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.
