Anatomy of the Vestibulocochlear Nerve (8th Cranial Nerve)
The 8th cranial nerve, also known as the vestibulocochlear nerve, is integral to our senses of hearing and balance. This nerve is bifurcated into two distinct parts: the cochlear nerve, which transmits sound signals, and the vestibular nerve, which manages balance. The cochlear nerve originates from the hair cells in the cochlea and extends to the cochlear nuclei in the brainstem. Conversely, the vestibular nerve fibers arise from sensory neurons in Scarpa’s ganglion and project to the vestibular nuclei in the brainstem.
Epidemiology of Acoustic Neuroma
Acoustic neuroma, or vestibular schwannoma, is a benign tumor affecting approximately 1.2 per 100,000 individuals annually in the United States. It arises from the Schwann cells that form the myelin sheath of the vestibular nerve. This tumor is characterized by its slow growth rate and the potential to remain undetected within the internal auditory canal for years.
Pathophysiology of Acoustic Neuroma
Acoustic neuromas typically develop from the vestibular portion of the 8th cranial nerve, not the cochlear part, which is why the term “vestibular schwannoma” is medically more accurate than “acoustic neuroma.” As they expand, they may extend to the cerebellopontine angle—a critical area adjacent to the brainstem—potentially leading to significant neurological complications if unchecked. Large tumors in this region are referred to as cerebellopontine angle tumors.
The tumor often develops slowly and can remain in the bony ear canal for many years. It is referred to as intra-meatal if it is (still) entirely inside its place.
Rate of Growth
Acoustic neuromas typically develop at a pace of 2 mm annually, while they can occasionally grow more quickly and frequently cease growing altogether. In fact, after diagnosis, up to 60% of acoustic neuromas do not develop at all.
Clinical Manifestations
Early Signs
Loss of hearing
A loss of hearing on one side is the initial and most prevalent sign. The majority of the time, hearing loss happens gradually and softly. People who are impacted frequently become aware of the hearing issue very late or by coincidence, such as when making a phone call or at a standard checkup. Above all, high-frequency hearing problems are apparent; one finds that the sound of the birds has altered or is absent all of a sudden. A hearing loss of some kind affects 90% of those who have an acoustic neuroma.
Balance/Vertigo
Vertigo and poor balance are only the third most common symptoms of an acoustic neuroma, even though these tumors mostly arise from the upper section of the balance nerve. They manifest as vertigo, wavering dizziness, and shaky gait. Acoustic neuroma patients frequently deny ever having such a hazy sense of instability, most commonly in the dark and with abrupt head and body movements, until they are specifically asked.
The capacity to maintain balance may decline when the vestibular section of the nerve is squeezed, but because this normally happens gradually, the brain adjusts and makes up for the shift. Because of this, many people seldom even realize when their equilibrium changes. Alternatively, the patient may show ataxia-related symptoms.
Another precursor might be facial paresthesia.
Except for those who have neurofibromatosis, the symptoms often appear beyond the age of 30 due to the tumor’s gradual growth.
Tinnitus
Tinnitus, sometimes known as “ringing in the ears,” is a frequent companion to deteriorating hearing abilities. Tinnitus is the perception of hearing ringing, buzzing, hissing, chirping, whistling, or other noises. The level of the noise might fluctuate and it can be either intermittent or constant. Without hearing loss or any other symptoms at all, tinnitus may even be the first sign of an illness. Tinnitus is mostly prevalent in the high-frequency range, similar to hearing loss. during Acoustic Neuroma instances.
Later Signs
Other cranial nerves and arteries that nourish the brain and enter the brain through the apertures in the skull base may become inoperable if an acoustic neuroma develops towards the base of the skull.
A facial motor failure results from a problem with the 7th cranial nerve (the facial nerve), which controls, among other things, the facial muscles. The production of tear fluid (causing Dry Eye) and secretions from the nose and mouth are commonly impacted, and facial paralysis, also known as facial palsy, may result. Two-thirds of the tongue’s ability to taste may eventually be affected as well.
Trigeminal neuralgia results from damage to the Trigeminal Nerve, the 5th cranial nerve, which affects sensation. Because this cranial nerve travels farther from the cerebellopontine angle, these symptoms are less prevalent.
The 9th cranial nerve (Glossopharyngeal Nerve) and 10th cranial nerve are comparable in this regard (Vagal Nerve). These nerves can get damaged, which can result in painful swallowing, taste impairments in the back portion of the tongue, and other issues.
Diagnostic Approaches
- MRI: The gold standard for diagnosing acoustic neuroma, offering detailed images of the cerebellopontine angle and internal auditory canal.
- CT Scans: Used less frequently, typically in scenarios where MRI is contraindicated.
- Audiometry: Essential for assessing the type and severity of hearing loss associated with these tumors.
Management
Since acoustic neuromas are benign and often develop extremely slowly, it is frequently chosen to observe growth and refrain from taking any additional action, at least initially. A conservative strategy with observation, including repeated MRI tests, may be the sole course of treatment recommended for patients with extremely tiny, asymptomatic tumors, elderly patients, and patients with other critical medical conditions. However, therapeutic interventions may be necessary as the tumor grows or symptoms worsen. Treatment options include:
- Microsurgery: To remove or debulk the tumor.
- Radiotherapy: Techniques like Gamma Knife, Cyberknife, or proton beam therapy help control tumor growth non-invasively.
Physical therapy
Following surgery, it has been shown that individualized vestibular therapy integrating adaptation, habituation, balance, and mobility improves balance.
To enhance results in terms of the facial range of motion, symmetry, and function, physiotherapy is advised in cases where the patient has sustained facial palsy following surgical excision of the acoustic neuroma.
