Vestibular schwannoma (acoustic neuroma)
The term "acoustic neuroma" is considered anatomically inaccurate because the tumor originates from Schwann cells of the vestibular portion of the vestibulocochlear nerve (cranial nerve VIII). In the early stages, it slowly destroys the vestibular fibers; however, pronounced systemic vertigo usually does not occur because the central nervous system has time to compensate for the deficit. Hidden asymmetry of labyrinth function at this stage can be detected using the caloric test.
As the mass grows, it compresses the cochlear nerve fibers. This leads to clinically significant hearing loss, with high-frequency perception affected first. Notably, no direct correlation has been established between tumor size and the severity of hearing loss.
Adjacent structure compression syndrome
Further expansive growth of the tumor in the cerebellopontine angle leads to involvement of neighboring cranial nerves and brain structures:
- Cranial nerves V and VII: compression of the trigeminal and facial nerves causes characteristic symptoms. For example, dysfunction of the chorda tympani (chorda tympani) leads to impaired lacrimation and taste sensitivity.
- Brainstem and cerebellum: involved in later stages, which is accompanied by severe neurological deficit.
To confirm the diagnosis, MRI (the primary neuroimaging method), audiometry, and auditory brainstem response (ABR) testing—which records prolonged impulse conduction time along the nerve—are used. Stereotactic radiosurgery (Gamma Knife, linear accelerator), which is comparable in efficacy to open surgery, is increasingly used as a treatment.
Vestibular syndrome and nystagmus
Unilateral damage to the vestibular nerve creates asymmetry in vestibular nuclear activity. The central nervous system perceives this as movement toward the healthy side (where activity is higher), provoking the vestibulo-ocular reflex and nystagmus.
- The fast component of nystagmus is directed toward the healthy side.
- The slow component is directed toward the affected side.
The patient tends to keep their eyes closed and turns their head so that the affected ear is higher (avoiding irritation). Nausea, a tendency to fall toward the lesion, and oscillopsia—an illusion of environmental oscillation—are observed. Nystagmus is easier to detect using Frenzel goggles, which eliminate visual fixation.
Differential diagnosis of vertigo
Cerebellopontine angle nerve lesions must be differentiated from other vestibular pathologies:
- Vestibular neuritis: acute viral nerve damage. Accompanied by rotational vertigo lasting several days, nausea, and nystagmus, but hearing is not impaired. There are no symptoms of other cranial nerve or brainstem involvement.
- BPPV: episodes of systemic vertigo (up to 60 seconds) triggered by rapid head movements. The cause is canalithiasis (detachment of otoliths and their entry, most often, into the posterior semicircular canal). Movement of the crystals causes endolymph displacement toward the cupula ("piston effect"). Nystagmus in BPPV is subject to habituation.
- Central positional vertigo: occurs with lesions of the floor of the fourth ventricle (or the cerebellar nodulus). Unlike BPPV, nystagmus here does not depend on the speed of posture change, can change direction, and persists for a long time; a dissociation between nystagmus and the sensation of vertigo is observed.
Adjacent structures: the vagal system
Close to the cerebellopontine angle, a group of mixed nerves emerges from the cranial cavity through the jugular foramen: the glossopharyngeal (IX), vagus (X), and the cranial part of the accessory (XI) nerve. They form a unified "vagal system" with common nuclei in the brainstem:
- Nucleus ambiguus (nucleus ambiguus): gives off motor fibers to the constrictor muscles of the larynx and pharynx.
- Nucleus of the solitary tract (nucleus tractus solitarii): receives afferent impulses of taste sensitivity and signals from the carotid body.
Sensory fibers from this area travel as part of the medial lemniscus to the thalamus.
Briefly: answers to topical questions
Which branches does the facial nerve give off within the facial canal of the temporal bone before exiting through the stylomastoid foramen?
Within the facial canal of the temporal bone, before exiting through the stylomastoid foramen, three main branches arise from the facial nerve.
- Greater petrosal nerve (n. petrosus major) — provides parasympathetic innervation to the lacrimal gland and nasal cavity glands.
- Stapedius nerve (n. stapedius) — innervates the stapedius muscle (m. stapedius) to regulate tympanic membrane tension.
- Chorda tympani (chorda tympani) — provides taste sensitivity to the anterior 2/3 of the tongue and parasympathetic innervation to the salivary glands.
Which cranial nerve nuclei project onto the floor of the rhomboid fossa?
Nuclei of cranial nerves V through XII project onto the floor of the rhomboid fossa.
- Nuclei of cranial nerves IX, X, XI, XII project onto the upper triangle of the rhomboid fossa.
- Nuclei of cranial nerves V, VI, VII, VIII project onto the lower triangle of the rhomboid fossa.
- Hypoglossal nucleus forms the hypoglossal trigone.
- Dorsal nucleus of vagus nerve forms the vagal trigone.
- Vestibular and cochlear nuclei form the vestibular area.
Motor nuclei lie medially, sensory nuclei lie laterally, and autonomic parasympathetic nuclei occupy an intermediate position.
Class and exam questions
Why is vertigo rare in the slow growth of vestibular schwannoma?
The slow development of the deficit during the destruction of vestibular fibers has time to be compensated at higher levels of vestibular information processing in the central nervous system.
In which direction is the fast component of vestibular nystagmus directed?
The fast component always beats toward the healthy ear, while the slow component is directed toward the affected side.
How to differentiate BPPV from central positional vertigo?
In central lesions, nystagmus does not depend on the speed of body position changes, persists for a long time, may change direction, and there is often a dissociation between the severity of nystagmus and the vertigo itself.
What else to study on this topic
- Mechanisms of auto-induction of vestibular vertigo (head-pitch and rotation test)
- Pathogenesis of benign paroxysmal positional vertigo (BPPV)
- Anatomy and topography of the glossopharyngeal nerve
- Brainstem nuclei and their functional role
- Vestibular neuritis: clinical presentation and prognosis
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