Superior Semicircular Canal Dehiscence Syndrome (SCDS)
It is a condition characterized by the alteration of the labyrinthine bony capsule, inducing dehiscence in the Superior Semicircular Canal capable of creating a “third window” (also called Third Window Syndrome) in addition to the two naturally and anatomically present (Round and Oval Windows).
An abnormal communication is thus formed between the perilymphatic space and the middle ear and / or the temporal bone. In SCDS there is a wide symptomatic variability, due to the presence of both auditory and vestibular symptoms, associated in a very variable form. The Tullio Phenomenon and / or the Hennebert Sign may be present.
The Tullio phenomenon is characterized by the onset of dizziness, vertigo or oscillopsia as a consequence of a high intensity and low frequency sound stimulation.
The Hennebert Sign corresponds to the onset of nystagmus as a consequence of the application of positive pressure in the external auditory canal by means of a pneumatic otoscope or an impedance audiometry, sometimes also triggered by manual pressure on the tragus.
The Valsalva Manoeuvre can also cause the onset of more or less intense balance disorders.
Among the auditory symptoms that may be present, the following are the main ones:
• pulsating tinnitus
• bone conduction hyperacusis and low frequency transmission hearing loss.
Auditory symptoms often occur without vestibular symptoms (dizziness, vertigo, oscillopsia). It is probable that the auditory and vestibular association, also in terms of intensity of manifestation, is strictly dependent on the extent of dehiscence.
The aetiology considers a variety of causes, which can be congenital due to incomplete bone development, or head traumas inducing an increase in intracranial pressure or abrupt and sudden pressure changes in the middle ear.
SCDS was first described by Lloyd Minor in 1997, who detected via CT scan in patients affected by dizziness with Tullio’s phenomenon, the total or partial absence of the bony component that encapsulates the superior semicircular canal.
SCDS can be congenital or acquired (cholesteatoma, histiocytosis, head trauma, acoustic trauma, barotrauma, post middle ear surgery), as well as can be symptomatic or asymptomatic.
The ideal clinical evaluation includes an audiometric examination that can detect hyperacusis (increased auditory perception, which can be annoying) in the assessment of the audiometric bony pathway, and particularly relevant hearing loss on the severe tones of the audiogram. Concurrently, the Weber test localized on the side of the pathological ear is performed.
Moreover, a differential diagnosis from otosclerosis may be necessary. In this case, impedance audiometry is useful which, in the case of SCDS, shows a normal stapedial reflex and a Type A tympanogram bilaterally. The impedance measurement may also detect an alteration of the stapedial reflex at low intensity stimulation.
ABR of the VIII pair of cranial nerves will be normal.
In videonystagmoscopy, especially when the triggering the Hennebert sign which highlights a downbeat rotary nystagmus, a slow upward-facing phase is detected. With the application of the Valsalva manoeuvre, videonystagmoscopy can show a slow downward phase. The Mastoid Vibration Test (MVT) seems to be of some usefulness for the appearance of a rotary downbeat nystagmus with a greater amplitude as a result of stimulus on the pathological side.
Vestibular evaluation also benefits from FHIT and VHIT. Our experience at our Vestibology Medical centre has led to very good results with sound stimulation in Stabilometry.
The cVEMPs (with cervical stimulation) are also very significant, where much lower threshold values are found in the pathological ear with much higher amplitude values.
For diagnostic purposes, however, high-resolution CT of the temporal bone is useful. However, attention must be paid to possible false positives.
The typical patient with SCDS reports the onset of rotary vertigo with frequent association of oscillopsia or dizziness upon perception of high-pitched sounds or, following an alteration of the pressure in the middle ear, such as after sneezing or coughing fits.
However, SCDS has generally limited symptoms. Surgical treatment is required only in severe and decidedly disabling cases.