Otosclerosis is an autosomal dominant hereditary middle ear disease with variable penetrance which manifests itself with progressive hearing loss.
Otosclerosis is the main cause of hearing loss / acquired deafness, a dystrophic pathology of the labyrinth capsule affecting high frequencies. It is more often unilateral but with a capacity for bilateralization in up to 75-80% of patients and is the result of an alteration of the balance of bone resorption between osteoclasts and osteoblasts.
The result is a remodelling process that often involves the stapedius-vestibular joint and can lead to the fixation of the stapedial plate which determines clinical otosclerosis. According to some studies, histological otosclerosis is observed more frequently than clinical otosclerosis.
Very commonly, it manifests itself as a purely conductive hearing loss in the majority of patients. Only in a small percentage of cases that varies between 15 and 20% of (depending on the different case studies), can it manifest itself or evolve into a mixed hearing loss. Definitely rarer, below 4-5%, is neuro-transmissive hearing loss, which is generally a consequence of the otosclerotic involvement of the cochlea.
It occurs at a younger age, on average between the second and fourth decade of life and, in addition to the progressive and severe hearing loss, is characterized by the presence of tinnitus in a large number of patients. Females are more affected and subject to clinical worsening during pregnancy. Vertigo is decidedly infrequent in the initial stages of the condition but, in the more advanced stages, an initially sporadic dizziness may appear, which can progressively become disabling up to experiencing decidedly more frequent vertiginous syndromes.
However, the aetiology of otosclerosis remains not well defined. It is possible to speak of multifactorial aetiological components, among which genetic factors seem to have a certain importance, including the supposition of an alteration in the X chromosome, given the particular occurrence in female subjects. Source of debate is the influence exerted on the onset of otosclerosis by the measles virus (paramyxovirus), which appears to find receptors in the ear on which it easily binds. Other hypotheses concern autoimmunity, hormonal-metabolic and vascular disorders.
The classic manifestation of the active phase of the condition is Otospongiosis, a phase in which the bone of the otic capsule is replaced by otosclerotic foci, leading to the subsequent thickening and sclerotic evolution of the affected bone.
It is possible to distinguish otosclerosis into Fenestral and Retrofenestral Otosclerosis.
Fenestral otosclerosis is certainly the most typical and frequent form, characterized by the presence of otosclerotic foci on the lateral wall of the otic capsule, while in the Retrofenestral form the foci are decidedly more medial in the labyrinth capsule.
The anatomical site most often involved in clinical otosclerosis is the fissula ante fenestram, an insular fibrocartilaginous structure located within the endochondral layer of the otic capsule, between the oval window and the cochlea.
Essentially, in otosclerosis there is the formation of a neo-ossicle which prevents the motility of the stapes with consequent lack of or impaired sound transmission to the cochlea.
The diagnosis is based on audiometric and impedance tests, where in the former hearing loss will be detected, while in the latter there may be the absence of stapedial reflexes. The high-resolution petrous ear CT scan can be useful for the search for any otosclerotic foci, which are not always visible. In the otospongiosis phase, foci can be visualized on CT at high resolution as radiolucent areas located in the otic capsule, with involvement of the fissula ante fenestram in the fenestral form, an indication of reduced ossicle density. In retrofenestral otosclerosis, foci can show the so-called “double ring” sign, or areas of peri-labyrinthine radio transparency. Occasionally, detection may occur surrounding the cochlea. In rare cases, retrofenestral otosclerosis requires differential diagnosis with Paget’s disease and osteogenesis imperfecta, systemic pathologies affecting the temporal bone.
On the other hand , a High-resolution CT during the stage of advanced and overt otosclerosis, can detect a net increase in the density of foci, indeed present in greater numbers, in multifocal and with different phases of activity between themselves: some foci could be detected as in otospongiotic activity, others in the phase of consolidated otosclerosis. Unfortunately, especially in the otosclerotic relief more than in the otospongiotic one, the CT can expose to the detection of false negatives.
Various radiological classifications of otosclerosis have been proposed, among these we include:
Symons CT classification
• Grade 1: only fenestral otosclerotic foci
• Grade 2: Patchy cochlear otosclerosis with or without fenestral involvement
• 2A: otosclerotic foci at the basal turn
• 2B: otosclerotic foci around the lateral faces of the basal, middle, and apical cochlea
• Grade 3: Diffuse and confluent cochlear involvement, with or without fenestral involvement.
Rotteveel CT classification
• Type 1: only fenestral otosclerotic foci
• Type 2: retro fenestral disease, with or without fenestral involvement
• 2A: “double ring” effect
• 2B: restricted basal turn
• 2C: “double ring” effect and restricted basal turn
• Type 3: severe retrofenestral involvement, with unrecognizable otic capsule, with or without fenestral involvement.
Villon CT classification
• Type 1: involvement of the plantar plaque which is thickened (> 0.6 mm) and hypodense
• (1A), or with anterior fenestral hypodensity ≤ 1 mm (1 B)
• Type 2: anterior fenestral hypodensity> 1 mm without involvement of otosclerotic foci of the cochlear endosteum
• Type 3: anterior fenestral hypodensity> 1 mm with involvement of otosclerotic foci of the cochlear endosteum
• Type 4: anterior labyrinthine hypodensity, with foci around the cochlea
• (4A), or posterior hypodensity with localized foci in the semicircular canals or in the vestibule (4B).
The CT findings seem to have a good correlation with the audiometric evaluation of the condition, where in fact the clinical condition correlates with worsening progress, and with prognosis and post-surgical evaluation, although it is more extended towards the cochlea. Likewise, an involvement of the oval window would determine a not exactly favourable outcome of the stapedotomy.
The impedance test is based on the evaluation of tympanometry and endotympanic reflexes.
Of some usefulness in our experience, especially for differential diagnostic purposes and for the therapeutic choice, which is purely surgical based on stapedotomy, is the performance of Acoustic Evoked Potentials or ABR VIII pair of cranial nerves, which allow us to evaluate the state of health of the acoustic nerves, and therefore their functionality.
The stapedotomy involves the replacement of the stape, a bone component of the ossicular chain involved in the transmission of hearing and consisting of stape, incus and malleus, with a micro prosthesis. A possible complication of the surgery is the onset of a post-operative vertiginous syndrome. Such surgery, although performed to the best of subjective surgical skills and surgical technique, might be burdened by a deterioration of hearing in a small percentage of patients, which varies according to different experiences and schools, between 3 and 7% of cases.
Nevertheless, in which cases is stapedotomy indicated? Surgery is generally used to reduce hearing to at least 30-40 dB. If otosclerosis turns out to be cochlear, stapedotomy will not be indicated.
A non-minor detail is the fact that the evaluation of the transmission of sound by the ossicular chain is frequently intraoperative.
Medical therapy, on the other hand, is based on the intake of sodium fluoride, which is aimed above all at maintaining the clinical condition detected, especially in cases of cochlear otosclerosis where it is not possible to proceed with the surgical solution.