Key Points
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In congenital sensorineural hearing loss, approximately 20% of inner ears demonstrate a radiographically detectable abnormality.
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Many patterns of deformity appear to result from disturbance in the embryogenesis of the inner ear during the first trimester.
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Enlargement of the vestibular aqueduct is the most common anomaly, followed in frequency by semicircular canal and cochlear deformities.
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Cochlear anomalies apparent on imaging include aplasia, hypoplasia, incomplete partition, and common cavity.
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Semicircular canals may be absent (aplastic) or deformed (dysplastic).
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The eighth nerve may be aplastic or hypoplastic, especially when the internal auditory canal is narrow.
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Congenital ear malformations may be a feature of syndromes such as Pendred and branchio-oto-renal syndromes, CHARGE syndrome, and various trisomies.
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High-resolution T2-weighted magnetic resonance imaging is superior to computed tomography for demonstration of the endolymphatic sac and eighth nerve.
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Hearing loss associated with milder deformities is variable but tends to worsen over time.
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Stapes surgery for pseudoconductive hearing loss carries a high risk for dead ear and occurrence of cerebrospinal fluid (CSF) “gusher.”
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Predilection for stapes gusher due to modiolar defects may be inherited in an X-linked pattern.
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Head trauma may cause sudden decrease in hearing, transotic CSF leak, and recurrent meningitis.
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Surgery on the endolymphatic sac or middle ear windows has not been shown to improve or stabilize hearing.
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Several molecular diagnostic tools for genetic testing are available to complement imaging modalities.
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Cochlear implantation usually is successful but carries a heightened risk for CSF leak, meningitis, and facial nerve stimulation.
Development of the inner ear begins early in embryogenesis. By the end of the eighth week, the membranous labyrinth has assumed its characteristic convoluted shape. Gradual ossification of the otic capsule develops around the membranous labyrinth and is essentially complete by birth. Maturation of the sensory epithelium occurs long after formation of the membranous labyrinth, during the late second and early third trimesters. By weeks 26 to 28 of gestation, hair cell and auditory neural development are largely complete. Thus, the normal human fetus may be able to hear 2.5 to 3 months before birth.
Most inner ear malformations arise when formation of the membranous labyrinth is interrupted during the first trimester of pregnancy. This interruption may be either a result of an inborn genetic error or a consequence of a teratogenic exposure during the period of inner ear organogenesis between the fourth and eighth weeks of gestation. Genetic conditions may be either dominant or recessive and may manifest as sensorineural hearing loss (SNHL) alone or be associated with any of a number of syndromes. A partial list of syndromes associated with radiologically detectable inner ear malformation includes Pendred, Waardenburg, Apert, Wildervanck (cervico-oculoacoustic), branchio-oto-renal, and Alagille syndromes. Nonsyndromic familial inner ear malformations have also been described. Teratogenic influences known to affect inner ear organogenesis include in utero viral infection (e.g., rubella, cytomegalovirus infection), chemical teratogens (e.g., thalidomide), and radiation exposure. Abnormalities in otic capsule structure and deficiencies in the organ of Corti appear to arise as secondary effects of earlier errors in development of the membranous labyrinth. Derangement of the otic capsule ossification process alone does not appear to be a major mechanism in congenital hearing loss. Ossification of the labyrinthine lumen, however, is a common finding in early acquired deafness, typically arising as a consequence of meningitis.
Developmental damage to cochlear structure and function is not restricted to agents that cause gross structural malformations. Even in doses below those that would be ototoxic in the adult, aminoglycoside antibiotics administered in the animal equivalent of the human first trimester of pregnancy cause severe hearing loss in several species. This time frame corresponds to the maturation of the outer hair cells and initiation of the cochlear potentials. Human studies also document this. In 35 of the infants delivered by 72 women who received streptomycin prophylaxis for tuberculosis during the first 4 months of pregnancy, auditory deficits ranging from minor high-frequency threshold elevations to severe bilateral SNHL were noted. The risk is not restricted to the first trimester. Even at low doses, slight hearing losses were noted when aminoglycoside antibiotics were administered during the human equivalent of the last two trimesters of pregnancy, which has obvious implications for use of these drugs in pregnant women and in premature infants who have not yet reached functional cochlear maturity.
Congenital anomalies of the inner ear may be considered in two broad categories: malformations with pathologic changes limited to the membranous labyrinth and malformations that involve both the osseous and membranous portions of the labyrinth. Only patients with malformed otic capsules exhibit abnormalities on inner ear radiographs, which therefore may be diagnosed during life. By inference, children with congenital SNHL and radiographically normal inner ears may be assumed to possess anomalies limited to the membranous labyrinth or neural pathways. Although several types of membranous deformities have been described, their classification is not yet of clinical use because differentiation requires histopathologic examination. Among the deformities that affect the otic capsule, a variety of morphologic patterns may be recognized radiographically, and classification may have prognostic and even therapeutic importance.
Incidence
Most children with congenital sensory hearing loss, with the possible exception of auditory neuropathy, would have detectable abnormalities in their inner ears if they could be examined histologically. Because most children with profound bilateral SNHL have radiographically normal inner ears, it can be inferred that malformations limited to the membranous labyrinth predominate. The incidence of malformations reported varies, depending on the hearing level of the study population, the sophistication of imaging used, and the definition of abnormality used by the observer. As a rule of thumb, approximately 20% of cases of congenital SNHL will demonstrate inner ear malformation with modern imaging technology. In a series of 234 children who had SNHL of varying degrees of severity, Reilly found cochlear anomalies in only 4% of those evaluated by high-resolution computed tomography (CT). With improvements in CT imaging technology and greater awareness of inner ear deformities, Antonelli and colleagues found anomalies in 31% of 157 children with SNHL of variable degree. In another CT study, anomalies were found in 17% of 185 ears of children with SNHL and none in the ears of 309 children without SNHL. A large Korean study found a 22% incidence of anomalies on radiologic imaging in 590 ears with profound SNHL. Although it has been shown that the genetic etiologies of hearing loss can vary greatly among different ethnic groups, large cohorts of hearing loss patients from China and Korea showed similar incidences of anomalies (20% to 22%).
The incidence of deformities of the semicircular canals (SCCs) and inner ear aqueducts has been less well studied than that of cochlear deformities. In a series of patients with radiographically detectable malformations of the inner ear, the cochlea was involved in 76%, the SCCs were involved in 39%, and the vestibular aqueduct (VA) was affected in 32% of ears. The total is more than 100%, because many cases demonstrate abnormalities of more than one portion of the inner ear. In recent years, a heightened awareness of VA enlargement, combined with the greater sensitivity of axial CT scans in demonstrating this deformity, has led to a substantial increase in its detection. The rapid accrual of cases by clinicians interested in inner ear malformations suggests that enlargement of the VA will ultimately prove to be the most common radiographically detectable inner ear anomaly.
Among deaf children with radiographically normal inner ears, histopathologic studies indicate that cochleosaccular dysplasia (Scheibe dysplasia) is by far the most common deformity. Because of the paucity of pathologic specimens available for examination, it is impossible to estimate the relative frequency of the various membranous malformations.
Classification
The traditional nomenclature used to describe congenital anomalies of the inner ear involves a confusing array of eponyms that stem from the first reports of the various morphologic patterns, usually by 18th- or 19th-century authors. In this chapter, a descriptive classification system is used, along with the traditional eponymous terminology, to make this topic more logical, easier to learn, and more clinically relevant ( Box 13-1 ). In membranous malformations, this classification is based on histopathologic changes in the inner ear; in combined osseous-membranous deformities, radiographic appearance is used to distinguish among the various entities. Correct use of the terminology of pathoembryology is important, although these terms frequently are applied imprecisely in the earlier literature. Key terms are aplasia (complete lack of development), hypoplasia (incomplete development), and dysplasia (aberration in development). The classification scheme used in this chapter has proved to be of practical clinical use and has been employed in most published studies in recent years. Other classification schema that categorize observed anomalies have been proposed.
Malformations Limited to the Membranous Labyrinth
Complete membranous labyrinthine dysplasia
Limited membranous labyrinthine dysplasia
Cochleosaccular dysplasia (Scheibe)
Cochlear basal turn dysplasia
Malformations of the Osseous and Membranous Labyrinth
Complete labyrinthine aplasia (Michel)
Cochlear anomalies
Cochlear aplasia
Cochlear hypoplasia
Incomplete partition (Mondini)
Common cavity
Labyrinthine anomalies
Semicircular canal dysplasia
Semicircular canal aplasia
Aqueductal anomalies
Enlargement of the vestibular aqueduct
Enlargement of the cochlear aqueduct
Internal auditory canal anomalies
Narrow internal auditory canal
Wide internal auditory canal
Eighth nerve anomalies
Hypoplasia
Aplasia
Malformations Limited to the Membranous Labyrinth
In malformations limited to the membranous labyrinth, which account for more than 90% of cases of congenital deafness, the bony labyrinth is normal. In its most severe form, membranous dysplasia involves the entire labyrinth, including the cochlea, SCCs, utricle, and saccule. A limited form of membranous labyrinthine dysplasia, involving only a portion of the inner ear, also has been described.
Complete Membranous Labyrinthine Dysplasia
Complete membranous labyrinthine dysplasia was first described by Siebenmann and Bing and is extremely rare. It has been reported in association with Jervell and Lange-Nielsen and Usher syndromes.
Limited Membranous Labyrinthine Dysplasia
Cochleosaccular Dysplasia (Scheibe Dysplasia)
Incomplete development of the pars inferior is the most frequent histopathologic finding in congenital deafness. It was first described by Scheibe and commonly is known as cochleosaccular dysplasia . The spectrum of pathologic findings in this anomaly, which is confined to the cochlea and the saccule, has been well described. The organ of Corti is either partially or completely missing. The cochlear duct usually is collapsed, with Reissner membrane adherent to the limbus. Less commonly, the duct is distended, presumably as a result of endolymphatic hydrops. The stria vascularis typically is degenerated and may contain colloidal inclusions. Schuknecht described characteristic strial changes consisting of aplasia alternating with regions of hyperplasia and gross deformity. Cochlear changes may be severe in the basal turn and gradually lessen in intensity toward the apex, or they may be severe throughout. The saccule usually is collapsed and has degenerated sensory epithelium. In cochleosaccular dysplasia, the SCCs and utricle are normal. Auditory neuronal survival is variable but may remain normal into adulthood, at least in some cases. Cochleosaccular dysplasia also has been demonstrated in a number of animal species, including the deaf white cat, Dalmatian dogs, and various mouse mutants.
Cochlear Basal Turn Dysplasia
Dysplasia limited to the basal turn of the cochlea may be related to familial high-frequency SNHL. No description of membranous labyrinthine dysplasia limited to the pars superior was found in an extensive review of the literature. This outcome is not surprising, because affected persons probably are minimally symptomatic. They would have normal hearing and presumably would have compensated for their congenital vestibular deficit.
Malformations of the Membranous and Osseous Labyrinth
Congenital anomalies of the inner ear that deform the otic capsule are of special interest to the clinician because they may be recognized and differentiated during life through radiographic imaging. As discussed previously, only approximately 20% of congenitally deaf people demonstrate radiographically anomalous inner ears. The clinical manifestations and natural history of these deformities are highly variable. Although some patients are deaf from birth, most maintain some residual hearing into adulthood. Slowly progressive deterioration of hearing during childhood, with eventual stabilization, is common. Sudden decrements in hearing are frequent and may appear to be spontaneous or may be triggered by even minor head trauma. Presumably, most of these cases are due either to internal fistulization secondary to membrane rupture within the cochlea, with admixture of perilymph and endolymph, or to external fistulization to the middle ear. Fluctuating hearing loss is unusual in these patients, and endolymphatic hydrops is an atypical finding. In some patients, hearing may be best preserved in very high frequencies (>8000 Hz), which are not measured by conventional audiometry. Residual ultra-audiometric hearing should be suspected in hearing-impaired children who manifest substantially better auditory function than would be predicted by pure-tone results in the speech frequencies. Occasionally, malformation of the inner ears may be associated with normal hearing. This is especially the case for SCC anomalies. Vestibular symptoms, which occasionally are severe, are present in approximately 20% of patients. Retardation of motor development has been documented in some children with malformed inner ears, particularly those in whom SCCs are absent. A few patients experience vertigo when exposed to loud sounds—the so-called Tullio phenomenon.
A wide variety of morphologic patterns of inner ear malformation has been observed radiographically and may involve the cochlea, SCCs, or VA ( Fig. 13-1 ). Similar diversity has been observed on histologic analysis. A majority of combined osseous and membranous malformations appears to arise from a premature arrest in the development of one or more components of the inner ear ( Fig. 13-2 ). The strongest evidence for this theory comes from the resemblance of most malformed inner ears to the appearance of the inner ear during embryogenesis, particularly between the fourth and eighth weeks of gestation. As a general rule, the earlier the developmental arrest, the more severe the deformity and the worse the hearing will be.
Other anomalies cannot be explained by a premature arrest in development alone and appear to arise from an aberrant embryologic process. An example of this type of anomaly is a cochlea of normal length but abnormal size or coiling geometry. In humans, the inner ear is of adult size at birth and shows strikingly little variation in size among individual patients. Pappas and colleagues suggested that some children with congenital SNHL and apparently normal inner ear morphology on CT possess subtle abnormalities in the dimensions of inner ear structures. These investigators propose that such dimensional variations arise from a teratogenic insult during the second or third trimester, after the membranous labyrinth has formed but before it has reached adult size. Further study is needed to determine the clinical relevance of these observations.
Whereas some inner ear malformations involve only one portion of the inner ear, many patients have a combination of anomalies involving more than one component. Between the fourth and fifth weeks of development, the spheric otocyst develops three buds that ultimately form the cochlea, SCCs, and VA (see Fig. 13-2 ). An inner ear malformation may be limited to one of these anlages, involve a combination of two, or even affect all three.
The frequent coexistence of deformities involving the cochlea, SCCs, and VA has several possible explanations: (1) the anomaly is genetically predetermined; (2) an insult to the embryo occurred before the fifth week; or (3) each of the buds was susceptible to some teratogenic influence at a later stage of development. A majority of inner ear malformations are bilateral and symmetric. In cases in which radiographs detect an anomaly on only one side, the opposite “normal” inner ear has a hearing loss in approximately 50% of cases.
Before the evolution of high-resolution imaging technology, clinicians and histopathologists alike tended to lump these malformations together under the term Mondini dysplasia , after the first report by Carlo Mondini. We are indebted to the late Dr. Peter Phelps and Latin scholar Gordon Hartley for an English translation of Mondini’s original article. Mondini presented his work before the Academy of Sciences of the University of Bologna, describing the inner ear findings in a deaf 8-year-old boy who was struck on the foot by a wagon and later died of gangrene. The cochlea possessed only 1.5 turns and had a hollow apical cavity. An enlarged vestibule and VA also were noted. This deformity is the most common form of cochlear anomaly ( Table 13-1 ). Although numerous other distinct anatomic patterns of inner ear malformation are discernible radiographically and histologically, many workers continue to use the term Mondini dysplasia to describe them all. To avoid a confusing and overly broad nomenclature system, this designation is best reserved for the particular subtype of cochlear malformation first described by Mondini, whether or not it is associated with other inner ear malformations.
| Malformation | Incidence (%) |
|---|---|
| Incomplete partition (Mondini dysplasia) | 55 |
| Common cavity | 26 |
| Cochlear hypoplasia | 15 |
| Cochlear aplasia | 3 |
| Complete labyrinthine aplasia (Michel aplasia) | 1 |
Complete Labyrinthine Aplasia (Michel Aplasia)
The most severe deformity of the membranous and osseous labyrinth, complete labyrinthine aplasia, was first described by Michel. This malformation is exceedingly rare. Presumably, a developmental arrest occurs before the formation of an otic vesicle, resulting in a complete absence of inner ear structures. Complete labyrinthine aplasia has been reported in association with anencephaly and thalidomide exposure. An association with external ear abnormalities also has been reported. A purported case of Michel aplasia actually described a cystic inner ear of the common cavity type. This is but one example of the inaccurate use of traditional eponyms—a frequent occurrence in the literature. The incidence of complete labyrinthine aplasia is overestimated in the radiographic literature because it is confused with labyrinthine ossification. In the latter condition, which usually is acquired during life, a sizable and dense otic capsule is evident radiographically. In complete labyrinthine aplasia, the otic capsule is entirely absent ( Figs. 13-3 to 13-5 ). Such ears are, of course, uniformly deaf.
Cochlear Anomalies
Cochlear Aplasia
In cochlear aplasia, the cochlea is completely absent, presumably as a result of an arrest in the development of the cochlear bud at the fifth week of gestation (see Fig. 13-1 ). This morphologic pattern is rare. Radiographically, only a vestibule and SCCs (usually deformed) are present. To differentiate this anomaly from labyrinthine ossification, it is necessary to assess the amount of otic capsule bone anterior to the internal auditory canal (IAC). In cochlear aplasia, the otic capsule is absent, whereas in osseous obliteration, it is dense and of normal dimensions. Ears with cochlear aplasia are devoid of auditory function.
Cochlear Hypoplasia
An arrest during the sixth week of gestation results in a hypoplastic cochlea consisting of a single turn or less. This deformity comprises approximately 15% of all cochlear anomalies. Radiographically, a small bud of variable length (usually 1 to 3 mm) protrudes from the vestibule ( Fig. 13-6 ). The vestibule frequently is enlarged, with accompanying semicircular malformations in approximately one half of cases. Small cochleae lacking a modiolus or other internal architecture have been described histologically. Hearing is variable in these ears and may be remarkably good in view of the minute size of the cochlea. The variability of hearing presumably is accounted for by the degree of membranous labyrinthine development within the truncated cochlear lumen.
Incomplete Partition (Mondini Deformity)
Arrest at the seventh week of gestation yields a cochlea that has only 1.5 turns. This is the most common type of cochlear malformation, accounting for more than 50% of all such deformities. Radiographically, the cochlea is smaller than normal and partially or completely lacks an interscalar septum ( Figs. 13-7 and 13-8 ). Although the cochlea usually measures 8 to 10 mm vertically, it is typically in the 5- to 6-mm range in incomplete partition deformity. Care must be exercised in counting the number of cochlear turns radiographically because this may be difficult to determine even using high-resolution CT. The radiographic diagnosis depends more on cochlear size and the absence of a scalar septum than on the number of cochlear turns perceived. Histologically, incomplete partition appears to be the radiographic correlate of classical Mondini dysplasia ( Fig. 13-9 ). In numerous reported cases, a small cochlea with 1.5 turns possessing an apical scala communis secondary to deficiency in the osseous spiral lamina has been described. Sennaroglu and Saatci have subtyped the incomplete partition deformity into three variants ( Fig. 13-10 ). Type I lacks the entire modiolus and interscalar septa and demonstrates a cystic appearance. Type II has a normal base turn but a cystic apex (Mondini type). Sennaroglu and Saatci have proposed a type III variant with deficient modiolus and partial interscalar septation at the cochlea’s periphery. Organ of Corti development is variable, as is the auditory neural population. As might be expected, auditory function also is variable, ranging from normal to profound SNHL. The mean hearing threshold (three-tone average) in a group of 41 ears with incomplete partition was 75 dB. SCC deformities accompany incomplete partition of the cochlea in approximately 20% of cases.
Common Cavity
A deformed inner ear in which the cochlea and vestibule are confluent, forming an ovoid cystic space without internal architecture, may be explained by an arrest at the week 4 otocyst stage. Alternatively, this deformity may result from aberrant development at a later stage. An empty ovoid space typically longer in its horizontal dimension is seen radiographically. Although the size of the cyst may vary, it averages 7 mm vertically and 10 mm horizontally. It is quite easy to misdiagnose a dysplastic lateral SCC as a common cavity deformity. The key to differentiating between the two is that a common cavity cochlea lies predominantly anterior to the IAC on axial-plane CT and a dysplastic vestibular system lies posterior to it. Histologically, an ovoid or spherical smooth-walled cystic cavity containing primordia of the membranous labyrinth has been described. Sensory and supporting cells may be differentiated into recognizable organs of Corti that are scattered peripherally around the walls of the cyst. Neural population usually is sparse or absent. Hearing is usually, but not invariably, poor.
Cochlear Hyperplasia
First reported in 2006, three temporal bones with an extra half apical turn have been described histologically. Neither the clinical features nor the radiographic appearance of this anomaly has yet been reported.
Labyrinthine Anomalies
Semicircular Canal Dysplasia
Dysplasia of the lateral SCC is a common type of inner ear malformation ( Fig. 13-11 ). Approximately 40% of ears with a malformed cochlea will have an accompanying dysplasia of the lateral SCC. Occasionally, dysplasia of the lateral SCC exists as the sole inner ear malformation and may not lead to any symptoms. During the sixth week of development, the budding SCC normally forms a semicircular evagination from the vestibular anlage. The central portion of the pocket-shaped protrusion adheres, leaving a peripheral semicircular tube. When this central adhesion fails to occur, SCC dysplasia results ( Fig. 13-12 ; see also Fig. 13-2 ). SCC dysplasia occasionally takes the form of a small bud, rather than the more common half-disk shape, presumably because of a slightly earlier timing of the developmental insult. The lateral SCC is deformed more often than the posterior or superior SCC, apparently because it forms earlier in embryogenesis. The typical radiographic appearance of SCC dysplasia is that of a short, broad cystic space confluent with the vestibule ( Fig. 13-13 ). However, more subtle forms of dysplasia also exist: in a series of patients with hearing loss, the widths of bony islands of the superior and lateral SCC, in addition to cochlear heights, were significantly smaller than those of normal patients.
Numerous histologic descriptions of SCC dysplasia exist in the literature. The half-disk–shaped cavity may contain a rudimentary crista ampullaris. The utricle and saccule may be distended, collapsed, or entirely absent. Caloric responses in SCC dysplasia are functionally absent or reduced in most cases, but a few ears may have normal responsiveness. Ears with malformations limited to the vestibular system often have normal or near-normal hearing. When the cochlea also is abnormal, sensory hearing levels tend to be impaired, to a variable degree. SCC dysplasia appears to have an association with conductive hearing loss, presumably because of inner ear micromechanical factors rather than stapes fixation.
Semicircular Canal Aplasia
SCC aplasia is only one-fourth as common as SCC dysplasia. It is usually associated with cochlear anomalies. Presumably, SCC aplasia arises from a failure in the development of the vestibular anlage before the sixth week of gestation. Most cases are syndromic, with a predominance of CHARGE syndrome ( c oloboma, h eart defects, a tresia of the choanae, r etardation of growth and development, g enital abnormalities, and e ar abnormalities or hearing loss).
Semicircular Canal Dehiscence
As a result of the frequent use of radiographic imaging for the work-up of patients with congenital hearing loss and the increased awareness of superior canal dehiscence syndrome in adult patients, pediatric patients with dehiscence of the SCCs are more commonly being identified. Similar to SCC dysplasia, most reported cases of pediatric SCC dehiscence lack vestibular symptoms, although both conductive and sensorineural hearing loss have been reported. Data from one study suggest that the incidence of hearing loss, both conductive and sensorineural, was not higher in patients with SCC dehiscence. Because this condition frequently leads to conductive hearing loss and vestibular symptoms in the adult population, more longitudinal studies are needed to determine the significance of this radiologic phenomenon in children.
Malformations of the Vestibular and Cochlear Aqueducts
Enlargement of the Vestibular Aqueduct
Experience suggests that enlargement of the VA is the most common radiographically detectable malformation of the inner ear. In earlier literature its incidence was underestimated, partly because of a lack of awareness but mostly because enlargement of the VA could be visualized only by lateral polytomography at a time when most studies were confined to the anteroposterior plane. The advent of high-resolution CT in the axial plane has made assessment of the VA much easier. The diameter of a normal VA, when measured halfway between the common crus and its external aperture, is between 0.4 and 1 mm. Enlargement of the VA is diagnosed when its diameter exceeds 2 mm, although enlarged VAs may exceed 6 mm in width. Whereas the VA is well visualized on axial CT ( Fig. 13-14 ), the dilated endolymphatic sac is better seen with T2-weighted MRI ( Fig. 13-15 ). The sac often is seen to be enormously enlarged on MRI, sometimes measuring 2 cm or more in diameter. In many cases, VA enlargement accompanies malformation of the cochlea or SCC. It also may be the sole radiographically detectable abnormality of the inner ear in a child with hearing loss. This condition is commonly referred to as the large VA syndrome, after Valvassori and Clemis’s first descriptions.
The VA derives from a diverticulum formed in the wall of the otocyst during the fifth week of gestation. It begins as a short, broad pouch but gradually elongates and thins until it achieves its characteristic J shape of adulthood (see Fig. 13-2 ). With normal development, the VA is vascular and rugose in appearance, features which are both thought to be important for normal physiologic function. A premature arrest in development would be expected to produce a VA that is abnormally short and broad. However, an increasing body of evidence suggests that VA enlargement does not stem from an early arrest of sac development but rather is an acquired deformity. Histologic analysis of specimens with an enlarged VA demonstrate that the sac and aqueduct are thin-walled and lack both vascularity and the rugose features.
Three observations suggest that a large VA stems from an abnormal communication between the subarachnoid space and the fluid chambers of the inner ear. The bone surrounding the VA may show signs of erosion, a finding that is inconsistent with a stable congenital deformity. Serial magnetic resonance images show variability in both the size and signal characteristics of the enlarged sac. The presence of cerebrospinal fluid (CSF) under pressure, with consequent “gusher,” within the inner ear has been observed in ears with large VAs during both cochlear implantation and stapedectomy. Furthermore, the existence of abnormal CSF in the inner ear structures has been frequently observed with CT or MRI. An enlarged VA typically consists of a defect of the cochlear modiolus at the distal end of the IAC (modiolar deficiency). For CSF to become confluent with the endolymphatic space, the CSF fistula (subarachnoid space to perilymph compartment) must be accompanied by a second fistula joining the endolymph and perilymph spaces.
The large VA syndrome typically is bilateral and often associated with other radiographic anomalies of the inner ear. Affected children can present with various degrees of hearing impairment ranging from mild to profound, although at least 40% eventually develop profound SNHL. The typical and dreaded progression of this syndrome is the gradual deterioration of auditory function through childhood into adolescence and early adulthood. As with other inner ear malformations, there is a tendency to suffer sudden decrements of hearing, particularly after head trauma. Conductive hearing loss may be present but is likely to be from intracochlear micromechanical disturbances (also known as the third window effect) rather than from impairment of ossicular mobility. Stapes surgery is best avoided because of the risk of CSF otorrhea. Vestibular complaints in these patients have frequently been reported. Large VA syndrome has been observed to occur in familial clusters and can be associated most notably with Pendred, branchio-oto-renal, or Waardenburg syndrome. However, other syndromes may also exhibit the radiographic abnormality of an enlarged vestibular aqueduct. Fifty percent of patients with a unilateral large VA experience contralateral hearing loss, suggesting that genetic mutations are a common cause of large VA syndome.
The natural course of many patients with large VA syndrome is fluctuating yet progressive hearing loss (51% stable, 28% fluctuated, and 21% progressively worsened). Limited studies have found that steroid administration is potentially helpful in preventing the progression of hearing loss. Surgical manipulation of the endolymphatic sac has been attempted in patients with large VAs. In a sizable series of shunt procedures, there was a high incidence of postoperative hearing loss. Furthermore, a comparison of the operated and contralateral unoperated ears revealed no evidence of hearing stabilization after surgery. Recently, surgical obliteration of the enlarged endolymphatic sac with muscle and fascia has been advocated. Results in a multicenter study indicated that the procedure not only lacks efficacy but actually injures half of the operated ears. Cochlear implantation has been quite successful in both adults and children with large VAs.
Enlargement of the Cochlear Aqueduct
Enlargement of the cochlear aqueduct (CA) frequently is mentioned in the otologic literature because of its purported association with stapedectomy gusher and transotic CSF leak. Despite an active interest in the field and an extensive review of published radiographic images, the authors have never seen a radiograph that convincingly demonstrated CA enlargement. Most cases presented as enlargement of the CA are misinterpretations of the wide internal funnel that opens into the posterior fossa. In healthy subjects, the radiographic diameter of this aperture averages 3 to 4 mm but ranges from radiographically invisible to more than 10 mm ( Figs. 13-16 and 13-17 ). For enlargement of the CA to be diagnosed radiographically, the intraosseous portion coursing toward the vestibule must be enlarged beyond 1 mm, the practical resolution limit of contemporary CT scanners. If criteria analogous to those for enlargement of the VA are used, then an enlarged CA must have a diameter exceeding 2 mm throughout its course between the inner ear and the posterior fossa.
The preponderance of evidence suggests that the human CA usually is functionally patent. In most persons, however, it can neither transmit sudden large pressure changes to the inner ear nor allow free flow of spinal fluid in any significant quantity (e.g., with outflow of perilymphatic fluid from a perilymphatic fistula or an open vestibule). This observation reflects two anatomic features of the aqueduct: a narrow diameter of the bony channel and the presence of a fibrous tissue meshwork occupying and baffling the lumen. Schuknecht and Reisser evaluated more than 1400 temporal bones in the Massachusetts Eye and Ear Infirmary collection and found no CA that exceeded 0.2 mm at its narrowest point. This series included 29 congenitally malformed inner ears. Of note, the CA was absent or nonpatent in 21 of these dysmorphic ears.
Some persons have less than normal or no fibrous tissue within the lumen, and the bony diameter is wider than normal. An opportunity may then exist for free flow of CSF from the oval window through the CA. According to Poiseuille’s law, fluid flow through a tube varies with the fourth power of the radius, suggesting the possibility of free flow in an individual with a large-diameter unimpeded channel, as discussed by Allen. Several histologic studies of congenital inner ear malformations have detected slightly larger CAs, particularly at their lateral orifice at the vestibule. Even in these malformed inner ears, however, the aqueduct diameter remained well in the submillimeter range. From a clinical standpoint, a CA less than 1 mm in diameter is undetectable radiographically. Despite the theoretic possibility that a widely patent CA may cause a stapedectomy gusher, most gushers can be attributed to abnormal connections between the IAC and the vestibule.
Developmental Anomalies of the Internal Auditory Canal
Wide Internal Auditory Canal
Unlike congenital narrowness of the IAC, a congenitally large IAC may be an incidental finding in healthy individuals ( Fig. 13-18 ). When a large IAC (>10 mm in diameter) accompanies a malformation of the inner ear, it does not, as an independent variable, correlate with the level of hearing. One report suggests that a wide IAC may be correlated with hearing loss. The primary importance of detecting enlargement of the IAC is its association with spontaneous CSF leak and the occurrence of gusher during stapes surgery. Because hearing after stapedectomy complicated by CSF gusher (often erroneously called perilymph gusher ) frequently is poor, a CT scan should be obtained before stapedectomy to address congenital fixation. Dilation of the IAC, especially when the partition between the lateral end of the canal and inner ear appears deficient, should contraindicate stapedectomy.
Narrow Internal Auditory Canal
A narrow IAC may indicate a failure of eighth cranial nerve development. When a patient has normal facial function and an IAC less than 3 mm in diameter, it is likely that the bony canal transmits only the facial nerve ( Fig. 13-19 ). A narrow IAC may accompany inner ear malformations or may be the sole radiographically detectable anomaly in a deaf child. Interestingly, in patients with atresia of the IAC, the facial nerve may take an aberrant course to establish facial motor function. A narrow IAC has been considered a relative contraindication to cochlear implantation, because it suggests that the eighth nerve may be insufficiently developed to conduct an auditory signal. After cochlear implantation, some patients with narrow IACs have experienced facial pain and twitching without useful auditory sensation. The literature also describes rare cases of bilateral, duplicated IACs that were associated with labyrinthine anomalies.
Anomalies of the Eighth Nerve
Hypoplasia and aplasia of the eighth nerve are often, but not inevitably, associated with congenital narrowness or even absence of the IAC. Similarly, although eighth nerve maldevelopment frequently accompanies malformation of the inner ear, the presence of a normal cochlea and SCCs does not guarantee normal development of the audiovestibular nerve. High-resolution, thin-section MRI with T2-weighted sequences currently is the best means of assessing the fine anatomy of the eighth nerve in the IAC. Such a study is warranted before cochlear implantation when the bony IAC is narrow on the CT scan, with severe types of inner ear malformation, and in syndromes known to be associated with maldevelopment of the eighth nerve (e.g., CHARGE syndrome and Möbius syndrome). Unilateral cochlear nerve aplasia, sometimes familial, is increasingly recognized as an important cause of congenital unilateral profound SNHL. The IAC frequently is normal. Increasingly refined magnetic resonance techniques may reveal hypoplasia of the auditory nerve as more common than previously recognized.
Molecular Diagnoses Associated with Congenital Inner Ear Malformations
Inner ear malformations have been described in an increasing number of recognized syndromes associated with SNHL. Some of the more common examples are described and a more comprehensive list can be found in Table 13-2 . As a group, Waardenburg syndrome patients can demonstrate absence of any of the three SCCs (17%) and cochlear hypoplasia (8%). In Waardenburg syndrome type I, the labyrinth is frequently normal and the IAC is occasionally narrow (11%). In Waardenburg syndrome type II, maldevelopment of the posterior SCC has been described. SCC agenesis or hypoplasia also has been noted in CHARGE syndrome. In branchio-oto-renal syndrome, which is most commonly caused by mutations of the EYA1 gene among others, hypoplasia of the cochlea, sometimes accompanied by large VA, has been reported. In Pendred syndrome, large VA is a common feature, sometimes accompanied by modiolar deficiency. Otic capsule anomalies also have been described in trisomy 18 and Usher syndrome. Down syndrome (trisomy 21) is associated with SCC dysplasia and a variety of other inner ear malformations.
