SSCD
Superior Semicircular Canal Dehiscence (SSCD) is described as the absence of the bony covering of the superior semicircular canal.
This creates a virtual third mobile window,
other than the oval and round existing ones,
and the sound is transmitted to the vestibular system causing vertigo by excitation of the cupula creating dizziness and nystagmus.
This third window also leads the sound away from the cochlea and hence causes conductive hearing loss in some individuals.
It also may remain asymptomatic.
Superior canal dehiscence probably arises from failure of postnatal bone development.
The usual bilateral nature of the thinning and the mature lamellar bone on the margins of the thin areas point towards a developmental cause(4).
However other factors may intervene in the pathologic process as the clinical manifestations usually appear in middle aged patients.
Some authors hypothesize that patients with this disorder fail to develop a normal thickness of bone overlying the superior canal and that this abnormally thin layer of bone may become disrupted by a traumatic event or by pressure from the overlying temporal lobe or dura (6) (8).
Clinical symptoms may be unsteadiness associated with loud noises and/or pressure changes. Tullio phenomenon defined as vertigo and nystagmus precipitated by loud noises and the Hennebert sign (vestibular symptoms with pressure changes) are classically described in this entity.
Associated features are hearing loss and chronic balance problems.
Some patients have exclusively vestibular symptoms and signs; some have both auditory and vestibular manifestations; and still other patients have exclusively auditory complaints.
This condition is diagnosed not only on the basis of vestibular and auditory symptoms,
but also on radiological findings such as absence of bone overlying the Superior Semicircular Canal.
Very symptomatic patients with balance alterations,
unsteadiness or others incapacitating vestibular or auditory symptoms may benefit from surgical repair of the dehiscent canal through middle fossa or transmastoid approach.
TEGMEN TYMPANI DEFECT
The tegmen tympani is formed by the petrous and squamous portions of the temporal bone.
There is a suture between these two portions called the petrosquamous suture,
which is unossified in youth and ossifies in adult age.
The tegmen tympani extends posteriorly to roof the tympanic antrum and anteriorly to cover the semicircular canals.
Its lateral edge corresponds with the remnants of the petro-squamous suture.
The medial part of the tegmen tympani develops form the otic capsule during chondral ossification,
and the lateral portion of the tegmen tympani develops by membranous ossification.
Congenital defects of the tegmen tympani develop ventrally to the geniculate ganglion and may be due to incomplete ossification of the tegmental process of the otic capsule.
Inadequate closure of the petrosquamous suture may be a factor as well.
TTD can lead to meningoencephalocoele and/or cerebrospinal fluid (CSF) leak and consequently recurrent meningitis,
convulsions,
paresis,
aphasia,
among other possible complications.
Meningoencephalocoeles are herniations of brain tissue through dehiscences of skull base.
ASSOCIATION
SSCD can be congenital or acquired,
unilateral or bilateral.
Its pathophysiology nowadays is not well understood but several hypotheses exist,
pointing out association between SSCD and TTD,
suggesting they might share a common mechanism and even be part of the same syndrome.
Some recent studies have observed that there is an association between these two entities concluding that the radiological absence of Tegmen Tympani is more frequent in ears that also have radiologic SSCD (2) (6).
Clinical symptoms may be conductive hearing loss and/or vertigo associated with Tullio phenomenon or Hennebert sign.
But,
considering this association,
there are also other symptoms and findings of extremely importance such as meningoencephaloceles or CSF leaks.
Radiologists must be aware of these possible findings when performing a conductive hearing loss study,
and if SSCD is found,
look further at TTD.