SUPERIOR SEMICIRCULAR CANAL DELHISCENCE
Superior semicircular canal dehiscence or SSCD is a condition which has only recently been identified; it was first described by Lloyd Minor in 1998. This condition affects the inner ear leading to auditory and vestibular symptoms. It is caused by a bony defect in the superior semicircular canal, which leads to the formation of a window between the inner ear and middle cranial fossa. This abnormal communication allows transmission of sound waves from the inner ear onward and can result in symptoms like vertigo, unsteadiness, falls, hearing loss and increased sensitivity to sounds. Oscillopsia or a feeling of moving up and down in the presence of loud sounds in a characteristic feature of SSCD. This feature is called Tullio’s phenomenon.
The inner ear is a closed hydraulic system of fluids with 2 functional windows. One is the oval window, which allows transmission of sound waves from the stapes (ossicular chain) to the scala vestibuli leading to vibration of the organ of the Corti. The 2nd window is the round window which allows release of sound energy in the form of waves from the scala tympani (inner ear) to the middle ear.
In SSCD, an additional window is created in the superior part of the superior semicircular canal allowing the sound energy to leak out from the closed system and transmit acoustic energy from the cochlear system (for hearing) to the vestibular system (for balance). Thus, loud sounds stimulate the vestibular system to cause vertigo and imbalance. Increased compliance of the inner ear leads to the signs and symptoms of SSCD which are described below.
The presentation of the symptoms dependS on the size of the dehiscence. Balance and hearing symptoms are seen in larger dehiscence while the smaller ones may present with either of them.
- Hearing loss
- Extreme sensitivity to sound: This symptom is due to enhanced bone conduction. The patient may complain of even the sound of their own footsteps, can hear their eyes moving, or a vibrating tuning fork placed at the elbow.
- Autophony or tympanophony: Hearing their own voice echo.
- Vertigo or unsteadiness in response to loud sounds (Tullio's phenomenon), straining.
- Tinnitus: This is usually pulsatile, like feeling your heart beat in the ear.
- Videonystagmography: Nystagmus may be elicited on Valsava manoeuvre. The nystagmus is usually vertical and may have a torsional component.
- Audiometry: Low frequency conductive hearing loss – this is due to the misdirected fluid and pressure dispersal leading to decreased basilar membrane displacement. Bone conduction is good, may even be better than 0 dB.
- Tympanometry: Dizziness on raising pressure, presence of stapedial reflex despite conductive hearing loss.
- VEMP (Vestibular Evoked Myogenic Potential): Lowering of thresholds is characteristic of SSCD.
- HRCT temporal bone: This is required for definitive diagnosis of SSCD and identifying site of breach to plan surgery. CT scan is done with reconstruction in the plane of the canal (Porschl’s plane) and 900 to the canal (Stenver’s plane) should be done. 0.5 mm cuts should be taken to avoid missing small defects.
After explaining the condition to the patient, 2 treatment options should be discussed.
- Conservative approach: For some patients, just understanding the medical condition and how it is causing the symptoms is sufficient to allay their anxiety and they are able to tolerate their symptoms.
- Surgery: This involves plugging the defect in the semicircular canal. This will close the 3rd window effect. This can be done by mastoid approach or middle fossa approach. However, there is a risk of some hearing loss in mastoid approach though most otologists are more comfortable in this surgery as compared to middle fossa approach.