Chapter 48. Congenital Disorders of the Middle Ear

Initial Stages of Development

Beginning at week 4, the tubotympanic sulcus develops as an extension of the endodermal epithelium of the first pharyngeal (branchial) pouch and eventually forms the middle ear canal and the eustachian tube. The tubotympanic recess has elongated and constricted to form the primordial tympanic cavity and eustachian tube by week 8. Simultaneously, the expanding end of the tubotympanic sulcus comes into proximity with the medial aspect of the ectodermal first pharyngeal cleft, the primordial external auditory canal. Although intimately related, the two linings remain separated by a layer of mesenchyme known as the pharyngeal membrane. This trilaminar relationship develops into the adult tympanic membrane, which comprises the outer cutaneous, middle fibrous, and inner mucosal layers. As the middle ear cavity expands, the tympanic sinus is created by the pneumatization of already ossified temporal bone. By 9 months, pneumatization of the tympanum and epitympanum is virtually complete. At the same time, the mastoid antrum is formed by the growth of the tympanic cavity into the mastoid portion of the temporal bone. The attachment of the sternocleidomastoid on the temporal bone promotes the formation of the mastoid process. Although the development of the mastoid air cells begins in fetal life, full maturation does not occur until age 2.

Early in development, the middle ear cavity is filled with loose mesenchyme that spans the gap between the primordial tympanic membrane and oval window. However, during the last 2 months of pregnancy, this mesenchyme is systematically reabsorbed, leaving the nearly mature ossicles suspended in the middle ear cavity. Beginning sometime between weeks 4 and 7, a condensation of neural crest ectoderm embedded within the mesenchyme begins to form the ossicles. Meckel cartilage, which is derived from the first pharyngeal (branchial) arch, gives rise to the head of the malleus and the body of the incus (ossicle portions above the tympanic membrane). The remainder of Meckel cartilage develops into the mandible and sphenomandibular ligament (Meckel ligament). The first pharyngeal arch is also associated with the mandibular division of the trigeminal nerve, the muscles of mastication, the tensor tympani muscle, and the tensor veli palatini muscle. The second pharyngeal arch gives rise to Reichert cartilage, which eventually forms the manubrium of the malleus, the long process of the incus, stapes suprastructure, and the tympanic portion of the stapes footplate (ossicle portions below upper limit tympanic membrane). The vestibular portion of the stapes footplate derives from the otic capsule. The facial nerve, the muscles of facial expression, the stapedius muscle, the upper portion of the hyoid bone, and the stylohyoid ligament are also derived from the second pharyngeal arch mesoderm. It is important to note that although the pharyngeal arches are mesenchymal, the ossicles are derived from neuroectoderm that is embedded within the mesenchyme. This partly explains the association between ossicular malformations and disorders of neuroectoderm.

Stapes

The stapes requires the longest period of development and is therefore the most frequently malformed. The earliest stages of development begin at 4 weeks, and ossification does not occur until week 26. Development of the stapes footplate is induced by a depression on the otic capsule, the lamina stapedialis. This occurs between weeks 6 and 9. Ultimately, the lamina stapedialis becomes the annular ligament and the vestibular portion of the footplate. Failure of this precise association between the stapes footplate and the lamina stapedialis may result in a malformed or atretic oval window.

The primordial stapes is characterized as a chondral ring. Resorption, periosteal erosion, and ossification shape this cartilaginous precursor into an adult-like ossified stirrup. As a result of this developmental process, the adult stapes is fragile; a “plate” of endosteal bone overlying the original layer of cartilage forms the head and base, and thin periosteal bone makes up the crura. This contrasts with the relatively dense incus and malleus, which form from the repeated layering of endosteal bone on a cartilaginous framework. Furthermore, in contrast to the stapes, the malleus and the incus do not undergo morphologic changes, which minimizes the complexity of the shaping process and the potential for error.

Malleus & Incus

The developmental process of the malleus and incus is rapid. The chondral elements reach adult size by week 15 and are fully ossified skeletal structures by week 25. Before the full development of the ossicular ligaments, projections from the endodermal lining of the middle ear cavity help to support the position of the ossicles. Invaginations of the endodermal lining between the ossicles also serve to separate the developing ossicles from each other and from the walls of the tympanic cavity. Failure of this results in ossicular fusion. The articulations between the ossicles develop early, with the incudomalleolar joint forming at 7 weeks. Adult size and relationships are fully established by the 9th month. Full ossicular mobility, however, does not occur until 2 months after birth, when the mesenchyme of the middle ear cavity is fully reabsorbed.

Stapedial Artery

The developing intracranial vasculature originates from six paired aortic arches and their associated arteries. During the 4th week of development, the stapedial artery arises from the hyoid artery (second aortic arch) near the origin of the proximal internal carotid artery (ICA) (third aortic arch). It enters the anteroinferior quadrant of the middle ear and courses over the promontory and through the primordial stapes to form the obturator foramen. It then proceeds anteriorly to pierce the horizontal facial canal and enter the cranial cavity. The artery subsequently divides into an upper (supraorbital) division and a lower (maxillomandibular) division. The supraorbital division provides the vasculature to the orbit and to the supraorbital areas early in fetal development. However, as the ophthalmic artery matures to assume these distributions, the supraorbital division largely involutes and persists as the middle meningeal artery. The maxillomandibular division exits the cranial cavity through the foramen spinosum and contributes to the fetal vasculature of the lower face, as well as to the inferior alveolar and infraorbital areas. By the third month, this division is largely replaced by branches of the external carotid artery. The proximal trunk of the stapedial artery normally atrophies, whereas the distal portion, the middle meningeal artery, persists and is supplied by the external carotid artery.

Jugular Vein Anomalies

Essentials of Diagnosis

• Dehiscence of the jugular bulb may lead to aberrant position within the middle ear.
• This may be asymptomatic or may lead to tinnitus or conductive hearing loss.
• Visible on CT and MRI/MRA.
• Avoidance is most prudent management.

Between the third and fourth weeks of development, paired cardinal veins first appear in the primordial neck. The cranial portion of the anterior cardinal vein ultimately gives rise to the internal jugular vein, whereas the cephalad portion forms the jugular bulb. The sigmoid sinus and the inferior petrosal sinus converge at the jugular bulb, which drains into the jugular vein in the neck. Normally surrounded by a layer of bone within the jugular fossa, the bulb is subject to congenital dehiscence and an aberrant position within the middle ear. A “high-riding” bulb may be defined anatomically as a bulb that rises above the inferior aspect of the bony annulus or the basal turn of the cochlea. It is present in 5% of temporal bone specimens and may be related to the poor pneumatization of the mastoid air cells and middle ear. The bony covering of the bulb may be thin or absent, resulting in dehiscence and protrusion into the middle ear cavity. Tinnitus, vestibular symptoms, and conductive hearing loss due to ossicular, tympanic membrane, or round window compression have been described. However, dehiscent jugular bulbs are often discovered incidentally on otoscopic examination. Typically, a blue mass is seen in the posteroinferior quadrant of the tympanic membrane.

Contrast-enhanced computed tomography (CT) scanning, magnetic resonance imaging (MRI), and magnetic resonance angiography (MRA) help delineate a vascular mass in the middle ear, whereas a high-definition temporal bone CT scan will reveal a bony defect in the floor of the hypotympanum. Venography may differentiate this lesion from other vascular masses in difficult cases. The lack of a fascial covering over the jugular bulb predisposes it to inadvertent laceration during myringotomy. Therefore, avoidance during middle ear surgery represents the most judicious management of these lesions.

Internal Carotid Artery Anomalies

Essentials of Diagnosis

• Agenesis, aneurysm, and aberrancy of the intratemporal carotid artery have been described.
• Symptoms include hearing loss, pulsatile tinnitus, aural fullness, otalgia, and vertigo.
• Pulsatile red mass is seen in the middle ear.
• Imaging studies differentiate this from other vascular lesions.

General Considerations

Anomalies of the intratemporal ICA are extremely rare. Typically, there is a female preponderance, and these anomalies first present in the third decade of life with conductive hearing loss, bloody otorrhea, headache, pulsatile tinnitus, or cranial nerve palsies. Conductive hearing loss is due to impingement by the aneurysm on the ossicles or tympanic membrane. Otoscopic exam may reveal a red and pulsatile mass in the middle ear or blood in the external auditory canal. However, it is presumed that most intratemporal aneurysms of the ICA are asymptomatic and go unrecognized.

Pathogenesis

The ICA normally enters the carotid canal in the petrous portion of the temporal bone medial to the styloid process. The initial vertical segment is anterior to the cochlea, separated from the internal jugular vein by the carotid ridge and from the tympanic cavity by a thin bony wall, 0.5 mm thick. When laterally displaced, this portion of the ICA is found in the hypotympanum with possible extension over the oval window. Displacement of the tympanic membrane and ossicles, as well as erosion of the cochlear promontory, may also be present. Although temporal bone studies have revealed an incidence of <1% of an aberrant carotid artery, gross and micro-dehiscences of the carotid canal have a reported incidence of 7% and 15%, respectively.

Multiple etiologies for an aberrant ICA have been proposed, including (1) agenesis of the bony carotid canal; (2) lateral traction of the ICA by persistent embryonic vessels (eg, stapedial artery); and (3) agenesis of the vertical ICA with compensatory vascular communication from branches of the developing external carotid artery (ECA) system. The latter theory also explains the association of aberrant ICAs with other vascular anomalies, such as persistent stapedial artery (PSA).

Clinical Findings

Symptoms and Signs

The presenting signs and symptoms of an aberrant ICA include pulsatile tinnitus, otalgia, aural fullness, vertigo, hearing loss (61% conductive, 6% sensorineural, and 33% normal) and a pulsating, red mass in the anteroinferior quadrant of the middle ear. There may be a right-sided predominance of this anomaly, and bilateral involvement has been described.

Although hypoplasia, agenesis, aneurysm, and aberrancy of the ICA have all been reported, the low incidence of these lesions demands a high clinical suspicion if disastrous complications are to be avoided. Agenesis and hypoplasia are most often found incidentally on radiographic imaging and may be unilateral or bilateral. These lesions may remain clinically silent since they may be well compensated by the vertebrobasilar, external carotid, or contralateral internal carotid systems. Alternatively, they may present with neurologic symptoms secondary to cerebral insufficiency or aneurysm formation. The latter occurs in 24–34% of cases.

Imaging Studies

Radiographic imaging is essential and should include high-resolution CT scanning of the temporal bones (Figure 48–1), MRA, and angiography. CT and MRA are noninvasive and may delineate the vasculature and bony anatomy. A temporal bone CT scan in patients with carotid agenesis shows the complete absence of the petrous carotid canal. Angiographic findings include persistent fetal branches of the ECA, such as the hyoid, caroticotympanic, inferior tympanic, and stapedial arteries, as well as other intracranial vascular anomalies. Findings suggestive of an aberrant ICA include the following: (1) a vascular mass in the hypotympanum, (2) an enlargement of the inferior tympanic canaliculus, and (3) a lack of bony canal wall over the vertical ICA. This last feature helps distinguish aberrancy from a glomus tumor.

Several clinicians advocate angiography as the gold standard in the diagnosis of vascular lesions of the middle ear. The classic angiographic finding of an aberrant ICA is identification lateral to a vertical line drawn through the lateral border of the vestibule. Angiography also allows for occlusion testing to define the adequacy of the contralateral carotid circulation if ligation is to be considered.

Differential Diagnosis

The rarity of ICA anomalies dictates that a broad differential diagnosis for vascular masses of the middle ear be considered. Also included in this list are glomus tympanicum, glomus jugulare, vascular tumors of the temporal bone, dehiscent jugular bulbs, arteriovenous malformations, and arterial fistulas.

Treatment

The treatment of aneurysms and aberrancy of the ICA should be determined on a case-specific basis. Most authors agree that if the patient's only symptom is pulsatile tinnitus or if the patient is asymptomatic, the lesions may be followed expectantly. Indications for definitive therapy include debilitating or progressive symptoms, the prevention of aneurysm formation, embolic phenomenon from an aneurysm, and the destruction of middle ear structures. Aneurysms may be embolized during angiography. Covering an aberrant vessel with fascia, a bone graft, or a Silastic (ie, polymeric silicone) sheet has been described but carries a significant risk of distal ischemia from compression. Inadvertent injury to an aberrant or aneurysmal ICA during myringotomy or middle ear surgery may result in severe hemorrhage. In these situations, the middle ear should be tightly packed. If this fails, surgical ligation of the internal or common carotid artery may be necessary to prevent exsanguination.

Persistent Stapedial Artery

Essentials of Diagnosis

• Usually asymptomatic, but may cause pulsatile tinnitus and hearing loss.
• May be associated with other anomalies and may complicate middle ear surgery.
• Retraction or avoidance may be the most prudent management.

A persistent stapedial artery (PSA) is a rare vascular anomaly of the middle ear. The reported prevalence of 0.48% in cadaveric studies of temporal bones is significantly less than the 0.02–0.05% found in surgical series.

Normally atrophied by 3 months of fetal development, the stapedial artery may persist as a 1.5- to 2.0-mm branch of the petrous ICA. As a result of this anomaly, the middle meningeal artery arises from the stapedial artery, and the foramen spinosum is absent. Although pulsatile tinnitus, conductive hearing loss, and sensorineural hearing loss have been described, most cases are clinically asymptomatic and found incidentally at the time of middle ear surgery. Case series have also noted multiple congenital anomalies associated with PSA, including aberrant ICA, Paget disease, anencephaly, anomalous stapes, anomalous facial nerve, thalidomide deformities, and trisomies 13 and 15.

Although the pathophysiology of these associations is poorly understood, an awareness of its possible existence at the time of surgery remains the most important aspect of treating PSA. Inadvertent transection during exploration of the middle ear may result in profuse hemorrhage. This has been described as a complicating factor for cholesteatoma surgery, stapes surgery, and cochlear implants. Some clinicians have described surgical ligation at the time of exploration, but this poses a theoretical risk of ischemic stroke. Generally, avoidance or retraction of a PSA is advocated.

Congenital Cholesteatoma

Essentials of Diagnosis

• Commonly present as small pearl in the anterosuperior quadrant of the mesotympanum.
• May result from persistence of fetal epidermoid rests.
• Often asymptomatic and not associated with a history of otitis media, tympanic membrane perforation, or eustachian tube dysfunction (white mass behind normal drum).
• Timely surgical removal is indicated to avoid complications.

General Considerations

Historically, congenital cholesteatoma has been defined as a middle ear cholesteatoma in the presence of an intact tympanic membrane without a history of perforation, otitis media, otorrhea, or otologic surgery. However, subsequent physicians have argued that these findings should not represent exclusionary criteria for congenital cholesteatoma, given the high incidence of middle ear infections or effusions in the general population.

Several features help distinguish acquired from congenital cholesteatoma. Patients with acquired lesions present in the setting of frequent episodes of otitis media, structural pathology of the tympanic membrane, eustachian tube dysfunction, and diseased mastoid cavities. The developing mass is frequently symptomatic, causing otorrhea, otalgia, and hearing loss, and on examination it is found to expand in direct continuity with a tympanic membrane perforation or retraction pocket.

In contrast, congenital cholesteatomas are not associated with a history of recurrent otitis media and develop in the setting of a normal tympanic membrane, a functional eustachian tube, and a well-aerated mastoid cavity. Furthermore, they are often clinically silent and discovered on routine examination.

Pathogenesis

Multiple theories have been put forth to describe the pathophysiology of congenital cholesteatoma. The presence of the epidermoid formation in the anterior epitympanum of the developing fetal temporal bone between weeks 10 and 33 of gestation has been described. This implies that congenital cholesteatoma of the anterosuperior quadrant may result from a failure of normal involution of this epidermoid tissue. This theory, however, does not account for posterior lesions. Proposed etiologies of posterior congenital cholesteatoma include the posterior migration of anterior epidermoid tissue, presence of amniotic cellular material in the middle ear, or ingrowth of external canal epithelium through a defect in the tympanic ring. To date, no single theory has been able to adequately account for the clinical spectrum of congenital cholesteatoma.

Clinical Findings

Symptoms and Signs

Lesions occurring “classically” in the anterosuperior quadrant of the mesotympanum account for anywhere from 27% to 67% of all cases. These typically present as small pearls adjacent to the long process of the malleus, with minimal ossicular involvement or hearing loss. Lesions in the posterosuperior mesotympanum, considered a minor variant in older series, develop near the incudostapedial joint and have recently been reported to account for 33–78% of all congenital cholesteatomas. These tend to be larger, with more frequent ossicular involvement and hearing loss. Bilateral congenital cholesteatoma (3% of cases), as well as extension into the epitympanum, sinus tympani, and facial recess, has been described.

Congenital cholesteatoma in any location is often clinically silent for years but may eventually present with a combination of tinnitus, vertigo, 30–40 dB conductive hearing loss, or sensorineural hearing loss. Although classically described as occurring in an ear free of structural pathology, a congenital cholesteatoma presenting at an advanced stage may perforate through the tympanic membrane or obstruct the eustachian tube and predispose to otitis media, making the distinction between a congenital and an acquired lesion difficult. No single symptom complex is diagnostic for congenital cholesteatoma, although the presence of a discrete, round white lesion seen in the anterosuperior quadrant of an otherwise normal tympanic membrane is suggestive. There is a male predilection, with a male-to-female ratio of approximately 2–3:1. The average age at presentation is 2–4 years for anterior lesions and 12 years for posterior lesions.

Special Tests

The diagnosis of a congenital cholesteatoma is clinical and based initially on the history and otoscopic examination. Audiometry is performed to evaluate and document preoperative hearing, and temporal bone CT scans help determine the extent of disease (Figure 48–2). A common finding in these studies is a well-aerated mastoid cavity, in contradistinction to patients with acquired cholesteatoma.

Treatment

The management of congenital cholesteatoma is timely surgical removal. Nonoperative intervention or observation may result in progressive growth of the lesion with progressive erosion of the ossicles. Although multiple surgical approaches have been advocated, the goal of complete extirpation remains universal. Most lesions located in the anterior mesotympanum may be successfully removed via a postauricular transcanal approach. Posterior or extensive lesions may also require an atticotomy or mastoidectomy to enhance exposure. In contrast to acquired cholesteatoma, congenital lesions result in minimal inflammatory reactions or adhesions between the matrix and the middle ear mucosa. A clear plane can be easily developed between the cholesteatoma and the surrounding mucosa of the middle ear or ossicles, especially if prior surgeries have not been performed. Cases in which a clear plane cannot be developed between the tympanic membrane and the cholesteatoma mass may necessitate removing the tympanic membrane and performing a lateral graft tympanoplasty.

The mastoid air cells in patients with congenital cholesteatoma tend to be well pneumatized, and bony resection to the limits of the lesion in cases involving the mastoid cavity often spares the majority of the air cells. Most clinicians, therefore, recommend intact canal wall procedures in patients with congenital cholesteatoma in an effort to prevent creating a large open cavity with its associated lifelong burden of care. Congenital cholesteatoma may be associated with ossicular erosion (most commonly the incus). Ossicular reconstruction can be staged with a “second look” procedure and ossicular reconstruction 6 months later.

Prognosis

Congenital cholesteatomas recur in approximately 30–55% of cases after surgical removal. The incidence of recurrence is notably higher in patients with involvement of the posterosuperior quadrant, attic, or mastoid. The mean time to recurrence ranges from 8 to 14 months in patients with disease limited to the middle ear, and 30 months for patients with more extensive disease. The high rate of recurrence in patients with a history of surgery for congenital cholesteatoma requires that these patients be followed clinically for a significant period of time.

Essentials of Diagnosis

• May occur in isolation or as part of a syndrome.
• Varying degrees of stable, conductive hearing loss may be present.
• Treatment is individualized based on the ossicular lesion, the patient's overall health, and the degree of hearing loss.

General Considerations

Ossicular anomalies may be unilateral or bilateral and may be associated with anomalies of the external ear (atresia) or the middle ear (facial nerve, stapedial muscle, tendon, or pyramidal eminence), or with a multiorgan syndrome (Treacher Collins or Goldenhar syndrome) (Table 48–1).

Classification

Historically, congenital malformations of the ear have been divided into major and minor types with the latter limited to the middle ear alone. Teunissen's classification system is based on the site of involvement and aids in determining the appropriateness for surgery (Table 48–2).

Surgical experience in patients with stapes ankylosis (Class I) and stapes ankylosis combined with ossicular anomaly (Class II) has been favorable, with a 73% rate of postoperative air–bone gap <20 dB. In contrast, patients with mobile footplates, ossicular discontinuity, epitympanic fixation (Class III), or dysplasia of either the round or oval windows (Class IV) are poor surgical candidates. Finally, multiple ossicular anomalies with subtle but significant variations have been described (Figure 48–3 and Table 48–3).

Clinical Findings

Symptoms and Signs

Isolated ossicular anomalies are rare, as is evident from large retrospective reviews of otologic practices documenting only dozens of cases. Approximately 1–2% of patients with congenital conductive hearing loss have isolated middle ear anomalies. In addition to this low incidence, other factors make accurate preoperative diagnosis difficult. With the exception of malleus–incus fusion, hypoplasia of the malleus, and middle ear aplasia, the otoscopic examination is unremarkable. Furthermore, audiometric evaluation demonstrates a similarly moderate-to-severe conductive hearing loss that is fixed over time with most anomalies. These factors mandate a high index of suspicion to ensure both an accurate diagnosis and an appropriate management.

A general examination of the patient is performed to evaluate the overall health and to search for any findings suggestive of a syndrome. Otoscopic examination of patients with anomalies of the malleus or combined ossicular anomalies may demonstrate loss of the tympanic membrane landmarks. Furthermore, the noninvolved ear should be evaluated for possible bilateral disease (25–40%).

Specials Tests and Imaging Studies

Audiometry demonstrates a stable moderate-to-severe conductive hearing loss, determines the severity of the air–bone gap, excludes bilateral or sensorineural disease, and can differentiate between ossicular discontinuity and fixation. A speech reception threshold worse than 30 dB has been defined as an indication for surgery. A high-resolution CT scan of the temporal bones may define the ossicular anomaly as well as the anatomy of the facial nerve and inner ear structures.

Treatment

Multiple factors are important in determining a patient's candidacy for operative intervention. Children with multisystem syndromes may be at a considerably higher anesthetic risk if there is involvement of the upper airway, heart, lungs, or kidneys. For children in overall good health, the indications, timing, and ideal method of surgical correction remain a source of controversy. The patient should be assessed for a delay in speech acquisition and the presence of cognitive and learning delays. Deferring surgery until at least the age of 5 is associated with a decreased incidence of otitis media, improved patient cooperation, and more sophisticated audiometric testing.

Surgery for unilateral disease in the setting of a normal contralateral ear can be either performed at the age of 5 or delayed until the patient is able to participate fully in the decision-making progress. Delaying surgery remains a source of controversy since the beneficial effects of binaural hearing on speech and development are continually being discovered. Finally, amplification with hearing aids, as a transition or an alternative to surgery, should be offered to the patient and family.

Malleus Anomalies

Although multiple anomalies of the malleus have been described, the incidence is lower than anomalies of the incus or stapes. Hypoplasia or aplasia of the malleus results from a failure of embryogenesis between weeks 7 and 25. Given the common pharyngeal arch origin, hypoplasia of the malleus is often associated with hypoplasia of the incus. An ossicular replacement prosthesis can be placed at the time of middle ear exploration in these cases. Fixation of the head of the malleus represents 80% of all isolated congenital anomalies of the malleus (Figure 48–4). Exploration of the temporal bone in these patients reveals bony bridges between the head of the malleus and the lateral epitympanum in 75–80% of cases. The term “malleus bar” has been used when this bridge connects to the posterior tympanic wall. Malleus fixation, in general, is presumed to result from failure of mesenchymal absorption and is correctable by either laser division of the connecting bridge or resection of the head and placement of a stapes to a manubrium ossiculoplasty prosthesis.

Incus Anomalies

Hypoplasia or aplasia of the incus typically occurs in conjunction with hypoplasia of the malleus but may occur in isolation. Ossicular replacement techniques used for patients with acquired erosion of the incus secondary to chronic otitis media are, similarly, effective for patients with these congenital lesions. The incus is also susceptible to fixation to the epitympanum. Treatment involves sectioning of the bony bridge with a laser.

Stapes Anomalies

Isolated congenital anomalies of the stapes represent approximately 40% of all congenital ossicular lesions. The stapes requires the longest period of embryologic development and, therefore, has the greatest potential for malformation. In addition, the stapes is derived from both branchial arch and otic capsule precursors, adding to the complexity of the development of this ossicle.

Numerous anomalies with variable morphologies exist. Congenital stapes footplate fixation is the most common isolated ossicular anomaly and is thought to result from ossification of a portion of cartilage in the annulus of the oval window. Footplate fixation also frequently occurs in BOR syndrome and Apert syndrome. Surgical therapy involves either stapedotomy or total stapedectomy. Stapes surgeons should be cautioned, however, due to a rare syndrome known as X-linked stapes gusher. Surgery in these patients may lead to a CSF leak as well as a total hearing loss, and as such it should be avoided. Patients are identified by a bulbous internal auditory canal on CT scan as well as an incomplete separation of the basal turn of the cochlea from the fundus of the internal auditory canal.

Although aplasia of the stapes is rare, multiple forms of hypoplasia that include small or absent crura and small, blob-like stapes have been described. The surgical options include total stapedectomy or stapedotomy. In contrast, isolated hyperplasia of the stapes is often an incidental finding that does not require therapy. This anomaly is thought to result from a failure of the resorption and remodeling that occurs during the final stages of stapes development and accounts for up to 20% of all ossicular anomalies.

Several crural anomalies have been described, including thin, absent, fused, and angled crura. The crura may also be replaced with a columella-like structure. Laser resection followed by stapes prosthesis replacement is effective in symptomatic crural lesions and columella stapes. The incomplete absorption of mesenchyme may result in bony bridges between the facial canal and either the head or the crus of the stapes that result in a symptomatic conductive hearing loss. Laser division of these bony attachments is effective. Equally effective is laser removal of a bony bar that spans the pyramidal eminence and stapes neck in cases of ossified stapedius tendon.

Multiple Ossicular Anomalies

Ossicular anomalies involving more than one ossicle occur as frequently as isolated anomalies. Complete agenesis of the ossicles occurs in conjunction with multisystem syndromes (eg, DiGeorge syndrome) and is not amenable to reconstruction. Fusion of the heads of the malleus and incus results from a failure of formation of the incudomalleolar joint at 7 weeks and is a common finding in aural atresia (Figure 48–5). Furthermore, the handle of the malleus is typically fixed to the atretic plate and posterior canal wall. This placement, in combination with the atresia of the external auditory canal, results in a maximal conductive loss.

Many management strategies have been proposed for appropriate surgical candidates. The incus–malleus complex may be removed and replaced with a partial ossicular replacement prosthesis. Alternatively, a combination of laser and a drill may be used to enlarge the canal and free the ossicular mass from the atretic plate and canal wall. Finally, the ossicular mass may be disarticulated from the stapes, remodeled, and used for reconstruction. All three ossicles may be fused either as a single mass or at specific articulation points (the malleus handle, the incus long process, or the stapes head). Although the treatment of complete ossicular fusion is limited secondary to fusion to the oval window, lesions involving fusion at single articulation points are amenable to reconstruction with a prosthesis.

One-third of all anomalies of the stapes are associated with anomalies of the long process of the incus. The incudostapedial joint forms during week 8 of fetal development as the incus precursor migrates to articulate with the future stapedial ring. The fibrous union of this joint results in a conductive hearing loss of approximately 30 dB and may be transmitted in an autosomal dominant fashion. Treatment options include incus removal and prosthesis replacement, stapedectomy, or cartilage interposition. Other congenital lesions of the incus and stapes include bony fusion and aplasia of the articular joint. Both lesions are amenable to laser remodeling followed by interposition grafts.

Essentials of Diagnosis

• Abnormal development of the oval window may be associated with failure of stapes insertion, a maximal conductive hearing loss, and abnormal position of the facial nerve.
• Round window aplasia is commonly associated with stapes ankylosis and results in unsuccessful stapedectomy.

Oval Window Anomalies

Hypoplasia or aplasia of the oval window is a rare anomaly and may occur in isolation or in conjunction with other anomalies. Failure of the normal association between the primordial oval window niche (the otic capsule) and the developing stapes footplate between the 5th and 6th weeks of development results in aplasia of the oval window and is most commonly associated with anterior displacement of the tympanic segment of the facial nerve. Additional anomalies of the stapes, round window, and inner ear may be present. A maximal conductive hearing loss (60 dB) is usually detected in early childhood, and an absent oval window may be visualized on high-resolution CT scanning (Figure 48–6). Radiographic imaging may also be used to confirm the presence of normal inner ear structures, determine the anatomy of the facial nerve, and detect any associated ossicular anomalies.

The management of oval window aplasia remains controversial. Options include hearing aids, vestibulotomy with prosthesis insertion, fenestration and piston insertion above the aberrant facial nerve, or fenestration of the horizontal semicircular canal. The success of all these different surgical approaches has been mixed. Furthermore, the facial nerve and the inner ear are at considerable risk for injury during these surgical approaches.

Round Window Anomalies

Aplasia or hypoplasia of the round window may be associated with endemic cretinism and mandibulofacial dysostosis. Non-syndromic cases of round window anomalies are rare, with fewer than 10 reports described in the literature. More commonly, the round window position and size may vary without functional consequence. The significance and management of round window aplasia remain unclear. During week 11 of fetal development, a condensation of connective tissue forms at the future site of the round window. This develops into a cartilage ring that prevents ossification of the round window niche. Failure of the development of this ring results in bony obliteration of the primordial niche.

Round window aplasia is often associated with stapes ankylosis and a 40-dB conductive hearing loss. When stapedectomy is unsuccessful in reversing this hearing loss, the absence of the round window may be diagnosed in retrospect. Although high-resolution CT scans may detect aplasia of the round window, most cases are diagnosed after unsuccessful stapedectomy. Attempts at surgical fenestration have met with poor results and carry a significant risk of sensorineural hearing loss. Therefore, amplification represents the most practical therapy.

We would like to acknowledge Abtin Tabaee, MD, Michelle Roach, BA, and Vicki Owczarzak, MD for their contribution to this chapter in the previous editions of CDT.