Acoustic neuroma vs vestibular schwannoma

Acoustic neuroma vs vestibular schwannoma

Acoustic neuroma vs Vestibular schwannoma

Acoustic neuroma and vestibular neuroma are terms used interchangeably but, in reality, imply the same condition. Although acoustic neuroma was traditionally more commonly used, it is a misnomer. The correct term should be cochleovestibular schwannoma, but because the tumour most commonly arises from the vestibular portion of the nerve, vestibular schwannoma is the closest and most commonly used nowadays.

The first description of an acoustic neuroma

In 1977, Professor Eduard Sandifort, a pathologist from Leiden, described the first acoustic neuroma. He described a “hard body” connected to the hearing nerve as” De duro quodam corpusculo, nervo auditorio adherente” After the appearance of more case reports in the early 19th century, the term “acoustic neuroma” soon became commonly used (1).

Prof Eduard Sandifort. Credit: Wikipedia
Prof Eduard Sandifort. Credit: Wikipedia

The cochleovestibular nerve

The cochleovestibular or eighth cranial nerve arises from the pons in the brainstem. It soon divides into two divisions: the cochlear decision for hearing and the vestibular division for vestibular function. The vestibular divison devides again in superior (upper) and inferior (lower) vestibular nerve.

Only 25% of tumours originate from the cochlear division. It is unknown which of the two vestibular branches is more commonly affected, so it is safe to assume that 50% originate from the superior nerve and 50% from the inferior vestibular nerve (2).

The nerve sheath

Neuroglia, or glial cells, serve several functions in the nervous system. They provide support and structure for neurons, help regulate the environment around neurons, and play a role in immune responses. They occur on the nerve near the brainstem.

Schwann cells are neuroglia found explicitly in the peripheral nervous system. They occur more laterally on the nerve. Their primary function is to produce the myelin sheath, a fatty substance that surrounds and insulates axons, speeding up the transmission of nerve impulses. Schwann cells also aid in nerve regeneration and repair after injury.

Schwann cell ( Modified from PB Pressbooks)

The transition zone

The glial-Schwann sheath junction, also called the Obersteiner-Redlich zone, refers to the area on the nerve where the glial cells and Schwann cells meet. The location of the transition zone can vary from patient to patient. It is commonly believed that this zone gives rise to the tumours, although it is not scientifically proven. More tumours arise laterally in the canal.

Schwann cells and schwannoma

Schwannomas are tumours that arise from Schwann cells. Until the development of the electron microscope in the 1960s it was not clear from which part of the nerve the tumours originated. We know now that the Schwann cell is responsible for these tumours.

Compared to the other cranial nerves, schwannomas most often affect the cochleovestibular nerve. More than 90% of cranial nerve schwannoma originate from the cochleovestibular nerve. The vestibular division of the cochleovestibular nerve has a higher density of Schwann cells than the cochlear division. In one study, the ratio was 3.2 : 1(2).

Schwannoma develop from schwann cells. (Modified from PB Pressbooks).

Vestibular schwannoma

Until it can be discriminated at an early stage, from which  division of the nerve the tumour originates, “ vestibular schwannoma” is the more accurate term. We know that the tumour is more likely to originate from the vestibular nerve and most likely to consist of Schwann cells. The preferred term therefore is vestibular schwannoma.