Third Window Syndrome: An Up-to-Date Systematic Review of Causes, Diagnosis, and Treatment
Article information
Abstract
Third window syndrome (TWS) is an inner ear condition caused by an additional compliant point in the otic capsule that disrupts auditory and vestibular functions. Superior semicircular canal dehiscence is the most common cause, presenting with hearing loss, vertigo, and autophony, significantly impairing quality of life. This study evaluated the pathophysiology, diagnostics, treatments, and recent advancements in TWS while identifying research gaps. Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, 70 studies from Embase, MEDLINE, Cochrane, and UpToDate databases were analyzed. TWS affects inner ear mechanics, enhancing bone conduction and reducing air conduction. Diagnosis involves clinical evaluations, high-resolution imaging, and functional tests such as vestibular evoked myogenic potentials, which are known for their high sensitivity and specificity. Management strategies range from vestibular rehabilitation and pharmacotherapy to surgical interventions, including transmastoid and middle cranial fossa approaches, which achieve over 75% success. Emerging minimally invasive techniques, such as underwater endoscopic ear surgery and round window reinforcement, show promise but carry risks like cerebrospinal fluid leakage and inconsistent symptom relief. Advancements in TWS management have improved outcomes, yet gaps remain, particularly in terms of false-positive imaging and long-term efficacy. Future studies should prioritize predictive models and minimally invasive techniques. A multidisciplinary approach is essential to improve patient care.
Introduction
The superior semicircular canal dehiscence (SSCD) syndrome was first described in 1998 by Minor, et al. [1], who analyzed eight cases of dizziness and oscillopsia associated with sound or pressure stimuli and confirmed the presence of the dehiscence using computed tomography (CT). Subsequent studies validated this pathology as a cause of vertigo and auditory disturbances, while surgical repair improved symptoms in many patients [2].
Studies on temporal bones have shown that 0.7% of the general population has a complete dehiscence and 1.3% has a thin bony layer in the superior semicircular canal [3]. Additionally, Cremer, et al. [4] documented the correlation between sound stimuli and vestibular responses through oculomotor measurements. Experimental studies in animals supported the pathophysiology of the syndrome, demonstrating changes in vestibular nerve activity under conditions of increased external auditory canal pressure [5].
Recent research has highlighted the presence of dehiscence in other semicircular canals, such as the posterior canal, with a higher prevalence in individuals with a history of vertigo [6].
Pathophysiology
In the inner ear, the oval and round windows act as compliant points, facilitating the transfer of sound pressure [7]. The creation of a “third window,” such as the SSCD or an enlarged vestibular aqueduct (EVA), disrupts this physiology, causing changes in pressure transmission within the inner ear [8]. This causes sound energy from air conduction to be directed toward the compliant point and weakened, while sound energy from bone conduction is amplified due to the increased compliance of the system [9,10].
These disorders lead to hearing loss, vertigo due to the Tullio phenomenon, nystagmus from pressure changes (Hennebert sign), and pulsatile tinnitus due to pressure transmission from cerebrospinal fluid (CSF) [11-13]. This pathology highlights the complex interaction between normal pressure regulation and auditory function. Furthermore, SSCD or an EVA causes pressure transmission from CSF to the cochlea, resulting in perceived pulsations and pulsatile tinnitus due to arterial waves [14].
Definition
The term “third window syndromes” (TWS) refers to the clinical entities arising from the creation of a third compliant point in the bony capsule of the inner ear, making the inner ear sensitive to changes in pressure from the outer and middle ear, as well as intracranial pressure.
This study aims to review the existing literature on TWS, its pathophysiology, diagnosis, and treatment, while highlighting recent advancements in these areas.
Materials and Methods
A literature search was conducted using the Embase, MEDLINE, Cochrane, and UpToDate databases until June 2024. The following keywords were used: “third window,” “third window syndrome,” “canal dehiscence,” “superior semicircular canal dehiscence,” “SSCD,” “superior canal dehiscence,” “superior semicircular dehiscence,” “canal dehiscence,” “vestibular aqueduct,” “enlarged vestibular aqueduct,” “EVA syndrome,” “third window AND symptoms,” “Meniere,” “Meniere’s disease,” “Meniere AND third window,” “cochlear aqueduct,” “enlarged cochlear aqueduct,” “DFNX2,” “X-linked deafness,” “stapes gusher,” “inner ear anatomy,” “ear anatomy,” “ear physiology,” “hearing physiology,” “VEMPs,” “cVEMPs,” “oVEMPs,” “vestibular evoked myogenic potentials,” “audiometry,” “air-bone gap AND [third window OR SSCD],” “[third window OR SSCD] AND hearing loss,” “air-bone gap AND SSCD,” “pseudoconductive hearing loss,” “autophony,” “pressure induced vertigo,” “[third window OR SSCD] AND diagnosis,” “[third window OR SSCD] AND imaging,” “[third window OR SSCD] AND [computed tomography OR CT],” “[third window OR SSCD] AND [high resolution computed tomography OR HRCT],” “third window lesions,” “third window AND differential diagnosis,” “[third window OR SSCD] AND prevalence,” “Tullio phenomenon,” “Tullio sign,” Hennebert sign,” “Valsalva maneuver,” “Toynbee phenomenon,” “ankle audiometry,” “[third window OR SSCD] AND treatment,” “[third window OR SSCD] AND surgical management,” “canal plugging.”
The research methodology was used according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines along with PICOS (Participants, Interventions, Comparisons, Outcomes, and Study design) criteria. English language, ability to extract data, and similarity with the subject were used as inclusion criteria.
Results and Discussion
Literature research led to 174 abstracts. After review, 78 articles were excluded for being non-English, not published yet, having a misleading title, or outcomes not discussed in the abstract. A total of 96 articles were fully reviewed, but 26 were excluded due to possible plagiarism, outcomes not discussed, being too concise, or having no meaningful analysis. In total, 70 studies were included (Fig. 1).
PRISMA flowchart.
Epidemiology
The prevalence of SSCD ranges from 0.5% to 1.4% in anatomical studies [3,15], while thinning of the canal wall is more commonly observed in older individuals due to reduced bone mass [16]. In high-resolution computed tomography (HRCT) imaging studies, prevalence ranges from 3% to 10%, though it is often overestimated due to technical limitations [17]. Although the bony wall is thinner in women, the prevalence of the condition is equal between the two genders [18].
Etiology
SSCD is the most common cause of a third window, resulting from incomplete development of the petrous bone or external factors such as trauma or increased intracranial pressure [19,20]. Less common causes include dehiscence in other semicircular canals [21] and an EVA, which occurs when its diameter does not decrease normally during development [22].
Bone erosion is also common in conditions such as chronic otitis media or trauma, particularly in the horizontal semicircular canal due to its proximity to the tympanic cavity [23]. Rarely, the DFNX2 mutation causes communication between the internal auditory canal and the inner ear, leading to hearing loss [24]. Other causes include Paget’s disease, which thins the cochlear capsule, and the “two-hit model,” where a thin bone fractures following trauma, the development of intracranial hypertension, diving, or even childbirth [25-27].
Medical history and symptoms
Patients present with a variety of symptoms, including dizziness, vertigo, instability, autophony, hearing loss, hyperacusis, pulsatile tinnitus, and a sensation of fullness [28]. Autophony involves an increased perception of one’s own voice and bodily movements due to increased bony conduction, while pulsatile tinnitus is often caused by adjacent blood vessels whose hydrostatic pressure is transferred to inner ear fluids through third window [29].
Dizziness is often caused by changes in pressure or loud sounds (Hennebert and Tullio signs) and may be accompanied by nystagmus, with episodes of falls or oscillopsia also having been recorded. The presence of a third compliance point makes the inner ear’s structure more susceptible either to atmospheric or sound pressure which is transferred to vestibular endolymph setting semicircular canal cupulae in movement [30,31].
Clinical assessment
The diagnosis requires a comprehensive otolaryngological examination to rule out other causes. Findings such as lateralization of Weber’s test to the affected ear and a potentially negative Rinne test are indicative but not pathognomonic.
Special tests, inducing nystagmus using Frenzel goggles or videonystagmography, are useful. Tullio sign is triggered by loud sounds, while Hennebert sign is elicited by pressure increases in the external ear. The Valsalva maneuver, either by equalizing middle ear pressure or increasing intracranial pressure, can confirm the diagnosis by inducing nystagmus. For greater accuracy, testing in the supine position is recommended to avoid false-negative results [32].
The Toynbee maneuver, performed by swallowing with closed nostrils, can also induce nystagmus in cases of TWS, although the underlying mechanism remains unclear [33].
Audiometry
Pure tone audiometry
The most common finding in audiometry is an air-bone gap at low frequencies (250–2,000 Hz), with the largest difference observed at 250 Hz (27.52 dB), which gradually decreases up to 2,000 Hz [34]. Larger SSCD are associated with greater gaps [35]. The bone conduction threshold is often better than normal or even negative at low frequencies. The Weber test using a 512 Hz tuning fork can confirm the gap, with the sound being perceived more intensely in the affected ear [36].
Ankle audiometry
The study by Verrecchia, et al. [37] proposed ankle audiometry as a diagnostic tool for SSCD. Using a device that generated controlled bone vibrations (125–750 Hz), it was found that thresholds were reduced in patients compared to the control group, with the greatest difference observed at 250 Hz. Following surgical repair, thresholds increased by 20 dB, supporting the test’s utility both for diagnosis and for assessing treatment outcomes.
Vestibular evoked myogenic potentials
Vestibular evoked myogenic potentials (VEMPs) are highly sensitive and specific tests for detecting TWS, particularly SSCD. Cervical VEMPs (cVEMPs) are recorded at a reduced threshold, with sensitivity and specificity >90%, while ocular VEMPs (oVEMPs) exhibit greater response and higher sensitivity [38,39]. The use of high frequencies, such as 4,000 Hz, improves accuracy, as otolithic organs are more sensitive to these frequencies. Studies have shown this approach achieves a 100% positive predictive value [40,41]. VEMPs can confirm clinical and radiological findings or assess postoperative outcomes. The cVEMP and oVEMP tests have been recognized as reliable diagnostic methods, with their values returning to normal levels following surgical repair [42].
Electrocochleography
Electrocochleography records the electrical activity of the cochlea and the vestibulocochlear nerve in response to sound stimuli. It involves placing an electrode on the tympanic membrane. Patients with SSCD exhibit an increased summating potential/action potential (SP/AP) ratio, which improves following surgical repair [43]. However, this test is now largely confined to research protocols due to technical challenges and the availability of more reliable alternative methods [44].
Imaging
Superior semicircular canal dehiscence
HRCT with slice thicknesses of 0.5–0.6 mm improves the sensitivity of diagnosing SSCD from 50% to 93% and reduces false-positive results [45]. For better visualization, reconstructions in the Pöschl and Stenver planes are recommended, although their effectiveness remains debated [46]. The dehiscence is typically found in the projection of the canal toward the middle cranial fossa and is often associated with a thin bony plate or meningocele. Increased pneumatization of the mastoid is correlated with a thinner bony covering and a higher risk of dehiscence [47]. Cone beam computed tomography (CBCT) is a newer technique mainly used in oral and maxillofacial radiology due to its ability to produce detailed 3D images with less radiation than CT. When properly used, it can exceed CT’s sensitivity in the identification of SSCD [48].
Horizontal and posterior semicircular canal dehiscence
Dehiscence of the horizontal and posterior semicircular canals is a rare cause of TWS and is often accompanied by other inner ear abnormalities. On HRCT, horizontal canal dehiscence is frequently associated with middle ear conditions such as chronic otitis media and cholesteatoma, due to proteolytic enzymes that erode the otic capsule [49]. Posterior canal dehiscence can be caused by a high-riding jugular bulb or paraganglioma, with characteristic imaging findings [50].
Enlarged vestibular aqueduct
An EVA creates a communication between the subarachnoid space and the otic capsule, leading to TWS symptoms. Normally, its width ranges from 0.4–1.0 mm, and it is considered pathological if it exceeds 1.5 mm on CT scans [51]. More recent criteria define a pathological width as >1.0 mm at the midpoint, >2.0 mm at the aperture, or >0.9 mm in the Pöschl plane [52]. Magnetic resonance imaging with T2-weighted sequences can reveal an enlarged endolymphatic sac up to 2 cm [53]. An EVA is associated with CSF leakage during surgical procedures and an increased risk of sensorineural hearing loss following trauma [54].
X-linked hearing loss DFNX2 mutation
This is a genetic disorder characterized by the absence of the cribriform plate between the basal turn of the cochlea and the fundus of the internal auditory canal, leading to progressive hearing loss. On HRCT, a triad of findings is observed: incomplete partitioning of the cochlea (type III), enlargement of the internal auditory canal, and an abnormal course of the facial nerve [24]. The condition is associated with CSF gusher during stapedectomy, necessitating avoidance of the procedure. Additional anomalies may include an enlarged facial nerve canal, cystic vestibule, oval window dysplasia, and stapes abnormalities such as stapes fixation, thickened footplate or single crus [55].
Paget’s disease
The hallmark imaging finding of Paget’s disease is a “cotton wool” appearance on skull radiographs. CT scans reveal irregular areas of sclerotic bone, cortical thickening, and exostoses, reflecting the coexistence of osteoblastic and osteoclastic activity [56].
Case reports
Rarely, dehiscences have been observed in the fallopian canal and carotid canal, causing third window symptoms due to bone loss in the otic capsule. Blake, et al. [57] reported two adults with unilateral hearing loss, autophony, and dizziness, where HRCT revealed a dehiscence between the cochlea and the labyrinthine segment of the facial nerve. In a 20-monthold infant with abnormal auditory brainstem responses, Koroulakis, et al. [58] identified a similar dehiscence. Kim and Wilson [59] described an ectopic position of the carotid canal in contact with the apex of the cochlea, with a bony defect diagnosed following a failed stapedectomy.
Differential diagnosis
TWS causes both auditory and vestibular symptoms, with its differential diagnosis including middle ear disorders, patulous Eustachian tube, vestibular migraine, vestibular schwannoma, Ménière’s disease, and perilymphatic fistula [10,36,60-64]. The differential diagnosis of TWS compared to other diseases according to symptoms, auditory thresholds, and diagnostic markers like oVEMP and cVEMP characteristics is presented in Table 1.
Differential diagnosis of third window syndrome compared to other diseases
Conservative treatment
In the initial management, conservative measures and strategies can be employed to alleviate symptoms. Lifestyle modifications are recommended, such as avoiding triggering stimuli like loud noise. Earplugs are particularly useful for preventing dizziness caused by the Tullio phenomenon. Vestibular rehabilitation appears effective in cases of low-intensity chronic dizziness. Symptom relief can be achieved with labyrinthine suppressants, H3 histamine receptor antagonists, or benzodiazepines [65]. Another proposed treatment is the insertion of ventilation tubes. The rationale behind this approach is to relieve middle ear pressure, which is transmitted to the inner ear due to its increased compliance with pressure changes. However, this method does not seem effective for intracranial pressure changes, such as those caused by head tilting [66].
Surgical intervention
In patients with severe chronic symptoms unresponsive to conservative measures, surgical management is performed. Improvements and considerations are noted for various causes of TWS such as superior and posterior semicircular canal dehiscence, EVA, X-linked hearing loss, and perilymphatic fistula [67-70] (Table 2). The most commonly applied surgical techniques involve closure of the dehiscence via the cranial fossa or mastoid approach, as well as reinforcement of the round or oval windows. In cases of bilateral TWS, such as SSCD, surgery is recommended initially for the ear with the more severe symptoms. However, restoration on one side can potentially worsen the other side, particularly in cases of thin bone. A possible explanation is that intracranial hypertension is redirected to the second compliant point. Despite the high effectiveness of bilateral correction in symptom control, there is a risk of developing oscillopsia due to the destruction of both vestibules [71].
Causes of TWS and related surgical outcomes
Regarding effectiveness, according to the study by Mekonnen, et al. [72], involving 350 patients who underwent surgical repair of SSCD, improvements were observed in 74.9% of cases for autophony, 62.9% for hearing, and 54.6% for dizziness. Unilateral repair showed slightly better outcomes than bilateral intervention, while repeat surgery increased improvement rates to 85%. Similarly, in the study by Mozaffari, et al. [73] involving 229 cases, unilateral intervention demonstrated better results in hyperacusis, hearing loss, dizziness, and instability compared to bilateral intervention, leaving open the question of the necessity for bilateral procedures. Postoperative CSF leakage was observed in up to 25% of cases, mostly managed conservatively. Risk factors include hypertension, obesity, and increased intracranial pressure [74]. Other studies are more conservative reporting lower rates of symptom relief after surgery and the possible need for a second intervention [75]. It has to be noted that plugging SSCD can lead to deterioration of bone conduction thresholds [76].
Repair of dehincence via the transmastoid approach
The transmastoid approach provides an alternative method for repairing SSCD through mastoidectomy, without the need for craniectomy or manipulation of the brain. It is less invasive and can be performed without a neurosurgeon, while offering similar effectiveness to the middle cranial fossa approach [77].
An innovative technique within this approach is underwater endoscopic ear surgery (UWEES), which utilizes an endoscope submerged in saline for enhanced visualization. This method is minimally invasive and effective in reducing autophony and dizziness. However, temporary hearing loss is frequently observed, with long-term improvement over time [78,79].
Repair of dehinscence via the middle cranial fossa
The first successful repair of SSCD was performed using muscle fascia. Today, various materials are used, including bone paste, cortical bone, wax, and hydroxyapatite cement.
The repair techniques include: 1) plugging: the dehiscence is filled with material (e.g., bone paste) and covered with fascia; 2) resurfacing: the dehiscence is externally covered with materials such as cortical bone and fascia; and 3) capping: a rigid material (e.g., hydroxyapatite cement) is wedged into the defect and secured with glue or bone paste.
Plugging and capping are more effective than resurfacing, but plugging is associated with more complications. However, some reviews find no significant differences in effectiveness among the methods [80,81].
Reinforcement of the round and oval windows
In 2014, an alternative approach for treating TWS was proposed, where instead of closing the dehiscence, reinforcement of the round or oval window is performed, leaving only two compliant points. The procedure involves creating a tympanomeatal flap, identifying the windows, and reinforcing one of them with cartilage from the tragus and fibrin glue [82]. By increasing the stiffness of the inner ear, this method reduces hyperacusis, dizziness, and the Tullio phenomenon, while cVEMP thresholds return to normal levels [83].
The effectiveness of this method has not been fully established, with outcomes often inconsistent. Despite positive results in some patients, preoperative prediction of success remains challenging [84]. The procedure has a favorable safety profile with mild complications, although a consistent threshold drop at 4,000 Hz has been observed, without clinical impact on speech audiometry [85].
Due to its low morbidity, this approach is recommended for elderly patients or those with comorbidities, as well as in cases with hearing loss in the contralateral ear, where preserving hearing is critical [86].
Future directions
Future research in TWS should prioritize several key areas to enhance patient care and outcomes. First, the development of predictive models for surgical outcomes is essential to optimize patient selection and tailor treatment strategies effectively. Standardizing and refining diagnostic criteria are equally important to reduce false-positive rates, which currently pose a challenge in accurately identifying TWS cases. Additionally, exploring long-term outcomes and patient satisfaction across both surgical and non-surgical interventions will provide valuable insights into the durability and effectiveness of current treatment modalities. Investigating the pathophysiological mechanisms underlying less common forms of TWS is another critical area, as it may uncover novel therapeutic options and broaden the scope of treatment. Finally, evaluating minimally invasive techniques, such as UWEES, in larger cohorts is vital to establish their safety, efficacy, and potential advantages over traditional approaches. These research directions collectively aim to address existing gaps in knowledge, refine clinical practices, and ultimately improve outcomes for patients with TWS.
Conclusion
TWS can be either congenital or acquired. By far, the most common cause is SSCD, due to its close proximity to the middle cranial fossa. Less common locations include the posterior and horizontal semicircular canals, an enlarged vestibular or cochlear aqueduct, X-linked hearing loss DFNX2, and Paget’s disease. Its prevalence in the general population is approximately 0.5%, increasing with age, but it does not differ between genders.
The pathophysiology involves the creation of a third compliant point in the inner ear, in addition to the two normal ones—the oval and round windows. This reduces the stiffness of the system, enhancing bone conduction while impairing air conduction.
Clinical suspicion is of high importance, while VEMPs are highly sensitive diagnostic tests for TWS. cVEMPs are elicited at lower thresholds, while oVEMPs exhibit larger amplitudes. HRCT and CBCT are an important diagnostic tool, enabling the identification of dehiscence, but they have a high rate of false positives, necessitating a combination of diagnostic methods.
The management of TWS has seen significant advancements, with treatments ranging from conservative measures to highly specialized surgical interventions. Initial management focuses on symptom relief through lifestyle modifications, vestibular rehabilitation, and pharmacological approaches, such as labyrinthine suppressants. Earplugs and strategies to avoid triggering stimuli are particularly effective for patients experiencing sound-induced dizziness. Surgical intervention is indicated in patients with refractory or severe symptoms. Techniques such as transmastoid or middle cranial fossa approaches have demonstrated success rates exceeding 75% for symptom improvement, particularly in vertigo and autophony. Novel approaches like UWEES and round window reinforcement offer promising results with reduced invasiveness, especially for elderly or high-risk patients. Comparative studies have yet to establish definitive superiority among surgical techniques, leaving room for further optimization of patient selection criteria and procedural protocols.
Despite these advances, gaps in the literature remain. The heterogeneity in patient outcomes points to the need for standardized evaluation frameworks and longitudinal studies to assess the durability of treatment benefits.
Notes
Conflicts of Interest
The authors have no financial conflicts of interest.
Author Contributions
Conceptualization: Ilias Lazarou, George Korres. Data curation: Ilias Lazarou, Giorgos Sideris. Formal analysis: Ilias Lazarou, Giorgos Sideris. Funding acquisition: Ilias Lazarou, Nikolaos Papadimitriou, Alexander Delides, George Korres. Investigation: Ilias Lazarou, George Korres. Methodology: Ilias Lazarou, Giorgos Sideris, George Korres. Project administration: Ilias Lazarou, George Korres. Resources: Ilias Lazarou, Nikolaos Papadimitriou, Alexander Delides, George Korres. Software: Ilias Lazarou. Supervision: Nikolaos Papadimitriou, Alexander Delides, George Korres. Validation: George Korres. Visualisation: Nikolaos Papadimitriou, Alexander Delides. Writing—original draft: Ilias Lazarou, Giorgos Sideris. Writing—review & editing: Giorgos Sideris, Nikolaos Papadimitriou, George Korres. Approval of final manuscript: all authors.
Funding Statement
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Acknowledgments
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