👂 CPA Tumours
The Cerebellopontine Angle, the vestibular schwannoma (“acoustic neuroma”), and how a single expanding tumour knocks out cranial nerves in strict anatomical sequence — CN VIII → CN VII → CN V → posterior fossa.
The Cerebellopontine Angle — Geography First
The Cerebellopontine Angle (CPA) is a wedge-shaped CSF-filled cistern in the posterior cranial fossa, formed at the junction of the cerebellum, the pons, and the petrous temporal bone. It is not a structure you can see on the surface of the brain — it is the space in the angle between these three structures, filled with CSF and traversed by several critical cranial nerves.
Understanding this geography is the entire basis of CPA tumour neurology. As a mass grows in this cistern, it predictably encounters each adjacent structure in sequence — and each encounter produces a specific, recognisable clinical sign.
🦴 The Internal Acoustic Meatus (IAM)
A bony canal in the petrous temporal bone through which CN VII (facial) and CN VIII (vestibulocochlear) travel together from the posterior cranial fossa into the inner ear. The vestibular schwannoma starts here — growing within this rigid bony tunnel before spilling into the CPA cistern. The confined space of the IAM means even a small tumour here compresses both CN VII and CN VIII.
🧠 The Cerebellum and Brainstem
The cerebellum (particularly the cerebellar hemisphere and flocculus) and the lateral surface of the pons bound the CPA posteriorly and medially. A large CPA mass will eventually compress these structures, causing ipsilateral cerebellar signs (ataxia, dysmetria, nystagmus) and ultimately life-threatening brainstem compression with hydrocephalus.
⚡ CN V — Trigeminal Nerve
The trigeminal nerve enters the posterior fossa at the level of the pons, just superior and medial to where CN VII/VIII enter the IAM. It sits in the CPA cistern and is vulnerable to compression from a growing mass that has already filled the IAM and is now expanding into the angle. CN V compression causes ipsilateral facial numbness and, critically, the afferent limb of the corneal reflex.
🔌 CN IX, X, XI — Lower Cranial Nerves
The glossopharyngeal (IX), vagus (X), and accessory (XI) nerves exit the posterior fossa through the jugular foramen, below and medial to the IAM. Only very large CPA tumours with significant caudal extension compress these nerves, producing dysphagia, absent gag reflex, hoarseness, and shoulder weakness — late signs indicating a tumour that has been left to grow unchecked.
Why the CPA Is the Most Common Posterior Fossa Tumour Site in Adults
The CPA accounts for approximately 5–10% of all intracranial tumours and is the most common site for tumours in the adult posterior cranial fossa. Three tumour types make up the vast majority: vestibular schwannoma (~80%), meningioma (~10%), and epidermoid cyst (~5%). Together these are the “CPA triad” and can often be distinguished radiologically before surgery.
The CPA is such a common tumour site because: (1) the IAM is a transition zone between peripheral and central nervous system — the Schwann cell–glial cell junction (Obersteiner-Redlich zone) at the nerve root entry zone is a site of predilection for schwannoma growth; (2) arachnoid cap cells are present throughout, allowing meningioma development; (3) epidermoid cysts arise from ectodermal inclusions along the embryological neural tube fusion line.