Vestibular schwannomas (VS) are the most frequent tumors of the cerebellopontine angle (CPA), presenting with unilateral hearing loss, tinnitus, and/or balance issues, overlapping symptoms of endolymphatic hydrops, thus complicating the clinical diagnosis. On MRI, VS demonstrate changes in signal intensity within the perilymph on MRI on sequences sensitive to protein variations (e.g., T2w GRE or FLAIR sequences) (14–16). The pathophysiological explanation for these signal changes, is that the tumor may induce biochemical alterations of the composition of the perilymph, through the secretion of several proteins (6, 17–20). For instance, Rasmussen et al. identified 314 proteins in total, 91 of which were detected in 12 out of 15 patients. In particular, the alpha-2-HS-glycoprotein (P02765, found in the extra-cellular region), was observed in all their patients and appeared to be an independent variable for tumor-associated hearing loss (18). Tumor-related changes in protein content of the perilymph may also indirectly affect the blood-perilymph barrier integrity (18, 21). All these osmotic alterations can lead to pressure changes in the perilymphatic and endolymphatic compartments of the inner ear, contributing to the occurrence of SEH (2).

In patients with VS, SEH is observed in approximately 20–30% of cases (14–16). Notably, secondary EH affects the contralateral ear in about 15% of these cases, suggesting a potential systemic component (14). Primary inner ear schwannomas, also known as intra-labyrinthine schwannomas, are much rarer than typical vestibular schwannomas and are regarded as a distinct clinical entity, because they originate from within the labyrinth itself (22). SEH is notably more prevalent in cases of primary inner ear schwannoma (Figure 1), occurring in 47–67% of such instances. In these cases, the proximity of the tumor to the peri-and endo-lymphatic spaces may facilitate the diffusion of tumor-secreted proteins, potentially exacerbating the hydrops (23, 24).

Meningiomas are slow-growing, benign tumors that typically arise from the meninges, and when they extend into the internal auditory canal (Figure 2), that can compress or displace critical inner ear structures. Patients may present with progressive hearing loss, tinnitus, vertigo, or balance disturbances, which overlap with those seen in SEH. Perilymphatic signal changes have also been described in cerebellopontine meningiomas extending into the internal auditory canal on T2W GE sequence, serving as an additional diagnostic feature to differentiate between the two tumors (6). In rare cases, posterior fossa meningiomas may affect the endolymphatic sac, which may lead to endolymphatic hydrops, due to mechanical obstructions like other posterior fossa neoplasms (25, 26).

Endolymphatic sac tumors (ELST) are rare, hyper-vascular locally aggressive papillary neoplasm that arise in the retro-labyrinthine region of the temporal bone (27). The majority of ELSTs occur sporadically, but approximately 30% of cases are associated with Von Hippel–Lindau’s disease (28). The clinical presentation of ELSTs is predominantly auditory with symptoms such as persistent sensorineural hearing loss (SHNL; sudden in nearly 50% of cases), along with tinnitus and vertigo. The SHNL in ELST is thought to result from invasion of the otic capsule, tumor-associated intralabyrinthine hemorrhage, or the development of endolymphatic hydrops on the side of the tumor (29). The contralateral side usually does not exhibit signs of hydrops (30).

Inner ear malformations, particularly those resulting in “pathologic third window lesions” like superior semicircular canal dehiscence syndrome (SSCD) and large vestibular aqueduct syndrome (LVAS), are known to disrupt the delicate fluid balance within the inner ear, altering perilymphatic pressure dynamics, thus contributing to the development of SEH. The mechanism behind this phenomenon may involve shifts in protein concentration that affect osmotic pressure between the endolymphatic and perilymphatic compartments, or direct pressure changes within the inner ear due to the presence of a third window. In SSCD, a dehiscence in the superior semicircular canal creates an abnormal third window, alongside the oval and round windows, through which sound and pressure stimuli can abnormally move the endolymph (Tullio phenomenon, with vertigo and nystagmus triggered by loud sounds). Secondary EH is a common finding in SSCD, with reported prevalence rates ranging from 27.3 to 80% of the cases (31, 32). Large vestibular aqueduct syndrome (LVAS) is a congenital condition characterized by an abnormally enlarged vestibular aqueduct (Figure 3), also associated with SEH. It is a recognized cause of unilateral or bilateral progressive sensorineural hearing loss among the pediatric and adolescent population. The exact etiology of hearing loss in patients with LVAS is still unknown. One significant hypothesis is that the hyperosmolar, protein-rich fluid present in the enlarged endolymphatic sac refluxes into the endolymphatic spaces of the cochlea, thereby damaging the hair cells, leading to SEH (31, 33).

Lateral canal vestibular dysplasia (LCVD) is a common congenital malformation of the inner ear, associated with vestibular dysfunction and varying degrees of hearing impairment, that primarily affects the lateral semicircular canal, an essential structure responsible for sensing horizontal head movements. The severity of dysplasia may vary from a short and broad lateral semicircular canal to a single fluid-filled cavity (Figure 4) confluent with the vestibule (34). Naganawa et al. demonstrated that patients with LCVD had a significantly increased vestibular endolymphatic ratio compared to controls, suggesting that the malformation affects endolymphatic fluid regulation within the vestibular system. Similarly, a strong negative correlation was found between the central bony island area and the size of the endolymph. The authors hypothesized that this increase in vestibular endolymphatic space is a result of the dysplasia impairing the normal clearance or absorption of endolymph, further contributing to the development of SEH (35). However, Niu et al. did not observe significant vestibular hydrops in patients with semi-circular malformations (36). Yun et al. expanded upon this, demonstrating that vestibular dysfunction in LCVD is not limited to the lateral canal, nor to hydrops alone, but can affect the broader vestibular system, leading to more complex vestibulocochlear symptoms (34). Thus, the observation of SEH in LCVD highlights the complex relationship between congenital malformations and secondary hydrops, where abnormal inner ear anatomy predisposes patients to fluid dynamic disturbances.

Surgical procedures may contribute to SHE in some cases. For instance, cochlear implantation, especially when associated with cochleostomy, has been associated with SEH in a portion of patients, with reports suggesting an incidence of 42–59% (38). This suggests that postoperative disruption of the normal inner ear balance could influence inner ear fluid dynamics, resulting in SEH. Several potential pathophysiological explanations have been suggested including a blockage of the endolymphatic system or the occurrence of cochlear inflammation (39). Furthermore, animal model studies in guinea pigs have shown that endolymphatic sac ablation can lead to mild hydrops within days post-surgery (40). This hydrops initially affects the saccule, endolymphatic sinus, and Reissner’s membrane, with progression to severe distension of Reissner’s membrane into the scala vestibuli, occurring within months (41).

Post-irradiation SEH is a potential complication following radiation treatment of the head and neck region, leading to disturbances in inner ear fluid homeostasis. Radiation-induced damage to the cochlear and vestibular structures can disrupt the delicate balance between perilymphatic and endolymphatic pressures, contributing to the onset of SEH. This fluid imbalance may manifest as hearing loss, vertigo, or tinnitus, with the severity and presentation of symptoms varying depending on the extent of inner ear damage. Additionally, post-irradiation SEH should be distinguished from other radiation-induced auditory conditions, such as sudden SNHL (42). While both conditions may arise following radiation therapy, they differ in their underlying pathophysiology and clinical course. SEH is typically associated with a more gradual onset of symptoms, often accompanied by episodic vertigo, whereas sudden SNHL tends to present suddenly without vestibular involvement (2). This underscores the importance of considering SEH as a potential diagnosis in patients presenting with auditory or vestibular symptoms post-radiation and highlights the utility of MRI for detecting SEH in these cases (42).

Noise-induced trauma may also contribute to the development of SEH. Acoustic trauma, such as high-intensity sound exposure, can generate pressure-striking forces on the membranous labyrinth, and disrupt endolymphatic fluid balance, potentially leading to SEH. For instance, mice exposed to 100 dB sound pressure level noise for two hours, developed EH and had reduced synaptic counts in the basal and middle regions of the cochlea and similar results were observed on guinea pigs (43, 44).

Blunt-force trauma (i.e., mechanical trauma to the inner ear), may also result in SEH. Physical impacts from blunt injuries can potentially disrupt the intricate structures of the inner ear, or cause pressure changes, both leading to changes in fluid dynamics, thus contributing to SEH (37). The severity and presentation of hydrops might vary depending on the extent of the trauma and its impact on the inner ear’s fluid compartments. Traumatic EH involves the accumulation of endolymph in the cochlear duct caused by traumatic insult. Possible mechanisms include: (1) a disruption in the normal perilymph-endolymph pressure relationship due to bony labyrinth fistulization; (2) direct trauma to the membranous labyrinth, potentially leading to fluid collection in the cochlear duct that may resolve over time; and (3) injury to the endolymphatic fluid drainage pathway, including temporal bone fractures involving the vestibule or the vestibular aqueduct. This can cause fibro-osseous blockage of the endolymphatic duct and surgical injury to the saccule with obstruction of the longitudinal flow of endolymph, resulting in SEH that may be delayed in onset and usually persists. The diagnosis of traumatic endolymphatic hydrops generally involves a history of trauma (e.g., barotrauma, blow to the head…); typical symptoms of EH (e.g., fullness, tinnitus, fluctuant hearing loss, and/or episodic vertigo); and characteristic electrocochleography findings (45).

SEH has also been reported in longstanding inner ear conditions such as otosclerosis, with prevalence rates ranging from 26 to 55%, whether the otosclerosis has been surgically treated or not (48, 49). While the precise mechanism by which SEH develops in otosclerosis remains unclear, preoperative EH has been suggested as a potential risk factor for inner ear disturbances following stapes surgery (50). Thus, acute episodes of sensorineural hearing loss or vertigo in patients with otosclerosis may be indicative of underlying SEH, though the condition does not always present with noticeable clinical manifestations. Thus, clinicians should maintain a high index of suspicion, especially when patients experience sudden changes in symptoms (Figure 6), as this may warrant an MRI evaluation to confirm the presence of SEH (51).

Spontaneous intracranial hypotension (Figure 7), characterized by reduced cerebrospinal fluid pressure, has been linked to the development of SEH (52–54). This association is thought to stem from changes in the pressure gradient between the perilymph and endolymph within the inner ear. Perilymphatic fluid is believed to communicate with the CSF via the cochlear aqueduct, and a drop in CSF pressure results in a corresponding decrease in perilymphatic pressure. When perilymphatic pressure falls below the endolymphatic pressure, it can lead to the dilation of the endolymphatic space, contributing to SEH. The underlying mechanism is hypothesized as a “vacuum phenomenon,” where negative pressure within the perilymphatic compartment causes an expansion of the endolymphatic space, promoting hydrops, highlighting the delicate balance of fluid dynamics in the inner ear and how intracranial pressure changes can influence inner ear function.