|If you have read any of the literature from the medical sites,
you saw that they discuss the cranial nerves that a tumor affects. Not being intimately
familiar with cranial anatomy, it was difficult for me to visualize what the texts and
physicians were talking about until I found this diagram (and the others of
CSF circulation and neck musculature) in the Anatomy Coloring
Book (c)1993 Kapit and Elson The text (and
the small diagram of the facial nerves) is from
A Glimpse of the Brain (c) 2001 Acoustic Neuroma Association This shows some of the complexity of the brainstem, and why the pressure from a tumor affects what it does.
|Historically, these cranial nerves are designated with Roman
numerals. Cranial nerves I and II are derived from the forebrain. All
others are brainstem derived.
ANATOMIC BASIS FOR ACOUSTIC NEUROMA SYMPTOMS
and inner ear structures are located within a bone called the petrous pyramid, part of the
base of the skull, that extends from the ear toward the center of the head. An opening in
the posterior surface of this bone, called the internal auditory meatus, leads to the
internal auditory canal, which contains the auditory, vestibular (balance), and facial
The acoustic neuroma starts near this opening and grows into the canal, which may cause it to enlarge from pressure. The tumor also may extend into the cranial cavity.
The tumor originates in the Schwann cells which form the (electrical insulation) sheath around the vestibular part of the eighth cranial nerve. (Hence the technically correct name for the tumor of Vestibular Schwannoma) As the tumor grows, it interferes with the function of the vestibular and cochlear (hearing) nerves. When the tumor becomes larger, it starts to compress the brain stem and fifth cranial nerve. A very large tumor may involve cranial nerves IV to X and markedly compress the brain stem and cerebellum.
Cranial nerves I, II, III, and XII are usually not involved with the tumor. The eleventh cranial nerve may rarely be involved with large tumors. Functions of the cerebral hemispheres are affected only in the rare circumstances when hydrocephalus (enlargement of the ventricles containing the cerebrospinal fluid) occurs due to obstruction of the normal circulation of the fluid.