DIFFERENTIAL DIAGNOSIS OF PERIPHERAL VESTIBULAR DISORDERS

Differential Diagnosis of Peripheral Vestibular Disorders

PART 9: Acoustic Neuroma

Acoustic neuroma (AN) is a misnomer, since it neither arises from the auditory division of the VIII^th cranial nerve nor is it a neuroma. The more precise term, but less commonly used, is vestibular schwannoma. This lesion represents 80-90% of all cerebellopontine angle tumors. It is a benign neoplasm arising from the schwan cells of the vestibular portion of the VIII^th cranial nerve. Interestingly, vertigo is not a common complaint with AN, occurring in less than one fifth of patients – mostly very small AN. Disequilibrium, or the chronic, more subtle feeling of imbalance, is a more frequent complaint. As opposed to vertigo, which is more common with smaller AN, disequilibrium becomes more common with increasing size of tumor. Nearly three fourths of patients with large (> 3 cm) AN will complain of disequilibrium.

Clinical Presentation

There is a tremendous amount of variability in the clinical presentation of AN and AN has been known mimic to several other auditory and vestibular syndromes. Consequently, AN remains in the differential diagnosis of anyone with unilateral symptoms relating to auditory dysfunction (hearing loss, tinnitus) or vestibular dysfunction (vertigo, disequilibrium). Additionally, many AN may be completely asymptomatic. However, the "classic" clinical presentation of a small AN is initially unilateral tinnitus and mild unilateral hearing loss. As the tumor grows the patient will note progressive hearing loss in the involved ear and may experience vertigo. The vertigo, however, is usually short lived and eventually resolves or is replaced by a much milder disequilibrium.

As the tumor becomes large, the trigeminal nerve may become compressed with variable symptoms – numbness, facial pain, and trigeminal neuralgia. As the brainstem becomes compressed hydrocephalus ensues with associated headaches and visual loss. If untreated, persistent growth results in lower cranial nerve dysfunction, ataxia, hemiparesis and respiratory arrest.

Symptoms:

Early: hearing loss, tinnitus, disequilibrium

Late: early signs, headache, trigeminal symptoms, hydrocephalus

Pathophysiology

AN have a variable growth rate with four distinct stages – internal auditory canal (IAC), cisternal, brainstem compressive, hydrocephalic. The tumor is felt to arise from the junction of central and peripheral myelin in the vestibular portion of the VIII^th cranial nerve. This is in the internal auditory canal. Growth of the tumor progresses from the IAC to the cerebellopontine angle cistern and attains the appearance of an ice cream cone. During the IAC and cisternal stages of growth, there is progressive compression of the vestibular and cochlear nerves. This correlates with progressive hearing loss, tinnitus, and disequilibrium.

The facial nerve, although quite compressed during these stages, very rarely demonstrates dysfunction. Continued growth is in the direction of the cerebellopontine angle cistern and results in compression of the brainstem and may also compress the lower cranial nerves. Further compression of the brainstem eventually results in collapse of the fourth ventricle and development of hydrocephalus. The terminal stage ends with herniation of the cerebellar tonsils.

Stages

Internal Auditory Canal
Cisternal
Brainstem Compressive
Hydrocephalic

Diagnosis

As mentioned, AN have a quite variable presentation and should be in the differential diagnosis of any unilateral process of vestibular or cochlear nerve dysfunction. Although there are many tests advocated as screening tools for AN, the gold standard for diagnosis is a thin-section MRI of the internal auditory canals with gadolinium contrast. The AN enhances markedly with gadolinium on a T1 weighted image. Other studies used for screening for AN, but less sensitive than MRI, include CT cisternography, ABR, ENG and retrocochlear audiometric studies. Other tumors in this region that may produce similar symptoms, but are less common include meningiomas, epidermoids, arachnoid cysts, schwannomas of the other lower cranial nerves, and lipomas.

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