Anatomy of the Cerebellopontine Angle: A Microscopic Perspective

33 Anatomy of the Cerebellopontine Angle: A Microscopic Perspective

Alvaro Campero, Juan F. Villalonga, Daniel Londoño Herrera, Amparo Sáenz, and Carolina Martins

Abstract

The cerebellopontine angle (CPA) is located between the upper and lower limbs of the cerebellopontine fissure. It is one of the anatomical regions with the highest density of eloquent structures in the cranial cavity, and therefore, an extremely complex region to address. Given that some relatively frequent pathologies are found in this region, every neurosurgeon must have a vast anatomical knowledge of it.

Advances in the field of this region’s surgical anatomy go hand in hand with advances in surgical techniques. It reflects an obstacle that two of the most brilliant neurosurgeons of the last century, Harvey Cushing and Walter Dandy, had to face.

In this chapter, we explain the surgical anatomy of this region by dividing it into two parts: continent and content. In the continent section, we explain in detail the constitution of each limit of the CPA. In the content section, we synthesize and exemplify the neurovascular complexes described some years ago by Albert Rhoton.

Keywords: cerebellopontine angle, anatomy, microscope, rule of three, neurovascular complex

33.1 A Brief Historical Review

In 1902, Henneberg and Koch introduced the term cerebellopontine angle (CPA) or “Kleinhirnbrückenwinkel.”1 The first successful intervention of the CPA is attributed to Charles Ballance in 1894.²

In 1913, three outstanding neurosurgeons from Europe (Horsley from London, von Eiselsberg from Vienna, and Krause from Berlin) met to share their surgical experience on CPA. The reported mortality rates were between 67 and 84%.¹ The lack of available tools explained this high mortality. The technique was similar, a unilateral suboccipital craniectomy, but they used the finger or a spatula to remove the tumor, and the CPA was later packaged with gauze.¹

Harvey Cushing crafted the term “bloody angle” due to the hemorrhagic complications that happened in this type of surgeries.¹ Cushing considered, among other factors, that the insufficient exposure, the complete resection, and the digital or spatula technique in the vestibular schwannomas were the cause of the high mortality rate. In 1917, he published “Tumors of the Nervus Acusticus and the Syndrome of the CPA,” where he reported a mortality rate of 20%. In a later study in 1936, the mortality rate improved up to 4%. These publications marked a milestone in CPA surgery, demonstrating a very significant reduction in mortality rates compared to its predecessors. Cushing accomplished this by: (1) increasing the exposure, improving the vision of the neurosurgeon and allowing greater decompression and manipulation of the cerebellum; he abandoned unilateral suboccipital craniectomy and used a bilateral suboccipital craniectomy; (2) routine use of ventricular drainage; (3) implementing more effective hemostasis techniques; and (4) changing the purpose of the surgery from total resection to intracapsular debulking and brainstem decompression. As a consequence of this new approach, despite the good early postoperative results, the tumor recurrence rate increased, and therefore the 5-year mortality rate was still as high as 54%.¹

Dandy and Cushing’s disciple developed a new resection technique that included intracapsular debulking first and a complete extracapsular resection second. Also resumed the original technique of unilateral suboccipital craniectomy initially described by Horsley. This variation of the technique triggered a famous disciple-master public conflict over the correct approach for the CPA tumors.²

During the second half of the 20th century, we witnessed great advances: the use of the microscope by Kurze and the introduction of specific monitoring techniques for the cranial nerve (CN) VII by Delgado in 1979.¹ It is worth mentioning the detailed, systematic, and didactic study of the anatomy of the CPA by Albert Rhoton.³

33.2 A Brief Introduction to the Anatomy of the CPA: A Detailed Look⁴ ^, ⁵

From a strictly anatomical point of view, the CPA is an infratentorial region delimited by the upper and lower limbs of the cerebellum-pontine fissure ( Fig. 33.1a).

Fig. 33.1Anatomy of the cerebellopontine angle (CPA) in a cadaveric specimen. (a) The limit of the CPA region. Anterior view of the brainstem and the upper and lower limbs of the cerebellum fissure. (b) Anterior view of the neurovascular components of the CPA region. (c) Posterior view of the neurovascular components of the CPA. (d) Lateral view of the left CPA where a neurovascular conflict between SCA and CN V is seen. AICA, anterior inferior cerebellar artery; BA, basilar artery; Chor. Plex, choroid plexus; CN, cranial nerve; F, foramen; Flocc, flocculus; Inf, inferior; CPF, cerebellopontine fissure; PICA, posterior inferior cerebellar artery; SCA, superior cerebellar artery; Sup, superior; VA, vertebral artery.

Professor Rhoton described the components of the CPA with his famous “rule of three”:

●Three cerebellar surfaces. The cerebellum has three surfaces which are named after the adjacent structures with which they have contact: tentorial, petrous, and suboccipital ( Fig. 33.1b).

●Three parts of the brainstem with three cerebellar peduncles. From an anatomical point of view, we can divide the brainstem into three parts: midbrain, pons, and medulla oblongata; each of these portions communicates with the cerebellum through three cerebellar peduncles: upper, middle, and lower ( Fig. 33.1a).

●Three fissures. The three fissures are formed by the embryological retraction of the cerebellum around the brainstem; from superior to inferior we find: the cerebellomesencephalic, the cerebellopontine, and the cerebellomedullary fissures ( Fig. 33.1a).

●Three arteries. There are three cerebellar arteries: the superior cerebellar artery (SCA), the anterior superior cerebellar artery (AICA), and the posterior inferior cerebellar artery (PICA) ( Fig. 33.1c).

●Three groups of veins. From a practical point of view, the venous system is divided into three groups: galenic, petrosal, and tentorial ( Fig. 33.1d).

33.3 Surgical Anatomy of the CPA Region

To our knowledge, the best method to understand the surgical anatomy of the CPA is systematizing its study into continent and content.

33.3.1 Continent: Walls of the CPA 4 ^, ⁵ ^, ⁶ ^, ⁷ ^, ⁸ ^, ⁹ ^, ¹⁰ ^, ¹¹ ^, ¹² ^, ¹³ ^, ¹⁴

The CPA is located between the petrous portion of the temporal bone (anterolateral wall) and the petrous portion of the cerebellum and the brainstem (posteromedial wall). The floor is formed by the inner surface of the occipital bone and communicates with the spine through the foramen magnum. The roof is formed by the lower portion of the tentorium cerebelli and communicates with the supratentorial space through the tentorial incisure.

Posteromedial Wall of the CPA

The posteromedial surface of the CPA corresponds to the anterolateral or petrosal surface of the cerebellum and the brainstem.

The petrosal surface of the cerebellum is divided into two parts by the cerebellopontine fissure: superior and inferior. The superior limb of the cerebellopontine fissure reaches the level of the CN V outlet. The inferior limb of the cerebellopontine fissure reaches the lateral recess of the fourth ventricle, where it communicates with the CPA through the foramen of Luschka and where the choroidal plexuses protrude below the flocculus ( Fig. 33.1a). There is also a series of indentations between the midbrain, the pons, and the medulla. The pontomesencephalic sulcus is a depression located between the mesencephalon and the pons. On the other hand, the pons is separated from the medulla by the pontomedullary sulcus. The pons continues on each side with the middle cerebellar peduncles, and the posterior root of the CN V emerges from the upper portion of the middle cerebellar peduncle just below the anterior angle of the cerebellum. The rootlets of the CNs VI, VII, and VIII arise from the pontomedullary sulcus: the CN VI near the midline and the CNs VII and VIII more laterally in the supraolivary fossette. Below is the anterolateral surface of the medulla oblongata, where the following structures are observed from medial to lateral: anteriomedian sulcus; pyramid; anterolateral (preolivary) sulcus (origin of CN XII), inferior olive, posterolateral (postolivary) sulcus (origin of the CNs IX, X, and XI), and the cuneate fasciculus ( Fig. 33.1a).

Anterolateral Wall of the CPA

The anterolateral surface of the CPA is the medial aspect of the petrous and mastoid portion of the temporal bone covered by the dura, which contains the internal auditory canal (IAC) and the jugular foramen (JF). The most crucial surgical component of this portion is the IAC, where the CNs VII and VIII pass ( Fig. 33.2a, b).