Shunt Procedures
Harold F. Schuknecht
Michael J. McKenna
EXTERNAL ENDOLYMPHATIC SHUNT
The external endolymphatic shunt operation purports to alleviate the vertiginous symptoms of Meniere’s disease by preventing the accumulation of excess endolymph by shunting it from a presumably incompetent endolymphatic sac to either the subarachnoid space or the mastoid. Ever since Guild (1) presented the theory of longitudinal flow of endolymph toward the endolymphatic sac and Portmann (2) proposed the shunt operation, the procedure has been steeped in controversy. There are strong advocates for shunting, many of whom have devised modifications based on some conceived notion for improving drainage, and there are almost as many skeptics who point out the improbability of achieving drainage by any shunt operation.
Portmann’s (2) original procedure consisted of removing part of the lateral wall of the sac, thus exposing the lumen to the mastoid. Yamakawa and Naito (3) modified the method by removing part of the medial wall of the sac to expose its lumen to the subarachnoid space, and House (4) devised a silicone tube that extended from the sac into the subarachnoid space. Subsequently, Shea (5), Paparella and Hanson (6), and many others (7, 8, 9, 10), not being enamored of the idea of opening into the subarachnoid space, proposed draining the sac into the mastoid using a variety of tubes or sculptured sheets of plastic material. Arenberg et al. (11) have advocated the use of a unidirectional valve, which is a modification of the valve designed for filtering the anterior chamber of the eye. Brackmann and Nissen (12) could show no significant differences between subarachnoid and mastoid shunts in the control of vertigo, with both procedures reporting a 70% success rate.
The endolymphatic sacotomy procedures have suffered some loss of credibility from the reports of Shambaugh (13) and Larouere and Graham (14) that merely decompressing the sac achieves the same success rate as opening it and the report of Bretlau et al. (15) that sham surgery limited to merely entering the mastoid is as successful as sac drainage procedures.
There are also several histopathologic observations that challenge the rationale of sac drainage procedures: (i) Any device implanted in the sac is almost certain to become encapsulated in fibrous tissue (16), which certainly would compromise its presumed function in promoting drainage. (ii) In some cases the endolymphatic sac is too small to be identified and drained (17). (iii) In some cases the endolymphatic duct is blocked by bone or fibrous tissue, which prevents endolymph from reaching the sac (18). (iv) In many ears the membranous labyrinths are severely distorted by dilation and collapse, causing blockage of longitudinal flow at the ductus reuniens, saccular duct, utricular duct, and sinus of the endolymphatic duct (18).
This preface should alert the reader to some significant flaws in the therapeutic concept of endolymphatic sac shunt procedures. We have used the shunt at the Massachusetts Eye and Ear Infirmary on a limited basis for patients with Meniere’s disease who were experiencing intractable and disabling vertigo in the presence of good hearing. The results are similar to the reports of others, that is, approximately 70% of patients are initially relieved of vertigo or are greatly improved. Our enthusiasm has been dampened, however, by the previously mentioned temporal bone findings and the high incidence of delayed postoperative failures.
From perusing reports in the literature, it seems quite clear that shunting into the subarachnoid space does not give better results than shunting into the mastoid and therefore does not warrant the increase in potential morbidity that comes with opening the subarachnoid space. If hearing is useless, it is generally preferable to ablate vestibular function by labyrinthectomy unless the patient is elderly, in which case a cochleosacculotomy may be preferable.
Indications for Endolymphatic Shunt Surgery
Surgical intervention for Meniere’s disease should be considered only for patients with disabling episodic vertigo that has failed to respond to medical management. Because endolymphatic shunt surgery is not an ablative procedure, it offers the advantage for potential alleviation of episodic vertigo without the loss of residual vestibular function in the operated ear. For this reason and because of a relatively low
incidence of surgically induced sensorineural hearing loss, it is often considered among the best first options for the management of recalcitrant Meniere’s disease. These advantages must be weighed against the relatively high failure rate of approximately 30%. Endolymphatic shunt surgery does not preclude other forms of surgical intervention, including labyrinthectomy and selective vestibular neurectomy, if the symptoms of episodic vertigo are not relieved. It is our practice to present patients with a thorough discussion of both endolymphatic shunt surgery and intratympanic gentamycin therapy as the first best options for the management of Meniere’s disease that has failed medical management and when the affected ear has serviceable hearing. For intratympanic gentamycin therapy, the advantages are that it can be performed in an outpatient setting without the need for a general anesthetic and that the success rate for the control of vertigo is superior to endolymphatic shunt surgery, in the range of 85% to 90%. The disadvantages of intratympanic gentamycin therapy are that it is an ablative procedure that results in loss of vestibular function in the treated ear and that it has a higher incidence of sensorineural hearing loss, in the range of 10% to 15%. Because endolymphatic shunt surgery is not ablative and has a relatively low incidence of associated sensorineural hearing loss, it may be considered as a viable treatment option for some patients with bilateral Meniere’s disease. In rare cases, endolymphatic shunt surgery may be considered in patients with Meniere’s disease in an only hearing ear. The indication for surgery in such cases is a progressive disease process with declining sensorineural hearing that has failed all forms of medical management, including immunosuppressive therapy, when it is clear that the active disease process poses a greater risk to hearing than the surgical intervention. Endolymphatic shunt surgery is contraindicated in cases of enlarged vestibular aqueducts or in ears with concurrent active chronic otitis media.
incidence of surgically induced sensorineural hearing loss, it is often considered among the best first options for the management of recalcitrant Meniere’s disease. These advantages must be weighed against the relatively high failure rate of approximately 30%. Endolymphatic shunt surgery does not preclude other forms of surgical intervention, including labyrinthectomy and selective vestibular neurectomy, if the symptoms of episodic vertigo are not relieved. It is our practice to present patients with a thorough discussion of both endolymphatic shunt surgery and intratympanic gentamycin therapy as the first best options for the management of Meniere’s disease that has failed medical management and when the affected ear has serviceable hearing. For intratympanic gentamycin therapy, the advantages are that it can be performed in an outpatient setting without the need for a general anesthetic and that the success rate for the control of vertigo is superior to endolymphatic shunt surgery, in the range of 85% to 90%. The disadvantages of intratympanic gentamycin therapy are that it is an ablative procedure that results in loss of vestibular function in the treated ear and that it has a higher incidence of sensorineural hearing loss, in the range of 10% to 15%. Because endolymphatic shunt surgery is not ablative and has a relatively low incidence of associated sensorineural hearing loss, it may be considered as a viable treatment option for some patients with bilateral Meniere’s disease. In rare cases, endolymphatic shunt surgery may be considered in patients with Meniere’s disease in an only hearing ear. The indication for surgery in such cases is a progressive disease process with declining sensorineural hearing that has failed all forms of medical management, including immunosuppressive therapy, when it is clear that the active disease process poses a greater risk to hearing than the surgical intervention. Endolymphatic shunt surgery is contraindicated in cases of enlarged vestibular aqueducts or in ears with concurrent active chronic otitis media.
Some patients who have undergone successful endolymphatic shunt surgery may develop a recurrence of symptoms in the operated ear years following surgery. If it is clear that the initial surgery resulted in a significant benefit, many of these patients will benefit from a revision endolymphatic shunt surgery.
Surgical Technique
Prior to endolymphatic saccotomy, an axial computed tomographic (CT) scan of the temporal bone is done to determine if there is sufficient space between the sigmoid sinus and the posterior semicircular canal to permit access to the area of the dura mater in which the sac is located. A separation of at least 3 mm is necessary to identify and enter the sac. Also, the CT scan will show the relationship of the posteromedial cell tract to the endolymphatic sac. This cell tract may lie between the endolymphatic sac and the dura, in which case the sac is surrounded by air cells and does not enter the dura until it reaches the sigmoid sinus. In such cases an unsuspecting surgeon may drill through the endolymphatic sac without ever having identified it.
The operation is performed under general anesthesia and normally requires 1 to 1.5 hours operating time. The mastoid is entered via a postauricular approach, and the lateral semicircular canal and incudal fossa are identified. Once exposed, the mastoid antrum is temporarily occluded with a piece of compressed Gelfoam to prevent spillage of bone dust into the middle ear and a resultant conductive hearing loss. The mastoid air cell system is sufficiently exenterated to identify and skeletonize the bony prominence of the sigmoid sinus. Bone is removed anterior to the sigmoid sinus and posterior to the posterior semicircular canal to expose the dura mater (Fig. 29.1). The endolymphatic sac can be identified as a thickened whitish-appearing area of the dura. A no. 11 Bard-Parker knife blade is used to incise the lateral wall of the sac (Fig. 29.2A). A dental excavator is introduced into the sac, and its anatomic configuration and size are explored. An implant of the surgeon’s choosing (e.g., Silastic or Teflon sheeting or tube) is introduced into the sac, and Gelfoam or tissue graft is placed over the implant. The author fashions a dumbbellshaped implant from Silastic sheeting,1 one lobe of the dumbbell to fit the sac and the other extending into the mastoid cavity (Fig. 29.2B). The expectation is that fibrous tissue will grip the narrow middle part of the implant and thereby prevent extrusion of the implant from the sac. The postauricular incision is closed in layers.
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