Turkish Neurosurgery 3: 68-72. 1993 Atilla: CT of 1ntracanalicular Acoustic Neuromas

Computerized Tomography Demonstration of PurelyIntracanalicular Acoustic Neuromas

SERHAN ATILLA, SEDAT ISIK. ERHAN T. ILGIT. MEHMET ARAÇ, HAKAN ÖZDEMIR

Department of Radiology. Gazi University. School of Medicine, Besevler 06510 Ankara, TÜRKIYE

Abstract: Three cases of purely intracanalicular acoustic neuromasnot causing bony erosion and/or enlargement of the internalacoustic canal walls diagnosed accurately by temporal bone CTexaminations are presented. It is suggested that temporal boneCT scans done with the appropriate technique is a sufficient and

INTRODUCTION

Acoustic neuroma. or schwannoma is a benigntumour arising from the schwann sheath of theeighth nerve in the internal auditory canal (lAC).Theusual clinical manifestations are sensorineural hear­

ing loss, tinnitus and vestibular disturbances; theseverity of the symptoms is dosely related to the sizeof the subarachnoid space in the cerebellopontineCIstern which is variable. The tumour is most com­

monly seen in the fourth and fifth decades of life andaccounts for 8-LO % of all intracranial , and 78 % of

all cerebellopontine angle tumours [3.12]. It is veryslow growing. at a rate of 5mm a year [8]. and takesapproximately about 2 years to mi the internalauditory canal. it is known to cause expansion of theIAC with smooth bony erosion. but there is no directrelatianship between the amount of bony erosionand size of the tumour.

Acoustic neuromas extending to the cerebellopon­tine CIsternout of the internal auditory canal are easi­ly diagnosed and demonstrated in routine camputedtomography (CT)examinations. When the tumour iscompletely canfined to the internal auditory canala definite diagnosis becomes harder to achieve. Atthe present time. although it has been stated that

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reliable diagnostic test for intrameatal acoustic tumours. needingconfirmation with MRI only in patients who have very narrowinternal auditory canals.

Key Words: Neuroma. Acoustic: Intracanalicular; Tomography.X-ray computed: Cerebellopontine angle

acoustic neuromas larger than 5 mm in diameter canbe detected with a high degree of accuracy with high­resolution CT [6]. the cammonly accepted belief isthat tumours smaller than 1.5 cm. in size completelyor mainly confined to the internal auditory meatuscannot be demonstrated by standard temporal boneCT examination; and the definitive diagnostic test isair-CT CIsternography or magnetic resonance imag­ing (MRI) [1.2.4.6.9.10,11].

Enlargement due to bony erosion of 2 mm ormore of any portion of one IAC compared to the cor­responding segment of the opposite side. andshortening of the posterior wall by at least 3 mm areaccepted as indicative of an acoustic neuroma in over90 % of cases [12].We are presenting three purely in­tracanalicular acoustic neuromas much smaller than

1.5 cm diagnosed carrectly by contrast enhancedhigh-resolution CT scans. which do not show theabove mentioned diagnostic bony erosion in the IACwalls. Brightness of the cantrast enhancement in theintracanalicular soft tissue and obliteration of the CSF

space inside the internal auditory canals were seenboth in axial and caronal sections. The diagnosis wasconfirmed by the Auditory Brainstem Response (ABR)test and MRI examinations.

Turkish Neurosurgery 3: 68-72. 1993 Atilla: CT of 1ntracanaJicuJarAcoustic Neurornas

middle and inner ear structures appeared normal andthe internal auditory canal wa11swere symmetricalshowing no bony erosion. The only questionable fin­ding was inability to see the fakiform crest on cor­onal images (Fig.l A). On examining the magnifiedsoft tissue images, a bare1y visible contrast enhance­ment in the left internal auditory canal with no ex­tension to the cerebellopontine angle was noticed(Fig.l B.C).Compared with the opposite side. contrastenhancement brightness and obliteration of the in­tracanalicular subarachnoid space was easier todetect. Especially the coronal scans were very helpfulin showing intracanalicular contrast enhancedfu11ness (Fig.l C). Supporting this finding with thefakiform crest erosion, a diagnosis of purely in­tracanalicular acoustic neuroma was made. Fo11ow­ing CT diagnosis. the ABRtest was applied, showinga destructive retrocochlear lesion on the left side. The

patient was also examined by MR! with the spin echotechnique using short and long sequences withGadolinium-DTPA injection, confirming the diagnosisof an intracanalicular acoustic neuroma (Fig.l D). The

y,

PATIENTS AND METHOD

A 35-year-old female with sensorineural hearingloss in the left ear was referred for temporal boneCT examination. Odiological test results suggesteda retrocochlear lesion. There was no tinnitus,vestibular dysfunction or involvement of othercranial nerves. On evaluation of the magnified boneresolution images in both planes, all the external.

CASE 1:

The patients were examined with using the GE9800 Quick Hilight CT system in both axial and cor­onal planes using the standard technique with 1.5mm thick contiguous sections. standard resolution.512x512 matrix size. 120 kVp. 100 mA. 4 sec. scantime. The patients were injected with 1.5 cc/kg non­ionic iodinated contrast medium. Bone review

algorithm images were also reconstructed to see thefine bony detail. Both the standard and bonealgorithm images were examined separately withmagnification factors ranging from 1.8 to 2.2.

Figure 1 A-D: (Case 1) CoronaJplane magiiihed bone review aigonrnm image ot ierr remporai Done with normal internal acoustic canalwalls. Note that normal extension of the faldform crest is not seen (A). Axial standard resolutian CT section from the levelof the left internal acoustic canal showing intracanalicular contrast enhancement with normal cerebellopontine dstern (B).

Magiiifjed coronal standard resolutian CT section of left temporal bone shows a contrast-enhanced soft tissue increaseobliterating the intracanalicular CSF space rC). Coronal plane short TRTE spin echo MR image with Gd-DTPA injeetion revealsa hyperintense acoustic neuroma in the left internal auditory canal (D).

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Turkish Neurosurgery 3: 68-72. 1993 Atilla: CT of Intracanalicular Acoustic Neuromas

Figure 2 A.B: (Case 2) Axial CT section through the level of 1ACs shows a contrast-enhanced acoustic tumour filling the right internalauditory canal (A). Magni{jed standard resolution coronal CT section of the right temporal bone reveals fullness of theintracanalicular space with no signi{jcant bony erosion (B).

Figure 3 A.B: (Case 3) Magni{jed soft tissue CT imageofleft temporal bone showing a contrast-enhanced intracanalicular acoustic neuromawith intact IAC walls (A). Contrast-enhanced coronal short TRshort TE MR image of the same patient confirming thediagnosis of intracanalicular acoustic neuroma on the left side (B).

patient refused an operation and decided to wait fora while.

CAS E 2:

A 28-year-old man with right-sided sensorinemalhearing loss and a doubtful odiological test result forretrocochlear lesion was referred for temporal boneCT examination. CT scans in the axial and coronal

plan es with direct intravenous contrast injectionshowed an intracanalicular soft tissue lesion on the

right side with no extension to the cerebellopontinecistern (Fig.2A.B).Soft tissue window settings reve al­ed marked contrast enhancement and obliteration

of the subarachnoid space in the right IAC suggestingthe diagnosis of intracanalicular acoustic neuroma.Bone review algorithm images were also examined

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far bony erosion in the IAC walls. The right IAC was1 mm wider than the left in the middle part. Theposterior wall of the right IAC was 0.5 mm shorterthan the left and the falciform crest was amputatedon the right side. MRI confirmed the diagnosis. Thepatient was operated using a translabyrinthine ap­proach and a schwannoma was excised from theright internal auditory canal.

CASE 3:

A 39-year-old male patient complaining of tinnitusand left sided hearing loss was examined forsuspected retrocochlear lesion. Odiological testsrevealed bilateral sensarinemal hearing loss moresignificant on the left side. Bone tissue window set­tings show ed no significant enlargement in any

Turkish Neurosurgery 3: 68-72. 1993

portion of the IAC. Soft tissue window settings ex­amin ed with magnification revealed a contrastenhanced intracanalicular acoustic neuroma on the

left side obliterating the cerebrospinal fluid areas ofthe IAC without extension to the cerebellopontineangle cistern (Fig.3 A). ABRtest and MRI confirmedthe diagnosis (Fig.3 B)and the patient was operatedusing the translabyrinthine approach.

DlSCUSSION

Acoustic neuroma is a benign. slow-growingtumour mak.ing surgical treatment necessary whenthe symptoms become severe. In the radiologicaldemonstration of acoustic neuroma, the main pur­pose for the radiologist is to diagnose it accuratelyand also ensure that there is no possibility of anegative surgical exploration. This may be ac­complished by the use of various different examina­tion techniques or evaIuation of follow-upexaminations.

The CT density of an acoustic neuroma is variablebefore contrast injection, and shows moderate con­trast enhancement mak.ing the use of contrast agentsessential. Tumours extending to the cerebellopontineangle dstern or causing significant erosion of the IACare easily demonstrated by routine CT scans. In ad­dition, we propose that tumours confined to the IACwhich do not cause a significant bony erosionregardless of size. can also be diagnosed accuratelyusing contrast enhanced high-resolution CT in the ax­ial and coronal planes if all the soft tissue and boneresolution views are examined by a radiologistfamiIiar with these images. Brightness of the contrastenhancement filling the IAC completely andobliterating the intracanalicular CSF spaces, detectedboth in axial and coronal planes suggests thediagnosis of intracanalicular acoustic tumour. The cor­onal plane is espedally very helpful in demonstratingintracanalicular fullness caused by the tumour. Ero­sion of the fakiform crest is another very importantearly sign of acoustic neuroma. Vascular loops whichmay be a possible cause of eighth nerve disordersor a neurovascular bundle entering the internalauditory meatus may sometimes simulate this lesion,especially in patients with narrow canals. Most caneasily be discriminated from a soft tissue mass, butin patients with narrow internal auditory canals (2-4

Atilla: CT of 1ntracanaJicuJarAcoustic Neuromas

mm). CT diagnosis of acoustic neuroma becomesharder needing confirmation with other diagnostictests [5]. It is no use performing CT pneumodster­nography which has no advantages over high­resolution CT scans done with standard techniquein the diagnosis of acoustic neuromas as it is an in­vasive examination procedure reported to give a have22 % false positive rate. The use of this procedureshould be confined to demonstrating intracanalicularvascular loops [5.11]. In patients with narrow inter­nal acoustic canals presenting doubtful images of in­tracanalicular tumour in standard temporal bone CTexaminations, MR! should be the procedure of choiceto confirm or rule out a soft tissue mass as it has a

diagnostic accuracy of 100% with no false positive ornegative results when examined with the appropriatetechnique using thin slices. and sagittaL. coronal andaxial views [6.7.10.11]. Considering cost and availabili­ty. we think that high-resolution temporal bone CTexaminations with standard technique is a suffidentand reliable examination modality for showing in­tracanalicular acoustic neuromas in patients withnormal-sized internal auditory canals.

Correspondence: Erhan T. Ilgit. MD.Gazi Üniversitesi Tip FakültesiRadyodiagnostik A.B,D.Besevler 06510 Ankara. TürkiyeTel: 2126565 / 273

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