it gives off the internal auditory branch that supplies the cochlea. The territory supplied by AICA includes cochlea, flocculus and area of the brainstem containing the nuclei of VII and VIII nerves.
4. Choroid plexus of fourth ventricle at foramen of Luschka
- The foramen of Luschka is just posteroinferior to the root entry zone of VIII nerve. Usually a tuft of choroid plexus of the fourth ventricle is seen protruding from it into the cerebellopontine angle cistern.
5. Flocculus of cerebellum – may be associated closely with the VII and VIII nerves in the cerebellopontine angle cistern

Embryology

The internal auditory canal develops as an independent event from the development of external, middle and inner ear. So its presence or absence is not connected to the development of external, middle or inner ear.

The internal auditory canal develops as a result of migration of VII and VIII nerves through this region. So the fewer the number of nerves, smaller the canal. If it is very small and only one nerve is seen, in most cases it is the VII nerve.

Jugular fossa

It lies between the temporal and occipital bone. It is partly divided into two parts by the jugular spine

• The smaller, anteromedial pars nervosa that transmits the cranial nerve IX, Jacobson nerve and inferior petrosal sinus and
• Larger, posterolateral pars vascularis which transmits cranial nerves X &XI, Arnold nerve, jugular bulb and posterior meningeal arteries

Erosion of the jugular spine is a sign of jugular fossa neoplasms.

Hypoglossal canal

It lies inferomedial to the jugular fossa and is within the condylar portion of the occipital bone. It transmits the cranial nerve XII and condylar vein.

On coronal CT, the jugular tubercle separating the hypoglossal canal and jugular fossa is visualized as an ‘eagle’s beak’. Lesions from the jugular fossa cause erosion of the outer superolateral surface of the jugular tubercle while lesions from the hypoglossal canal erode the inner inferomedial surface.

Foramen magnum

It is formed completely by the occipital bone, transmits the medulla oblongata, cranial nerve XI and the vertebral arteries.

Stylomastoid foramen

It is found between the mastoid process and styloid process of the temporal bone. It transmits the cranial nerve VII and directly extends to the parotid space.

Lesions of the posterior skull base

Approach

Pathological conditions of the posterior fossa can be divided based on their origin

• Lesions arising from above the skull base
• Lesions intrinsic to the skull base – bone tumours such as Aneurysmal bone cyst, chondrosarcoma, osteosarcoma, myeloma metastases
- • Other bone lesions such as LCH, FD and paget’s disease
• Lesions arising from below
- • From nasopharynx – nasopahryngeal angiofibroma, nasopharyngeal malignancy
- • Ecchordosis physaliphora (EP)
- • From sphenoid sinus – mucocele and malignancy may erode clivus and reach posterior cranial fossa

Lesions common to anterior, middle and posterior skull base are covered comprehensively under anterior skull base – Chapter 3.26.

Lesions unique to posterior skull base

• Cerebellopontine angle lesions
• Jugular fossa lesions
• Rare temporal bone lesions – endolymphatic sac tumours

Cerebellopontine angle lesions

CPA schwannoma

Schwannoma is the commonest lesion of the cerebellopontine angle in an adult (60%–90%).The cranial nerve VIII is the commonest cranial nerve to be affected by schwannomas. As they arise most commonly from the vestibular division, they are called vestibular schwannomas. They usually present as unilateral sensorineural hearing loss. Other symptoms include tinnitus and giddiness. The age of presentation is usually 40–60 years.

Vestibular schwannomas often arise from the Schwann cells at the glial – Schwann cell junction near the porus acousticus.

The intracanalicular portion of the tumour is ovoid or cylindrical in shape. The extracanalicular portion is spherical or globose, forming an acute angle with the posterior surface of petrous temporal bone. The classic appearance is that of ‘ice cream cone’, where the intracanalicular portion forms the cone and extracanalicular cisternal portion forms the ice cream.

The tumour is classified based on the size of the extracanalicular portion. Small – if extracanalicular portion is <5 mm, medium – if it is 5 mm to 2 cm, large – if 2–4 cm and giant – if it is > 4 cm.

Calcification is uncommon. Micro/macro-haemorrhage is common in larger lesions. Intra-tumoral cystic change seen in about 15% tumours. Peritumoural arachnoid cysts or trapped CSF can be seen.

Imaging features.

On CT, the lesion is hyperdense with respect to CSF and shows enhancement on contrast. Bony changes – Widening of internal auditory canal (difference of >2 mm when compared to the contralateral side) and flaring/funnelling of the porus acousticus are best appreciated on CT. Larger lesions show mass effect over the brainstem and cerebellum.

MRI is the investigation of choice.

Heavily T2 weighted thin section volume sequences (CISS/SPACE, FIESTA, 3D DRIVE) are used as screening tools.

Almost all acoustic schwannomas can be picked up in thin section post contrast T1 weighted images in axial and coronal planes (Figs. 3.28.1–3.28.7). The report should include a mention about the following important pre-surgical and prognostic information

• Size of the lesion (measurement in three planes, preferably in post-contrast images) – size of the intracanalicular and extracanalicular components.
• Presence of intratumoral cysts
• Presence and size of fundal CSF cap
• Tumour involvement of cochlear aperture

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