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Cerebellopontine angle (CPA) tumors account for 5 to 10 percent of all intracranial tumors. Because 85 to 92 percent of CPA tumors are acoustic schwannomas (vestibular schwannomas) the main focus will be on acoustic tumors. These tumors are usually slow growing and may initially present with only subtle clinical symptoms. Early diagnosis may limit surgical morbidity and facilitate facial nerve and hearing preservation. The clinician must, therefore, maintain a high index of suspicion and use appropriate audiologic, vestibular and radiographic studies to diagnose these tumors. Although radiographic imaging of CPA tumors is critical for accurate diagnosis and surgical planning. the initial evaluation should employ a thorough clinical examination and specific audiologic and vestibular testing.

Historical Perspective

The first report of an acoustic tumor recognized at autopsy was made as early as 1777 by Sandifort and the first successful operation for acoustic neuroma was performed by Sir Charles Ballance in 1894. In spite of these accomplishments, the characteristic symptoms of these tumors were not appreciated until 1917 when Harvey Cushing published his monograph. Tumors of the Nervus Acusticus and the Syndrome of the Cerebellopontine Angle.  In this treatise, Cushing described in detail the clinical history, operative reports and the postoperative results of 42 patients with large CPA tumors, the majority of which (71 percent) were acoustic schwannomas. Cushing himself did not realize that the first symptoms were hearing loss and tinnitus until after he had reviewed his own case material. The natural progression of symptoms in this series of patients with acoustic schwannomas was as follows: (1) auditory and labyrinthine dysfunction. (2) occipitofrontal pain, (3) cerebellar ataxia. (4) involvement of the adjacent cranial nerves, (5) evidence of increased intracranial pressure. (6) dysphagia and dysarthria, and (7) brain stem compression leading to depressed respiratory drive and death.

The late diagnosis of these tumors at that time was partly due to the limitations of diagnostic testing. Cushing was frustrated by the difficulty of establishing complete deafness or loss of vestibular function. Although clinical use of the tuning fork was described by Weber almost 100 years earlier. accurate audiologic testing was not available at the time of Cushing's monograph. Even though vestibular testing was also in its infancy, Cushing was able to detect vestibular dysfunction in 83 percent of his patients with caloric nystagmography. However, he could not distinguish between vestibular and cerebellar findings because his patients presented in such advanced stages. Cushing was also disappointed with the use of early imaging techniques. The use of roentgen imaging of the porus acusticus was introduced by Henschen in 1910, but the technique was not modified to fully evaluate the internal auditory canal (lAC) until 1917 (by Stenvers). The available radiographic techniques were able to demonstrate enlargement of porus acusticus in only 37 percent of the patients studied in Cushing's series.

Not only was his monograph a milestone in the recognition of the early symptoms of acoustic schwannomas, but Cushing's operative technique lowered the surgical mortality rate from 80 to 20 percent. His work, along with that of Walter Dandy, stimulated an interest in the treatment and diagnosis of CPA tumors. Dandy was able to further reduce the operative mortality and was a strong advocate of complete surgical resection of CPA tumors. Because of the expanding role of surgery in the treatment of these tumors, several large surgical series were reported in the 1950s and 1960s.

The management of acoustic schwannomas was further advanced by the work of William House and William Hitselberger; House published the results from over 200 cases of acoustic neuroma in 1968. House and his colleagues were able to diagnose tumors earlier because of the more precise methods of audiometric testing and x-ray imaging that had evolved since the time of Cushing and Dandy. Jerger had developed a method of audiologic testing based on von Bekesy's work which, along with the short-increment sensitivity index and tone decay test, allowed for the distinction between cochlear and retrocochlear hearing loss. Complete imaging of the temporal bone was available using plain x-ray filming with the Stenvers, Caldwell and Chamberlain-Towne views. Finally, precise preoperative evaluation of tumors was possible with iophendylate Pantopaque ) myelography.

Current technology has replaced many of the diagnostic tests employed by House and his colleagues. Routine audiologic testing and electronystagmography are still the mainstay of the evaluation of patients with unilateral hearing loss, unilateral tinnitus and vertigo. The more time-consuming audiometric tests, however, have given way to auditory brain stem response (ABR) testing. Like­wise, temporal bone polytomography has been replaced with computed tomography (CT ), CT air cisternography and gadolinium-enhanced magnetic resonance imaging (Gd­MRI ). Still, the early diagnosis of CPA tumors hinges on the clinician having a high index of suspicion and using an appropriate "cost-effective" diagnostic strategy to confirm his or her clinical impression.

Symptoms and Signs

Even with improvements in diagnostic testing in the late 1980s there was still, on average, a -1-year delay between the first symptoms of an acoustic neuroma and its subsequent diagnosis. Consequently, it is essential that the clinician be familiar with both the early and late symptoms and signs of acoustic schwannomas. The specific symptoms and signs of acoustic schwannomas include the following: (1) unilateral hearing loss, (2) unilateral nonpulsatile tinnitus, (3) vestibular dysfunction, (4) trigeminal hypoesthesia, (5) cerebellar dysfunction, (6) headache, (7) facial nerve dysfunction, (8) increased intracranial pressure, (9) lower cranial nerve palsy and (10) long-tract signs.

Unilateral Hearing Loss

The cardinal symptom of patients with acoustic schwannomas is a unilateral sensorineural hearing loss (SNHL) that develops over months and is associated with poor speech discrimination that is out of proportion to the acuity measured by pure-tone audiometer. Hearing loss is the initial symptom in 67 percent of patients. More than 95 percent of patients with acoustic schwannomas will manifest some hearing loss. although many patients may not be aware of it or may not mention this symptom to their physicians. The frequency of hearing loss as a complaint varies from 77 to 95 percent, depending on the size of the tumor.

Sudden SNHL is an uncommon presentation for an acoustic neuroma. A very small percentage of patients with sudden SNHL will have an acoustic neuroma. The exact incidence of acoustic neuroma in this population is not known. Shaia and Sheehy reported the incidence of acoustic neuroma to be less than 1 percent in 1220 patients with sudden SNHL. On the other hand, over 15 percent of patients with acoustic schwannomas may present with sudden hearing loss. Moreover, a detailed history may reveal that many more patients (over 25 percent) experience a sudden decrease in their hearing sometime during the course of their illness.

The mechanism of hearing loss from acoustic schwannomas probably involves either direct compression of the auditory nerve or interruption of the blood supply to the auditory nerve or cochlea. Direct compression of the nerve is related to tumor growth and should result in gradual hearing loss. Within the narrow confines of the lAC, the cochlear nerve may easily be compressed, producing a hearing loss prior to substantial tumor growth. Tumors arising outside the lAC, however, may demonstrate substantial growth before producing auditory symptoms. In tumors originating in the CPA the nerve is usually stretched over the tumor surface. Animal studies have shown that even brief retraction or compression of the cochleovestibular nerve leads to changes in the auditory threshold.

An interruption of the blood supply to the nerve or cochlea could theoretically cause either gradual or acute SNHL. Occlusion of the internal auditory (labyrinthine) artery, a branch of the anterior inferior cerebellar artery, has been proposed as a mechanism of sudden SNHL; however, this theory is controversial. Although the labyrinthine artery supplies both the cochlea and the vestibular organs, vertigo is present in only 57 percent of these patients. Furthermore, because of the spiral anatomy of this vessel, cochlear ischemia should have its greatest effect on the apical turn, producing a low-frequency or flat hearing loss. The audiometric pattern in this group of patients, however, is variable and low-frequency recovery has been documented.

Other theoretical, but unlikely, causes of hearing loss from acoustic schwannomas are the disruption of cochlear efferents within the vestibular portion of the nerve and an alteration of inner ear biochemistry by the tumor. Direct cochlear invasion by tumor is extremely rare.

Unilateral Nonpulsatile Tinnitus

Unilateral tinnitus is a common initial symptom, but is rarely the reason that a patient seeks medical attention. This is probably because the onset of tinnitus is accompanied by a more debilitating hearing loss. The characteristic description of tinnitus in acoustic neuroma is high-pitched, continuous and ipsilateral to the side of the lesion. The incidence of tinnitus is slightly lower than that of hearing loss with 53 to 70 percent of patients reporting this symptom at the time of presentation. Acoustic neuroma patients rarely complain of tinnitus occurring without hearing loss. The etiology of tinnitus in patients with acoustic neuroma is thought to be the same as that for hearing loss.

Vestibular Dysfunction

True vertigo is the sensation of movement or rotation of oneself or one's surroundings. In acoustic neuroma patients, the sensation of vertigo is usually abrupt in onset and occurs early in the clinical course. The symptom may last days to weeks and then resolve. often giving way to a more general sensation of unsteadiness or disequilibrium. Several studies of acoustic neuroma patients have found the incidence of vertigo to be 18 to 22 percent. Only 8 percent of patients reported by House had vertigo as an initial symptom. In other study, the incidence of vertigo was found to be much higher in smaller tumors, ranging from 27 percent of patients with small < 1 cm tumors to only 10 percent of patients with tumors greater than 3 cm. Conversely, disequilibrium was present in -18 percent of patients overall and was more common in patients with larger tumors. Patients with small <1cm tumors were found to have a 37 percent incidence of disequilibrium compared to a 71 percent incidence for tumors greater than 3 cm. Edwards and Paterson found that nystagmus was present in 92 percent of their patients, but such nystagmus may be due to either vestibular or cerebellar dysfunction.

The mechanisms of vestibular dysfunction from an acoustic neuroma are thought to be similar to those for hearing loss, namely, neural compression and vascular occlusion. The fact that more patients don't complain of vertigo is probably due to the slow growth of these tumors causing a gradual loss of peripheral vestibular input with compensation by the central vestibular nuclei. Sudden increases in tumor size or a vascular event may account for the onset of true vertigo. Disequilibrium occurs when the integration of peripheral vestibular, visual, proprioceptive and tactile information within the cerebellum is impaired. This may be due to the distortion of normal vestibular input from the affected vestibular nerve or from the direct compression of a large acoustic neuroma on the brain stem or cerebellum.

Trigeminal Nerve Symptoms

Trigeminal dysfunction in patients with acoustic neuroma is usually described as pareasthesia or hypaesthesia of the ipsilateral face, with the midface being the most frequent region affected. In early studies, the incidence of trigeminal symptoms was reported to be quite high (70 percent), with the an even higher incidence of trigeminal findings on direct testing (88 percent). In recent studies 28 percent of 300 patients had either facial or corneal hypaesthesia and a strong correlation existed between these findings and tumor size. Trigeminal findings in only in 14 percent of patients with tumors less than 2.5 cm, whereas tumors greater than 4.0 cm were associated with a 53 percent incidence of trigeminal signs. Similar results have been reported by Moffat et al. who found a 59 percent incidence of corneal hypaesthesia in patients with tumors greater than 2.5 cm. The correlation between trigeminal symptoms and tumor size is also supported by the fact that facial numbness is rarely an initial symptom of acoustic neuroma.

Facial pain is an unusual symptom of an acoustic neuroma. Erickson et al. reported in 1965 that 19 percent of the patients in their series complained of facial pain and that paroxysmal pain (tic douloureux) was present in 5.5 percent. Tic douloureux is even less frequent in more recent studies. This may be due to the relatively large tumor size in the earlier studies.

Trigeminal nerve symptoms are caused by the compression of the nerve between the superior aspect of the tumor and the tentorium. Paroxysmal pain may result from the displacement of a vascular loop onto the root entry zone. Motor fibers are generally resilient to the effects of compression and the trigeminal motor fibers are protected by the outer sensory fibers. Because of these relationships, masticatory dysfunction has not been reported in patients with acoustic tumors.

Cerebellar Dysfunction

The common cerebellar symptoms with acoustic neuroma are incoordination, ataxia and disequilibrium. Disequilibrium may be the result of either central (cerebellar) or peripheral (vestibular) dysfunction or a combination of the two. Cerebellar symptoms tend to occur late in the course of the disease, are of moderate intensity and are unrelenting. Ataxia and incoordination affect both the upper and lower extremities, but patients may be more apt to notice a disturbed gait than upper-extremity incoordination. In a review of 300 patients, Thomsen and Tos found an abnormal Romberg test, gait disturbance or dysdiadochokinesis in 45 percent of patients with acoustic schwannomas. These findings were present in 70 percent of patients with tumors greater than 4 cm, in 37 percent with tumors from 2.5 to 4.0 cm. and in only 22 percent of patients with tumors less than 2.5 cm. Of these signs, dysdiadochokinesis is the most specific test of cerebellar function in that it is not affected by peripheral vestibular dysfunction. Dysdiadochokinesis was noted in only 5.5 percent of patients with tumors less than 4.0 cm compared to 26 percent of patients with tumors greater than 4.0 cm. These differences are statistically significant (p < 0.00 I): therefore, specific tests of cerebellar function accurately reflect the size of the tumor.

Cerebellar compression by an acoustic neuroma generally occurs at the flocculus of the cerebellum, which along with the nodulus receives both primary and secondary vestibular input. These cerebellar structures are important for the control of posture. The flocculonodular syndrome is characterized by disequilibrium without successful central compensation that eventually leads to truncal ataxia. Nystagmus is usually present in these patients, but there is rarely individual limb tremor, incoordination or lateralization. Incoordination of the motor system may result from tumor compression of the lateral cerebellum or superior cerebellar peduncle, which functionally connects the cerebellum to the contralateral cerebral cortex. Compression at either of these sites is characterized by ipsilateral limb asynergy and intention tremor.

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The most common presenting symptom in acoustic neuroma patients with nonauditory complaints is headache, occurring in 24 percent. The headache associated with acoustic schwannomas may vary greatly in location, duration and intensity. The pain may occur early in the clinical course and progress in severity as the tumor grows. There is a predilection for the frontal and occipital regions, but the pain may be located in any region of the head. In their series of relatively large tumors. Edwards and Paterson found that 84 percent of patients had headaches at the time of presentation and 25 percent had headaches as an initial symptom. Selesnick and Jackler report a much lower incidence (19 percent) in their recent series. They also found a strong correlation between headache and tumor size. Patients with small intracanalicular tumors did not complain of headache, compared to 43 percent of patients with tumors greater than 4.0 cm.

The exact cause of headache in patients with acoustic schwannomas is not known. Because it generally precedes the onset of increased intracranial pressure, headache in this population is probably not due to hydrocephalus. Furthermore, acoustic neuroma patients with headache usually do not have other findings of increased intracranial pressure. One potential cause of headache is a localized irritation of the dura within the lAC or CPA, with a pain response mediated by dural branches of the trigeminal nerve.

Facial Nerve Dysfunction

In spite of its proximity within the lAC, facial nerve motor dysfunction is rare with small- to medium-size acoustic schwannomas. Dysfunction of the motor component of the facial nerve may present as a facial palsy or as hemifacial spasm. Edwards and Paterson reported an overall incidence of facial nerve symptoms of only 15 percent, but clinical facial weakness was found in 63 percent of those patients with relatively large tumors. In contrast, other reports reveal an overall incidence of facial palsy in approximately 10 percent of patients regardless of tumor size.

As an acoustic neuroma grows, the facial nerve is typically thinned and stretched over its anterior surface. Compression of the facial nerve against the porus acusticus and petrous bone may result in facial palsy: however, the motor fibers of the facial nerve are generally resilient to compression. Facial spasm may be caused by a displaced vascular loop against the root entry zone of the nerve. In addition to motor fibers, the facial nerve carries both sensory and autonomic fibers within the nervus intermedius. Hypaesthesia of the concha and external auditory canal, known as Hitselberger's sign, was originally reported in 95 percent of acoustic neuroma patients. In a subsequent study, however, Thomsen et al. reported an overall incidence of nervus intermedius involvement in only 48 percent of 300 patients with acoustic schwannomas. The most common finding in these patients was an abnormal naso­lacrimal reflex, which was present in 38 percent of patients: followed by altered taste in 29 percent and external meatus hypaesthesia in only 9 percent. Although the authors found a strong correlation between these findings and tumor size, the overall incidence was too low to aid in the diagnosis of these tumors.

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Increased Intracranial Pressure

Acoustic schwannomas that compress the brain stem may obstruct the fourth ventricle and lead to an elevation of intracranial pressure. Increased intracranial pressure may cause headache, nausea, vomiting, decreased visual acuity, diplopia, anosmia and obtundation. Early reports of acoustic schwannomas reported these advanced symptoms in 66 to 76 percent of cases. Fortunately, most tumors today are diagnosed prior to the onset of these grave findings. Radiographic evidence of hydrocephalus is found in less than 5 percent of patients with acoustic schwannomas. Clinical evidence of increased intracranial pressure may be slightly more common. Hamer and Laws noted that 9 percent of 76 patients with acoustic neuroma complained of diplopia and 12 percent had papilloedema. Diplopia in this situation is believed to be caused by the stretching of the abducens nerve.

Lower Cranial Nerve Palsy

Involvement of the ninth, tenth, eleventh or twelfth cranial nerve by an acoustic tumor may lead to the clinical symptoms of dysarthria, dysphagia, aspiration and hoarseness. Early reports of relatively large tumors documented these findings in approximately 30 percent of patients with glossopharyngeal and vagus nerve paralysis occurring most frequently. Compression of the lower cranial nerves results from extension of large acoustic schwannomas inferiorly toward the jugular foramen. Despite the proximity of the jugular foramen to the lAC, the clinical involvement of these nerves is quite rare in recent studies. This may be partly due to the resilience of motor fibers to gradual compression.

Long- Tract Signs

Involvement of the long tracts of the spinal cord is a late finding with acoustic neuroma. An initial sign of long-tract involvement may be hyperactivity of the ipsilateral deep tendon reflexes. Hyperreflexia was reported in 17 and 24 percent of patients in two early series, whereas extremity weakness was noted in only 5 and 9 percent, respectively, in these same studies. These findings are extremely rare in modem series.

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Symptom Progression

The progression of symptoms in patients with acoustic schwannomas may be divided into a series of stages that represent tumor growth within the anatomic confines of the lAC and CPA. Most authors recognize two stages of acoustic tumor growth: intracanalicular tumors, which are those confined within the lAC, and extracanalicular tumors that extend beyond the lAC into the CPA. The later group may be subdivided into two stages: cisternal and brain stem compressive. Hearing loss, tinnitus and vertigo develop early with intracanalicular tumors, whereas tumors arising outside the lAC may undergo substantial growth before producing auditory and vestibular symptoms. As the tumor enlarges within the CPA there is a progression of auditory symptoms, a transition from true vertigo to disequilibrium and the development of headache caused by local meningeal irritation. Late symptoms of the cisternal stage include mid-facial and corneal hypaesthesia. In addition to a progression of the previous symptoms, the stage of brain stem compression includes the development of hydrocephalus with its associated headache, visual loss and diplopia. Although, facial nerve dysfunction is rare, it may occur at any stage. With current diagnostic methods, most tumors are discovered during the first two stages.

In two studies of the average duration of symptoms prior to diagnosis, unilateral tinnitus or hearing loss was present almost 4 years before diagnosis and vertigo preceded diagnosis by 3.6 years. This was followed by a latent period until the development of headache 2.2 years prior to diagnosis and disequilibrium at 1.7 years before diagnosis. The involvement of other cranial nerves occurred later with the trigeminal nerve affected at 0.9 years and facial nerve dysfunction occurring at 0.7 years prior to diagnosis. Shiffman et al. correlated symptoms with tumor size in their analysis of 26 patients. They found that patients with tumors less than 2.5 cm in diameter presented with audiovestibular symptoms, patients with tumors 2.5 to 3.5 cm in diameter presented with audiovestibular and trigeminal symptoms, while any additional symptoms were associated with tumors greater than 3.5 cm. The symptom stages, therefore, reflect the size of the lesion and the length of time between the symptom stages is determined by the rate of tumor growth.

A clinically quiescent period exists between the onset of audiovestibular symptoms in the intracanalicular stage and the development of headache and trigeminal symptoms in the cisternal stage. Many studies agree that an average of 1.5 to 2.0 years may elapse between these symptom complexes. During this latent period the tumor may undergo significant enlargement, without an increase in symptoms. Thus, failure to recognize the early symptoms and obtain the appropriate diagnostic tests may lead to a significant delay in diagnosis and a corresponding increase in surgical morbidity.


Copyright © [2006 CNS Clinic - Neurosurgery - Jordan]. All rights reserved