Surgical Repair of SCDS
Two surgical approaches have been used - superior canal resurfacing and superior canal plugging. Although theoretically less traumatic to the membraneous labyrinth, resurfacing techniques have led to recurrent symptoms secondary to shifting or resorption of the bone and fascia reconstruction. Plugging with bone wax, bone paste, or fascia has been more commonly used to create a stable seal of the dehiscent superior semicircular canal.
MIDDLE FOSSA CRANIOTOMY
Middle fossa craniotomy is a safe surgical procedure and is the most common approach used to repair SCD. In many cases, the skull base, or tegmen, is low-lying, and will have multiple holes or defects that in some cases allows for brain to herniate into the ear / mastoid / middle ear. The craniotomy approach is ideal to 1) visual the SCD directly and 2) repair the tegmen defect(s) directly using bone and fascia.
If the skull base or tegmen is very low it can be difficult to safely reach the superior canal safely with a transmastoid approach to create two labyrinthotomies (as the SCD is not directly repaired) and then plug these two holes to indirectly repair the defect.
TRANSMASTOID APPROACH (Case #4)
Ideally, the transmastoid approach may be useful for a patient that has SCD and a skull base or tegmen that is NOT low-lying and has no other associated holes or brain herniating into the ear. In some cases, the SCD may be located along the SIDE of the canal (Case #4) and not on the TOP, as seen in most classic cases of SCDS. If the defect is located on the side of the canal away from the surgeon (posterior-medial), the SCD may be more difficult to visualize using a craniotomy approach.
The transmastoid approach is a safe approach and small studies have shown that this is a reasonable alternative surgical technique for SCDS, provided that the skull base / tegmen is not too low and that there is no evidence for large defects / holes in the skull base or brain sagging into the ear.
Case Report: a 29 year old otherwise health woman previously diagnosed with left-sided Meniere's disease
This patient complained of chronic dysequilibrium and episodic dizziness, worsened with straining and loud noises. Her eyes jump "up and down," causing the horizon to jump in the vertical plane. She notes an extremely violent episode of vertigo, in the vertical plane, during child birth. She has since had hearing loss, autophony, and sensitivity to loud noises and pressure on this left ear.
Serial audiograms before surgery showed a progressive severe sensorineural hearing loss on the left ear with a mild to moderate conductive component in the low frequencies, and present stapedial, or acoustic reflexes.
She had a history of congenital deafness in the right ear.
Vestibular Evoked Myogenic Potential Testing (VEMP) revealed markedly decreased thresholds in the left ear (50 to 60dB using tone burst testing), compared with the right (80 to 90 dB).
ABOVE: Temporal bone CT scans of the right (A,C) and left (B,D) superior semicircular canals. These were high resolution CT scans reformatted to the planes of Stenver (A,B) and Poschel (C,D). A very thin, but intact bony covering is see over the right superior semicircular canal (A,C) compared with a dehiscent left superior semicircular canal (B,D).
This patient was followed conservatively for several months with vestibular suppressants but her symptoms worsened. A big concern was that she was deaf in the right ear since birth, and so there was great reluctance to offer a left sided SCD repair. However she could no longer care for her young children and was unable to function normally, and so after careful counseling, she elected to proceed with a surgical repair of her left SCD via a middle fossa craniotomy approach.
A 4-5mm widely dehiscent SSC was encountered intraoperatively and was plugged with bone wax. Because the surrounding tegmen (skull base) is often very thin or also dehiscence, a bone graft was used to reconstruct the floor around and on top of the plugged canal.
She had a 2 day hospital stay and then was discharged to home in good condition.
Within about one month following her surgery, she noticed a complete resolution of her dizziness with improvement in her conductive hearing loss. She has returned to rock climbing, cycling, and caring for her young children.
Audiogram, left: This patient had congenital deafness of the right ear. On the left, she had a mixed hearing loss on the side of the SCD.
Audiogram, right: Postop hearing test showing closure of her air-bone gap following SCD plugging.
Below: 3-D reconstruction, lateral skull CT, showing the approximate size and shape of the middle fossa craniotomy needed to expose and repair a left-sided SCD.
Intraoperative Photo Summary:
Photo 1: middle fossa craniotomy performed, centered over the external auditory meatus, approximately 4 x 5cm. The tegmen represents the floor of the middle fossa, which is exposed using an extradural dissection to reveal the dehiscent left superior semicircular canal. The dura and the temporal lobe of the brain are retracted superiorly. A split calverial bone graft is harvested from the bone flap.
Photo 2: bone wax is use to plug the superior canal dehiscence
Photo 3: bone paste, harvested while turning the middle fossa craniotomy bone flap, is used to resurface the plugged superior semicircular canal.
Photo 4: pressed temporalis fascia graft is used to cover the bone paste and plugged superior semicircular canal
Photo 5: split calverial bone graft followed by a second piece of fascia is used to resurface the remaining skull base floor. This is completed by using fibrin glue before removing the middle fossa retractor.
Not shown: once the skull base reconstruction is complete, a mini-titanium plate cranioplasty is performed, followed by multiple layer tissue and skin closure.