Magnetic Resonance Imaging of the Large Vestibular Aqueduct Barry E. Hirsch, MD; Jane
L.
Weissman, MD; Hugh D. Curtin, MD; Donald B. Kamerer, MD
The large vestibular aqueduct syndrome describes an abnormally large endolymphatic duct and sac with associated sensorineural hearing loss. This entity was originally reported in 1978 and has since been identified as a finding in children with progressive hearing loss. The original description of the large vestibular aqueduct employed hypocycloidal polytomography of temporal bone. Subsequent reports studied patients identified with this syndrome using computed tomographic scans. We report magnetic resonance imaging of two patients diagnosed with the large vestibular aqueduct syndrome. The magnetic resonance imaging and computed tomographic scans are compared and the significant findings on magnetic resonance imaging are reviewed. This should assist the otolaryngologist and radiologist with establishing the appropriate diagnosis. (Arch Otolaryngol Head Neck Surg. 1992;118:1124-1127) \s=b\
of sensorineural hearing loss and anomalies of the has long been ThegenitalAbnormalities have of the vestibular association
con¬
inner
ear
ognized.
rec¬
aqueduct
also been identified and categorized as obliterated, fili¬ form, and large.1 Valvassori and Clemis2 subsequently coined the term the large vestibular aqueduct syndrome in 1978 when they described the clinical features of 50 patients evaluated by tomography for inner ear dysfunc¬ delin¬ tion. Various reports were subsequently the audiometrie and and clinical radiologie findings eating course.3"5 Imaging studies utilized for their evaluation in¬ cluded tomograms and thin-section high-resolution com¬ puted tomography (CT). Brogan et al6 compared threedimensional magnetic resonance imaging (MRI) with high-resolution CT to examine the endolymphatic duct in five normal subjects. We report the findings on MRI and CT of two patients evaluated for hearing loss and found to have the large vestibular aqueduct syndrome.
published
REPORT OF CASES
56-year-old woman was evaluated because of episodic dizziness. Acute symptoms had been present
Case 1.—A recurrent
Accepted for publication June 17, 1992. From the Departments of Otolaryngology (Drs Hirsch, Curtin, and Kamerer) and Radiology (Dr Weissman and Curtin), University of Pittsburgh (Pa) School of Medicine and The Eye & Ear Institute of Pittsburgh. Reprint requests to The Eye & Ear Institute, Suite 500, 203 Lothrop St, Pittsburgh, PA 15213 (Dr Hirsch).
previous month consisting of a sensation of swaying or moving with associated nausea lasting from 15 minutes to 1 hour. Symptoms would occur once or twice daily. She described a spinning sensation lasting a few seconds with position changes. In addition, she complained of a balloonlike sensation in the right ear with pulsating tinnitus described as "hammering." She was for the
profoundly deaf since the age of 3 months. This was attributed to a
viral infection. She
was
sent to
our
service for consultation
re¬
garding her symptoms and the abnormalities noted on MRI in the cerebellopontine angles. Examination showed a 56-year old woman who communicated with sign language. Facial function was normal, as was her ear examination. An audiogram confirmed a profound sensorineural hearing loss. Vestibular testing revealed a right-beating nystagmus in posi¬ tional tests. There was no evidence of paroxysmal positioning ver¬ tigo. Caloric testing demonstrated a right-directional preponder¬ ance with absent responses to ice water in the right ear. Platform posturography testing resulted in abnormal responses in sensory tests 5 and 6. Vertical axis rotational testing confirmed a rightdirectional preponderance. The MRI scan showed bilateral pos¬ terior fossa abnormalities separate from the internal auditory ca¬ nals. Magnetic resonance images (Gyrex [Elscint Ine, Hackensack, NI] 0.5T) showed collections of approximately cerebrospinal fluid (CSF) signal intensity in the cerebellopontine angle cisterns (Fig 1 ). These were low signal on ,-weighted images (Fig 1, left), and high signal on T2-weighted images (Fig 1, right). Thin axial and coronal CT images (General Electric [Milwaukee, Wis] 9800) demonstrated the markedly enlarged osseous canal for the vestibular aqueduct on each side (Fig 2, right). The endolymphatic sacs were enlarged, CSF-density collections that were outlined medially by thin, en¬ hancing dura (Fig 2, left) and inferolaterally by the sigmoid sinus (Fig 3). The sigmoid was slightly flattened by the sac. The dilated endolymphatic sacs seen on CT correlated with the fluid collection
On MRI, a connection between the CSF collection and the operculum could not be demonstrated on either side. The co¬ chlear aqueducts were of normal size (not shown). The patient was scheduled for a right-sided endolymphatic sacmastoid shunt in anticipation of reducing her pulsatile "hammer¬ ing" tinnitus and alleviating vestibular complaints. The sac was initially aspirated with a needle and syringe, which yielded clear fluid and collapse of the outer membranous layer. Opening of the sac revealed an enlarged cavity. Suctioning fluid from the cavity collapsed the lumen. However, the cavity quickly refilled to the point that continuous drainage was ongoing. A connection be¬ tween the sac and CSF was considered to be present. The opening of the sac was sutured closed and the area packed with fat, per¬ forming a compression rather than a shunt. Postoperatively, the patient no longer complained of pounding tinnitus in the right ear and was free from her previously described episodic vertiginous attacks. She has been asymptomatic for the past 2 years. seen on MRI.
Downloaded From: http://archotol.jamanetwork.com/ by a Karolinska Institutet University Library User on 05/24/2015
1.—Axial magnetic resonance images. Left image, T,-weighted image demonstrates the large
Fig
endolymphatic sacs (solid white arrows). Flow ar¬ tifact creates signal in each sigmoid sinus (open arrows). Right image, T2-weighted image shows the very large left endolymphatic sac (arrow¬ heads). The sac has a very high signal, compara¬ ble to a signal from the vitreous of the globe (V). Because the patient was tilted in the scanner, the right side is not well seen.
Fig 2.—Axial computed tomographic images. Left image, Soft-tissue algorithm demonstrates the large endolymphatic sac (arrow) seen on both sides but labeled only on the right. Enhancing
dura defines the medial border of the sac (small arrowheads), and the enhancing sigmoid sinus runs posterior to the sac on each side (large arrowhead). Right image, The osseous vestibular aqueduct is markedly enlarged (white arrows). The vestibules (highlighted arrows) are slightly
dysplastic.
Case 2.—A
30-year-old project
manager
was
evaluated for
new-onset left-sided tinnitus with associated decreased
3.—Coronal computed tomographic image. Soft-tissue algorithm shows the low-density sac (S) outlined by dura medially (arrowheads). On the right, the sigmoid sinus (arrows) runs between the sac and the bone.
Fig
hearing;
he complained of these symptoms for the previous 7 weeks. He had used a hearing aid in his left ear since the age of 5 years when he was diagnosed with a severe hearing loss. He had been told he lost his hearing at the age of 2 years from a febrile illness. The patient had a profound hearing loss in the right ear with reports of fluctuation in his left ear during upper respiratory illnesses. He also noted progressive loss occurring with minor head trauma (street hockey) and during seasonal changes in the spring and fall. His speech was appropriate for the degree of his reported hearing loss. His examination was otherwise normal. An audiogram confirmed a bilateral profound loss (Fig 4). A metabolic screen was normal. Pending a CT scan, a tapering course of pred¬ nisone was started, which was of no benefit to his hearing. Thin axial CT images were obtained through tlie temporal bones (General Electric 9800) after intravenous contrast. On each side, the osseous channel for the vestibular aqueduct was markedly dilated (Fig 5, left). Tlie dilated aqueduct opened into an enlarged endolymphatic sac that lay along the posterior surface of the temporal bone. The dilated sac was approximately the same den¬ sity as CSF (Fig 5, right). The enhancing sigmoid sinus was iden¬ tified posterior to the endolymphatic sac (Fig 5, right). The MRI images (General Electric Signa 1.5T) demonstrated that the area presumed to be the sac was essentially isointense with CSF. This was apparent on the T,-weighted, proton-density, and T2-weighted sequences. On one T2-weighted image, there was a suggestion of a connection between the dilated sac and vestibule, but this was not ideally seen on the 5-mm-thick scans of the MRI study. Unlike the adjacent dura, the sac did not enhance after gadolinium. (Figs 6 and 7).
Downloaded From: http://archotol.jamanetwork.com/ by a Karolinska Institutet University Library User on 05/24/2015
Audiometer 500
2S0
1000
2000
4000
Q&_% >
Oat· of Last Audio
ß 000
*j â^-^" Q O -
RIGHT
7W-.
MASK.
LEFT
MASK.
UQo
e^— cm:Tit
ter?
Fig 4.—Audiogram in case 2. A 30-year old man with profound deafness diagnosed with large ves¬ tibular aqueduct syndrome. ^ -
^s 750
(simboli
ö *;
tì:
^ jv:
Hi*^rr
side
Fig 5.—Axial computed tomographic images. Left image, Bone algorithm demonstrates the dilated vestibular aqueduct on each side (D) extending back from the vestibule (highlighted arrows). Right image, Soft-tissue algorithm shows the di¬ lated endolymphatic sac (large arrow), seen here better on the left side. The sac is a fluid-density structure adjacent to the contrast-enhanced sig¬ moid sinus (small arrows).
COMMENT imaging is a
Magnetic ity that provides significant resonance
relatively new modal¬
information about the anat¬ of the and head temporal bone region and detailed omy differentiation of tissue signal intensities. Thin-section CT remains an essential diagnostic tool for defining the bony anatomy of the temporal bone and otic capsule. It remains to be seen if further advances in MRI will provide greater resolution and definition of the fluid compartments of the membranous labyrinth. The learning curve for reading and interpreting MRIs continues to rise as we come to better understand the biophysical properties of normal and pathologic states. This is exemplified by patient 1, who had an image obtained at the time when MRI first became rou¬ tinely available. Review of the MRIs confirmed that the in¬ creased signal on T2-weighted images evident in the pos¬ terior fossa was anterior to the sigmoid sinus and posterior to the internal auditory canals, consistent with the location of the endolymphatic duct and sac. The incidence of the large vestibular aqueduct syndrome in the general population is difficult to assess accurately. In a study of 2683 patients evaluated with tomography and CT for a variety of otologie problems, 26 patients (1 %) were found to have a large vestibular aqueduct (diameter, >1.5 mm).3 The original report by Valvassori and Clemis2 stud¬ ied 3700 patients referred for inner ear tomography. They also found 50 patients (1.3%) with an enlarged vestibular
aqueduct.2
An early derangement in the embryogenesis of the en¬ dolymphatic duct leads to development of the large vesti¬ bular aqueduct.4 The clinical features of the syndrome be¬ come manifest in early childhood. In both of our cases, the patients were told their hearing loss as children was sec¬ ondary to a virus and high fever. Their subsequent eval¬ uation provided objective evidence of a congenital anom¬ aly. The large vestibular aqueduct syndrome has been recently defined to be a congenital anomaly of the tempo¬ ral bone that predisposes the development in early child¬ hood of an acquired sensorineural hearing loss.5-7 The cause of hearing loss in the large vestibular aque¬ duct syndrome is unclear. The large duct and sac, in and of themselves, do not cause the hearing loss, and are likely a marker of other otopathology. Simpson8 suggested that the hearing loss may be caused by a concurrent maldevelopment of the cochlear membranous labyrinth. One theory proposed that hyperosmolar proteins contained in the large endolymphatic sac reflux into the cochlear duct, causing osmotic-induced damage to the neuroepithelium.5 This theory was supported by Jackler and De La Cruz,4 who felt that the hearing loss was a result of fluctuating increases in CSF pressure transmitted via the endolym¬ phatic sac and duct to the membranous labyrinth. We con¬ cur with the concept of inner ear membrane pathology but speculate on a different mechanism in view of the opera¬ tive findings in case 1. The incessant welling up of clear fluid following the opening of the endolymphatic sac re-
Downloaded From: http://archotol.jamanetwork.com/ by a Karolinska Institutet University Library User on 05/24/2015
6.—Axial ,-weighted magnetic resonance images. The enlarged endolymphatic sacs (white
Fig
are approximately the same signal inten¬ sity as cerebrospinal fluid (C).
arrows)
7.—Axial magnetic resonance images. Left image, ,-weighted image, after gadolinium. The sac, which does not enhance, is seen best on the left side (white arrow). Both sigmoid sinuses enhance after contrast (black arrows). Right im¬ age, Proton density image at the same level dem¬ onstrates the left sac (white arrow), approximately the same signal intensity as cerebrospinal fluid.
Fig
Slow flow in each (black arrows).
fleeted an abnormal communication with a large clear fluid reservoir, specifically CSF. Though speculative, the source of such communication would most likely be through the lateral wall of the internal auditory canal or the cochlear aqueduct. The CT scan showed a normal cochlear aque¬ duct, thus implicating the lateral wall of the internal audi¬ tory canal (spiral foraminous tract and cribrose macula) as the likely source of CSF. This patent communication between the CSF and the vestibule can be theorized to damage the membranous labyrinth by fistula formation from the perilymphatic to the endolymphatic space. An¬ other explanation would be collapse of the membranous labyrinth, increasing the size of the perilymphatic space. In summary, we have reviewed two patients with pro¬ found hearing loss and imaging studies demonstrating large vestibular aqueducts and endolymphatic sacs. The sacs were nearly isointense with CSF on Tr and T2weighted imaging. Pulsation of the brain causes dephasing of protons in CSF and slightly decreases the signal inten¬ sity on T2-weighted images. It is possible, as suggested by some of the images, that the signal intensity of the dilated endolymphatic sacs is somewhat greater than that of CSF on T2-weighted sequences, as the sacs may be more protected from brain pulsations. The sacs' location in the posterior fossa adjacent to the sigmoid sinus and posterior to the internal auditory canal raises the suspicion of the large vestibular aqueduct syndrome, even though the MRI does not show the endolymphatic ducts well. It is impor¬ tant that the radiologist and otolaryngologist not misin-
sigmoid
sinus creates
signal
terpret the enlarged
sacs as some other pathologic condi¬ tion. In particular, acoustic neuromas should enhance after intravenous contrast. If the associated osseous abnormal¬ ities were not recognized, the large endolymphatic sac could be mistaken for CSF surrounding the cerebellar hemisphere. Confirmation of the large vestibular aque¬ ducts can be obtained with thin-section CT. The large vestibular aqueduct syndrome may actually be a manifestation of an abnormal communication between the CSF and bony labyrinth. Examination with temporal bone histologie sections may enhance our understanding of the pathophysiology of this unusual clinical entity.
References 1. Clemis J, Valvassori G. Recent radiographic and clinical observation on the vestibular aqueduct. In: Pulec J, ed. Meniere's Disease. Philadelphia, Pa: WB Saunders Co; 1968:339-346. 2. Valvassori GE, Clemis JD. The large vestibular aqueduct syndrome.
Laryngoscope. 1978;88:723-728.
3. Emmett JR. The large vestibular aqueduct syndrome. Am J Otol. 1985; 6:387-415. 4. Jackler RK, De La Cruz A. The large vestibular aqueduct syndrome.
Laryngoscope. 1989;99:1238-1243. 5. Levenson MJ, Parisier SC, Jacobs M, Edelstein
aqueduct syndrome
in children. Arch
115:54-58. 6. Brogan M, Chakeres
DR. The large vestibular Otolaryngol Head Neck Surg. 1989;
DW, Schmalbrock P. High-resolution 3DFT MR Am J Neuroradiol. 1991;12:1-11. 7. Arcand P, Desrosiers M, Dube J, Abela A. The large vestibular aqueduct syndrome and sensorineural hearing loss in the pediatric population. J Otolaryngol. 1991;20:247-250. 8. Simpson C. In discussion: Jackler RK, De La Cruz A. The large vestibular aqueduct syndrome. Laryngoscope. 1989;99:1238-1243.
imaging of the endolymphatic duct and soft tissue of the otic capsule.
Downloaded From: http://archotol.jamanetwork.com/ by a Karolinska Institutet University Library User on 05/24/2015
L.
Weissman, MD; Hugh D. Curtin, MD; Donald B. Kamerer, MD
The large vestibular aqueduct syndrome describes an abnormally large endolymphatic duct and sac with associated sensorineural hearing loss. This entity was originally reported in 1978 and has since been identified as a finding in children with progressive hearing loss. The original description of the large vestibular aqueduct employed hypocycloidal polytomography of temporal bone. Subsequent reports studied patients identified with this syndrome using computed tomographic scans. We report magnetic resonance imaging of two patients diagnosed with the large vestibular aqueduct syndrome. The magnetic resonance imaging and computed tomographic scans are compared and the significant findings on magnetic resonance imaging are reviewed. This should assist the otolaryngologist and radiologist with establishing the appropriate diagnosis. (Arch Otolaryngol Head Neck Surg. 1992;118:1124-1127) \s=b\
of sensorineural hearing loss and anomalies of the has long been ThegenitalAbnormalities have of the vestibular association
con¬
inner
ear
ognized.
rec¬
aqueduct
also been identified and categorized as obliterated, fili¬ form, and large.1 Valvassori and Clemis2 subsequently coined the term the large vestibular aqueduct syndrome in 1978 when they described the clinical features of 50 patients evaluated by tomography for inner ear dysfunc¬ delin¬ tion. Various reports were subsequently the audiometrie and and clinical radiologie findings eating course.3"5 Imaging studies utilized for their evaluation in¬ cluded tomograms and thin-section high-resolution com¬ puted tomography (CT). Brogan et al6 compared threedimensional magnetic resonance imaging (MRI) with high-resolution CT to examine the endolymphatic duct in five normal subjects. We report the findings on MRI and CT of two patients evaluated for hearing loss and found to have the large vestibular aqueduct syndrome.
published
REPORT OF CASES
56-year-old woman was evaluated because of episodic dizziness. Acute symptoms had been present
Case 1.—A recurrent
Accepted for publication June 17, 1992. From the Departments of Otolaryngology (Drs Hirsch, Curtin, and Kamerer) and Radiology (Dr Weissman and Curtin), University of Pittsburgh (Pa) School of Medicine and The Eye & Ear Institute of Pittsburgh. Reprint requests to The Eye & Ear Institute, Suite 500, 203 Lothrop St, Pittsburgh, PA 15213 (Dr Hirsch).
previous month consisting of a sensation of swaying or moving with associated nausea lasting from 15 minutes to 1 hour. Symptoms would occur once or twice daily. She described a spinning sensation lasting a few seconds with position changes. In addition, she complained of a balloonlike sensation in the right ear with pulsating tinnitus described as "hammering." She was for the
profoundly deaf since the age of 3 months. This was attributed to a
viral infection. She
was
sent to
our
service for consultation
re¬
garding her symptoms and the abnormalities noted on MRI in the cerebellopontine angles. Examination showed a 56-year old woman who communicated with sign language. Facial function was normal, as was her ear examination. An audiogram confirmed a profound sensorineural hearing loss. Vestibular testing revealed a right-beating nystagmus in posi¬ tional tests. There was no evidence of paroxysmal positioning ver¬ tigo. Caloric testing demonstrated a right-directional preponder¬ ance with absent responses to ice water in the right ear. Platform posturography testing resulted in abnormal responses in sensory tests 5 and 6. Vertical axis rotational testing confirmed a rightdirectional preponderance. The MRI scan showed bilateral pos¬ terior fossa abnormalities separate from the internal auditory ca¬ nals. Magnetic resonance images (Gyrex [Elscint Ine, Hackensack, NI] 0.5T) showed collections of approximately cerebrospinal fluid (CSF) signal intensity in the cerebellopontine angle cisterns (Fig 1 ). These were low signal on ,-weighted images (Fig 1, left), and high signal on T2-weighted images (Fig 1, right). Thin axial and coronal CT images (General Electric [Milwaukee, Wis] 9800) demonstrated the markedly enlarged osseous canal for the vestibular aqueduct on each side (Fig 2, right). The endolymphatic sacs were enlarged, CSF-density collections that were outlined medially by thin, en¬ hancing dura (Fig 2, left) and inferolaterally by the sigmoid sinus (Fig 3). The sigmoid was slightly flattened by the sac. The dilated endolymphatic sacs seen on CT correlated with the fluid collection
On MRI, a connection between the CSF collection and the operculum could not be demonstrated on either side. The co¬ chlear aqueducts were of normal size (not shown). The patient was scheduled for a right-sided endolymphatic sacmastoid shunt in anticipation of reducing her pulsatile "hammer¬ ing" tinnitus and alleviating vestibular complaints. The sac was initially aspirated with a needle and syringe, which yielded clear fluid and collapse of the outer membranous layer. Opening of the sac revealed an enlarged cavity. Suctioning fluid from the cavity collapsed the lumen. However, the cavity quickly refilled to the point that continuous drainage was ongoing. A connection be¬ tween the sac and CSF was considered to be present. The opening of the sac was sutured closed and the area packed with fat, per¬ forming a compression rather than a shunt. Postoperatively, the patient no longer complained of pounding tinnitus in the right ear and was free from her previously described episodic vertiginous attacks. She has been asymptomatic for the past 2 years. seen on MRI.
Downloaded From: http://archotol.jamanetwork.com/ by a Karolinska Institutet University Library User on 05/24/2015
1.—Axial magnetic resonance images. Left image, T,-weighted image demonstrates the large
Fig
endolymphatic sacs (solid white arrows). Flow ar¬ tifact creates signal in each sigmoid sinus (open arrows). Right image, T2-weighted image shows the very large left endolymphatic sac (arrow¬ heads). The sac has a very high signal, compara¬ ble to a signal from the vitreous of the globe (V). Because the patient was tilted in the scanner, the right side is not well seen.
Fig 2.—Axial computed tomographic images. Left image, Soft-tissue algorithm demonstrates the large endolymphatic sac (arrow) seen on both sides but labeled only on the right. Enhancing
dura defines the medial border of the sac (small arrowheads), and the enhancing sigmoid sinus runs posterior to the sac on each side (large arrowhead). Right image, The osseous vestibular aqueduct is markedly enlarged (white arrows). The vestibules (highlighted arrows) are slightly
dysplastic.
Case 2.—A
30-year-old project
manager
was
evaluated for
new-onset left-sided tinnitus with associated decreased
3.—Coronal computed tomographic image. Soft-tissue algorithm shows the low-density sac (S) outlined by dura medially (arrowheads). On the right, the sigmoid sinus (arrows) runs between the sac and the bone.
Fig
hearing;
he complained of these symptoms for the previous 7 weeks. He had used a hearing aid in his left ear since the age of 5 years when he was diagnosed with a severe hearing loss. He had been told he lost his hearing at the age of 2 years from a febrile illness. The patient had a profound hearing loss in the right ear with reports of fluctuation in his left ear during upper respiratory illnesses. He also noted progressive loss occurring with minor head trauma (street hockey) and during seasonal changes in the spring and fall. His speech was appropriate for the degree of his reported hearing loss. His examination was otherwise normal. An audiogram confirmed a bilateral profound loss (Fig 4). A metabolic screen was normal. Pending a CT scan, a tapering course of pred¬ nisone was started, which was of no benefit to his hearing. Thin axial CT images were obtained through tlie temporal bones (General Electric 9800) after intravenous contrast. On each side, the osseous channel for the vestibular aqueduct was markedly dilated (Fig 5, left). Tlie dilated aqueduct opened into an enlarged endolymphatic sac that lay along the posterior surface of the temporal bone. The dilated sac was approximately the same den¬ sity as CSF (Fig 5, right). The enhancing sigmoid sinus was iden¬ tified posterior to the endolymphatic sac (Fig 5, right). The MRI images (General Electric Signa 1.5T) demonstrated that the area presumed to be the sac was essentially isointense with CSF. This was apparent on the T,-weighted, proton-density, and T2-weighted sequences. On one T2-weighted image, there was a suggestion of a connection between the dilated sac and vestibule, but this was not ideally seen on the 5-mm-thick scans of the MRI study. Unlike the adjacent dura, the sac did not enhance after gadolinium. (Figs 6 and 7).
Downloaded From: http://archotol.jamanetwork.com/ by a Karolinska Institutet University Library User on 05/24/2015
Audiometer 500
2S0
1000
2000
4000
Q&_% >
Oat· of Last Audio
ß 000
*j â^-^" Q O -
RIGHT
7W-.
MASK.
LEFT
MASK.
UQo
e^— cm:Tit
ter?
Fig 4.—Audiogram in case 2. A 30-year old man with profound deafness diagnosed with large ves¬ tibular aqueduct syndrome. ^ -
^s 750
(simboli
ö *;
tì:
^ jv:
Hi*^rr
side
Fig 5.—Axial computed tomographic images. Left image, Bone algorithm demonstrates the dilated vestibular aqueduct on each side (D) extending back from the vestibule (highlighted arrows). Right image, Soft-tissue algorithm shows the di¬ lated endolymphatic sac (large arrow), seen here better on the left side. The sac is a fluid-density structure adjacent to the contrast-enhanced sig¬ moid sinus (small arrows).
COMMENT imaging is a
Magnetic ity that provides significant resonance
relatively new modal¬
information about the anat¬ of the and head temporal bone region and detailed omy differentiation of tissue signal intensities. Thin-section CT remains an essential diagnostic tool for defining the bony anatomy of the temporal bone and otic capsule. It remains to be seen if further advances in MRI will provide greater resolution and definition of the fluid compartments of the membranous labyrinth. The learning curve for reading and interpreting MRIs continues to rise as we come to better understand the biophysical properties of normal and pathologic states. This is exemplified by patient 1, who had an image obtained at the time when MRI first became rou¬ tinely available. Review of the MRIs confirmed that the in¬ creased signal on T2-weighted images evident in the pos¬ terior fossa was anterior to the sigmoid sinus and posterior to the internal auditory canals, consistent with the location of the endolymphatic duct and sac. The incidence of the large vestibular aqueduct syndrome in the general population is difficult to assess accurately. In a study of 2683 patients evaluated with tomography and CT for a variety of otologie problems, 26 patients (1 %) were found to have a large vestibular aqueduct (diameter, >1.5 mm).3 The original report by Valvassori and Clemis2 stud¬ ied 3700 patients referred for inner ear tomography. They also found 50 patients (1.3%) with an enlarged vestibular
aqueduct.2
An early derangement in the embryogenesis of the en¬ dolymphatic duct leads to development of the large vesti¬ bular aqueduct.4 The clinical features of the syndrome be¬ come manifest in early childhood. In both of our cases, the patients were told their hearing loss as children was sec¬ ondary to a virus and high fever. Their subsequent eval¬ uation provided objective evidence of a congenital anom¬ aly. The large vestibular aqueduct syndrome has been recently defined to be a congenital anomaly of the tempo¬ ral bone that predisposes the development in early child¬ hood of an acquired sensorineural hearing loss.5-7 The cause of hearing loss in the large vestibular aque¬ duct syndrome is unclear. The large duct and sac, in and of themselves, do not cause the hearing loss, and are likely a marker of other otopathology. Simpson8 suggested that the hearing loss may be caused by a concurrent maldevelopment of the cochlear membranous labyrinth. One theory proposed that hyperosmolar proteins contained in the large endolymphatic sac reflux into the cochlear duct, causing osmotic-induced damage to the neuroepithelium.5 This theory was supported by Jackler and De La Cruz,4 who felt that the hearing loss was a result of fluctuating increases in CSF pressure transmitted via the endolym¬ phatic sac and duct to the membranous labyrinth. We con¬ cur with the concept of inner ear membrane pathology but speculate on a different mechanism in view of the opera¬ tive findings in case 1. The incessant welling up of clear fluid following the opening of the endolymphatic sac re-
Downloaded From: http://archotol.jamanetwork.com/ by a Karolinska Institutet University Library User on 05/24/2015
6.—Axial ,-weighted magnetic resonance images. The enlarged endolymphatic sacs (white
Fig
are approximately the same signal inten¬ sity as cerebrospinal fluid (C).
arrows)
7.—Axial magnetic resonance images. Left image, ,-weighted image, after gadolinium. The sac, which does not enhance, is seen best on the left side (white arrow). Both sigmoid sinuses enhance after contrast (black arrows). Right im¬ age, Proton density image at the same level dem¬ onstrates the left sac (white arrow), approximately the same signal intensity as cerebrospinal fluid.
Fig
Slow flow in each (black arrows).
fleeted an abnormal communication with a large clear fluid reservoir, specifically CSF. Though speculative, the source of such communication would most likely be through the lateral wall of the internal auditory canal or the cochlear aqueduct. The CT scan showed a normal cochlear aque¬ duct, thus implicating the lateral wall of the internal audi¬ tory canal (spiral foraminous tract and cribrose macula) as the likely source of CSF. This patent communication between the CSF and the vestibule can be theorized to damage the membranous labyrinth by fistula formation from the perilymphatic to the endolymphatic space. An¬ other explanation would be collapse of the membranous labyrinth, increasing the size of the perilymphatic space. In summary, we have reviewed two patients with pro¬ found hearing loss and imaging studies demonstrating large vestibular aqueducts and endolymphatic sacs. The sacs were nearly isointense with CSF on Tr and T2weighted imaging. Pulsation of the brain causes dephasing of protons in CSF and slightly decreases the signal inten¬ sity on T2-weighted images. It is possible, as suggested by some of the images, that the signal intensity of the dilated endolymphatic sacs is somewhat greater than that of CSF on T2-weighted sequences, as the sacs may be more protected from brain pulsations. The sacs' location in the posterior fossa adjacent to the sigmoid sinus and posterior to the internal auditory canal raises the suspicion of the large vestibular aqueduct syndrome, even though the MRI does not show the endolymphatic ducts well. It is impor¬ tant that the radiologist and otolaryngologist not misin-
sigmoid
sinus creates
signal
terpret the enlarged
sacs as some other pathologic condi¬ tion. In particular, acoustic neuromas should enhance after intravenous contrast. If the associated osseous abnormal¬ ities were not recognized, the large endolymphatic sac could be mistaken for CSF surrounding the cerebellar hemisphere. Confirmation of the large vestibular aque¬ ducts can be obtained with thin-section CT. The large vestibular aqueduct syndrome may actually be a manifestation of an abnormal communication between the CSF and bony labyrinth. Examination with temporal bone histologie sections may enhance our understanding of the pathophysiology of this unusual clinical entity.
References 1. Clemis J, Valvassori G. Recent radiographic and clinical observation on the vestibular aqueduct. In: Pulec J, ed. Meniere's Disease. Philadelphia, Pa: WB Saunders Co; 1968:339-346. 2. Valvassori GE, Clemis JD. The large vestibular aqueduct syndrome.
Laryngoscope. 1978;88:723-728.
3. Emmett JR. The large vestibular aqueduct syndrome. Am J Otol. 1985; 6:387-415. 4. Jackler RK, De La Cruz A. The large vestibular aqueduct syndrome.
Laryngoscope. 1989;99:1238-1243. 5. Levenson MJ, Parisier SC, Jacobs M, Edelstein
aqueduct syndrome
in children. Arch
115:54-58. 6. Brogan M, Chakeres
DR. The large vestibular Otolaryngol Head Neck Surg. 1989;
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imaging of the endolymphatic duct and soft tissue of the otic capsule.
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