Use of a new device, the [MicroWick.sup.TM], to deliver medication to the inner ear.

From the Ear Research Foundation, Sarasota, Fla.

Reprint requests: Herbert Silverstein, MD, Ear Research Foundation, 1901 Floyd St., Sarasota, FL 34239. Phone: [phone]; fax: [phone]; e-mail: [e-mail]

Abstract

A new procedure for delivering medication directly to the inner ear has been developed. This delivery system, called the [MicroWick.sup.TM], involves the use of a small wick that is inserted through a tympanic membrane vent tube into the round window niche. Once the wick has been inserted, the patient can self-administer eardrops into the ear canal, where they are absorbed by the wick and transported to the round window membrane and to the inner ear fluids. Inserting the wick is a minor procedure that is performed in the office. This paper describes the indications for and use of the MicroWick.

Introduction

Perfusion of the inner ear with various medications such as gentamicin and steroids is becoming a more popular form of treatment for inner ear disease. [1-8] There are three advantages to placing medications directly into the inner ear: 1) the physician can treat the diseased ear directly without affecting the rest of the body; 2) systemic side effects of the drug are avoided; and 3) a higher concentration of medication can be delivered directly to the site. The current indications for the direct treatment of inner ear disease are for uncontrolled vertigo in Meniere's s disease, sudden viral deafness, and autoimmune inner ear disease.

Three techniques have been developed thus far to inject medication directly into the inner ear. One route is through the tympanic membrane into the middle ear. [9] Another route is through a tympanostomy opening opto Gelfoam that has been placed in the round window niche. [3] The third route is through a microcatheter that has been inserted into the round window niche. [1] The microcatheter technique is performed in the hospital, and both the catheter and the procedure are expensive.

A safer, simpler, and more cost-effective procedure for the treatment of inner ear disease has been developed. This method involves the use of a small wick that is inserted through a tympanic membrane vent tube into the round window niche. The patient places eardrops into the ear canal and they are absorbed by the wick and transported to the round window membrane and to the inner ear fluids. The new device is called the [MicroWick.sup.TM]. Inserting the wick is a minor procedure that can be performed in the office.

Insertion of the device

Anesthesia. Before inserting the MicroWick, the ear must be anesthetized. There are several ways to accomplish this. One way is to inject the ear with lidocaine 1% and adrenaline (1:100,000) buffered with sodium bicarbonate (ratio: 1 ml of sodium bicarbonate to 9 ml of lidocaine). The buffering greatly diminishes the initial pain that is usually felt by the patient. Another method is to use drops of a fresh mixture of 180 mg of topical tetracaine base powder with 0.2 ml of isopropyl alcohol. This solution will anesthetize the tympanic membrane in 10 minutes and will last for several hours. [10,11] Finally, a third method is to apply phenol directly to the tympanic membrane. But because topical anesthetics do not numb the skin of the external auditory canal, infiltrative lidocaine and adrenaline are usually preferred when performing otoendoscopy.

Tympanostomy and otoendoscopy. Before inserting the MicroWick, the surgeon may perform an otoendoscopy of the middle ear. The surgeon can use either a [CO.sub.2] laser to make a 2-mm bloodless opening over the round window niche or a myringotomy knife to make a 4-mm incision parallel to the posterior canal wall at the level of the umbo. The round window lies approximately 3.44 mm (SD [plus or minus] 0.68) posterior and inferior to the malleus at a 113[degrees] (SD [plus or minus] 9.9[degrees]) angle drawn from the tip of the umbo. [12] In a normal tympanic membrane, the round window is usually seen as a dark shadow beneath the tympanic membrane.

A small 30[degrees] otoendoscope, 1.7 mm in diameter, is introduced to determine whether adhesions or membranes are present over the round window membrane that would obstruct or impede the flow of medication into the inner ear; this occurs in approximately 20% of cases. [3] When such membranes are present, they are easily removed with a small pick. The adhesions are approximately 1 mm from the round window membrane.

Next, a silicone vent tube Silverstein with an inner diameter of 1.42 mm and a 1-mm flange is inserted into the tympanostomy opening. The round window niche is seen through the tube opening (figure 1). The vent tube prevents the swelling caused by the MicroWick from enlarging the opening in the tympanic membrane.

Inserting the wick. The Micro Wick is made of polyvinyl acetate. It is 9 mm long and 1 mm thick (figure 2), narrow enough to fit though a 1.42-mm vent tube. Before insertion, the wick should be rolled and compressed with gloved fingers to reduce its diameter. When the target area is free of blood and fluid, the MicroWick is inserted through the vent tube by placing its distal end into the round window niche (figures 3, 4).

After 4 weeks, the vent tube and the wick are removed with a pick as a single unit. Pulling the wick through the vent may cause it to break. The tympanostomy can be closed with a [3-mm.sup.2] piece of Gelfilm impregnated with an antibiotic ointment. The tympanic membrane usually heals in 2 to 4 weeks. Anesthesia is usually not necessary when removing a silicone vent tube or when placing the Gelfilm patch.

Delivering medication to the inner ear

The surgeon delivers the first dose of the prescribed medication onto the MicroWick with a 27-gauge, 1.5-in needle on a tuberculin syringe. The patient is then given a prescription for the medication to self-administer and instructions to return to the office weekly for evaluation.

Menier's disease. For patients with Meniere's s disease, sterile techniques are used in the pharmacy to prepare a 10-ml dilute solution of gentamicin eardrops. Usually 10 mg/ml is prepared and poured into a sterile dropper bottle. A lower concentration (5 mg/ml) is usually used when hearing is better than 30 dB pure-tone average.

The patient is instructed to lie supine for 15 minutes with the head turned and then self-administer 3 drops of medication into the ear canal three times a day. At the end of each week, the patient returns to the office and hearing is evaluated by audiometric testing of air, bone, and discrimination, and vestibular function is tested by bithermal air and ice air electronystagmography (ENG). Depending on the test results and the patient's symptoms, treatment is either continued or terminated. The goal of treatment is to obtain a 100% reduced vestibular response to both bithermal and ice air caloric ENG testing without producing a loss of hearing.

The usual duration of treatment is 1 to 3 weeks. If hearing has significantly diminished during treatment but vestibular function is still present, then the dose of gentamicin can be reduced to 5 mg/ml or the treatment can be temporarily stopped. Retreatment with a low dose is possible after the hearing stabilizes. Gentamicin relieves attacks of Meniere's disease in 80% of cases.[13] If the vertigo attacks continue, the physician can perform either a posterior fossa vestibular neurectomy or a labyrinthectomy, depending on the level of hearing.

Sudden deafness and autoimmune inner ear disease.

A solution of dexamethasone 24 mg/ml is prepared at a pharmacy and put in a sterile dropper bottle. Best results are obtained when treatment is initiated as early as possible. Treatment is usually prescribed for no more than 4 weeks. The decision whether or not to prescribe oral prednisone prior to dexamethasone treatment rests with the individual physician. If the patient cannot tolerate high-dose steroids because of concomitant hypertension, diabetes, or ulcer, the physician can proceed directly with intratympanic dexamethasone. If the patient can tolerate oral steroids, then it is up to the patient and the physician to decide which treatment will be tried first.

The patient should administer 3 drops three times daily and lie in the supine position with the head turned for 15 minutes after instilling the medication. In addition, antibiotic eardrops should be instilled once a day to prevent infection. Hearing is assessed after 2 weeks, at which time the physician should determine whether to continue or terminate treatment.

Complications

There have been no cases of permanent tympanic membrane perforation or persistent infection that precluded the completion of treatment with the MicroWick. There have been two cases where the vent tube and wick extruded during treatment and needed to be replaced.

Discussion

The MicroWick device is simple and easy to use. The advantage of using the laser technique to make the opening in the tympanic membrane is that it makes a bloodless opening, thereby minimizing the chance of fluid coming into contact with the wick or endoscope. Any fluid that comes into contact with the MicroWick will cause it to swell immediately and will preclude proper placement in the round window niche. When blood or fluid in the area cannot be removed, the procedure can be terminated after the endoscopic view of the middle ear has been made and the vent tube has been inserted. The MicroWick can be placed several days to a week later, after the ear has dried.

The advantage of looking into the round window niche with the endoscope is that the surgeon can detect any obstructing membranes that may be present, which occurs in approximately 20% of the cases. [3] The membranes can be removed with a small pick and the microscope. The round window membrane niche should be completely open to accept the MicroWick.

The MicroWick can still be placed if a laser and endo scope are not available. Phenol or a tetracaine-based solution can be used to anesthetize the tympanic membrane, after which the vent tube can be inserted directly over the round window niche. The placement of the incision in the tympanic membrane is critical to the successful insertion of the MicroWick into the round window niche. Usually the round window niche can be seen through a normal tympanic membrane as a dark shadow beneath the tympanic membrane. If the tympanic membrane has been scarred as a result of a previous infection, determining the exact location beneath the tympanic membrane can be difficult. With the laser, a small opening can be made in the tympanic membrane over the suspected site, and the location of the 2-mm opening can be adjusted so that it is directly over the round window niche. A vertical myringotomy posterior to the umbo will allow for the proper placement of the vent tube over the round window niche.

Cautions. Certain cautions need to be kept in mind when using the MicroWick technique. For example, occasionally the opening in the tympanic membrane will not be directly over the round window niche. But even if this occurs, medication will still enter the middle ear and diffuse through the round window membrane. Also, when using a steroid to treat inner ear disease, antibiotic eardrops must be administered once a day to prevent middle ear infection. Finally, pulling the MicroWick through an implanted vent tube may cause it to break off in the middle ear. This will require injecting the ear with lidocaine and removing the remaining wick material.

But the advantages of this delivery system outweigh any potential drawbacks. For example, placement of the MicroWick is easy, safe, and inexpensive. Moreover, self-treatment of inner ear disease with otic medication is well accepted by patients, and it obviates the necessity for frequent return office visits. The medication can be kept in contact with the round window membrane for long periods, which is more efficient than administering blind injections through the eardrum in the surgeon's office. Another advantage is that most of the medication is not lost down the eustachian tube, which can occur when medication is injected blindly into the middle ear. With self-administered gentamicin, vestibular function has been completely ablated (no response to ice air caloric testing) in 55% of cases, and hearing has been maintained at pretreatment levels in 80%. [14] Because most patients with Meniere's disease have a sensorineural hearing loss prior to treatment, a further hearing loss has not been a major complain t, especially when vertigo attacks have been relieved. Finally, the MicroWick can be safely left in place for 4 weeks.

In the future, new medications might be developed to treat certain types of sensorineural hearing loss and to reduce tinnitus. In such an eventuality, the MicroWick would allow the patient to self-administer these drugs directly to the inner ear.

Note

The MicroWick is produced and distributed by Micromedics Inc. of Eagan, Minn. The device is scheduled for distribution in September 1999. ([MicroWick.sup.TM] patent pending Feb. 17, 1999; [MicroWick.sup.TM] trademark applied for April 2, 1999.)

References

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(12.) Silverstein H, Durand B. Location of the round window niche in relationship to the malleus handle [in preparation].

(13.) Silverstein H, Arruda J, Rosenberg SI, et al. Direct round window membrane application of gentamicin in the treatment of Meniere's disease. Otolaryngol Head Neck Surg 1999;120:649-55.

(14.) Silverstein H. Self-treatment of inner ear disease using otic medication delivered by the Silverstein [MicroWick.sup.TM]. Presented at the 4th International Symposium on Meniere's Disease. Paris, April 1999.