Conductive Hearing Loss
Conductive hearing loss is typically associated with absent acoustic reflex thresholds, which when combined with a normal tympanogram, likely indicate an ossicular pathology. When conductive hearing loss is accompanied by a normal tympanogram and fully or partially present acoustic reflexes, it creates enough of a diagnostic discrepancy to prompt a recheck of the test results. So, what could this clinical scenario indicate? Most clinicians would consider the possibility of superior semicircular canal dehiscence (SSCD). But what if the common symptoms of SSCD such as autophony, Tullio phenomenon, pulsatile tinnitus, or aural fullness are not reported during the case history?
Review of the literature shows that SSCD is not the only otologic cause of acoustic reflex thresholds being fully or partially present in conductive hearing loss. Other otologic disorders that have been documented to exhibit this condition include large vestibular aqueduct syndrome (LVAS), dehiscence of a facial nerve, fracture of the stapes, congenital malformations, parilabyrinthine fistula secondary to middle ear pathology, ossicular discontinuity (in rare cases), and other third-window disorders such as posterior semicircular canal dehiscence, cochlear-carotid dehiscence, and abnormal thinning on the scala vestibuli side of the cochlea (Laryn. 2008 Nov; 118 (11): 2059; Amer J Neurorad. 2017 Jan; 38 (1): 2; Open J Clin Diagnostics. 2014 Sep; 4: 177; Oto Head Neck Surg. 2016; 154 (2): 343).
If the acoustic reflexes are fully absent, there is a high likelihood that the pathology is related to an ossicular chain disorder in the presence of a normal tympanic membrane (Laryn. 2008). If acoustic reflexes are present with a conductive hearing loss and normal tympanic membrane, the above mentioned otologic disorders, especially third-window disorders, should be suspected and further investigated. Hong, et al., recommended that the presence of even one acoustic reflex in conductive hearing loss should prompt further evaluation to rule out a third-window disorder (Otolaryngol Head Neck Surg. 2016). They further explained that acoustic reflexes are an effective way to provide additional information on the status of the middle ear, minimizing the risk of any unnecessary exploratory middle ear surgery on a patient who may have a third-window disorder instead of an ossicular pathology. The authors also recommended for patients to be further evaluated via computed tomography (CT) of the temporal bone to ensure an accurate medical diagnosis.
To further complicate matters, it is possible to have an ossicular disorder with partially present acoustic reflexes. This finding is extremely rare but has been documented in several case studies. Sakamoto, et al., described a case of ossicular discontinuity with partially present acoustic reflexes (Open J Clin Diagnostics. 2014 Sep; 4: 177). An exploratory middle ear surgery revealed that the long head of the incus was not present within the ossicular chain. The cause was determined to be related to a formation of strong fibrous bands that replaced the long head of the incus, creating a sound transmission pathway that maintained intact acoustic reflexes. The authors also concluded that spontaneous myringostapediopexy and fibrous band formation connecting the stapes to the tympanic membrane could cause acoustic reflexes to remain partially intact in the presence of ossicular pathology.
Isenberg and Tubergen documented a case of congenitally missing stapes that was associated with conductive hearing loss and the presence of acoustic reflexes (Arch Otolaryngol.1980; 106 (3): 179). Ebert, et al., presented a case wherein a dehiscent facial nerve interrupted the mobility of the stapes, resulting in mixed hearing loss and partially intact acoustic reflexes (Laryn. 2008). Acoustic reflexes can also change based on the stage of a middle ear disease. For example, acoustic reflexes may be fully or partially present in the early stages of otosclerosis (Oto Head Neck Surg. 2016). As the stapes’ mobility becomes more fixed, acoustic reflexes become progressively absent, resulting in the typical finding of absent acoustic reflexes, as in the case of a true conductive hearing loss.
Acoustic reflexes serve as a very useful diagnostic tool in the assessment of middle ear function. True middle ear pathology typically results in absent acoustic reflexes, but there are rare cases that demonstrate intact acoustic reflexes in the presence of conductive hearing loss and normal tympanometry. In these cases, the diagnostic utility of acoustic reflexes becomes apparent, particularly in the accurate diagnosis of third-window disorders and other rare otologic pathologies.