Use of the malleus handle as a landmark for localizing the round window membrane.

Abstract

Localization of the round window membrane is desirable in order to allow a more direct application of medication to the inner ear. A new procedure has been developed to deliver direct, near-continuous inner ear perfusion to the round window with the Silverstein Micro Wick. In this office procedure, the wick is inserted through a tympanostomy tube into the round window niche. Accurate localization of the round window is a necessary component of this procedure. In an effort to ascertain the precise location of the round window, we examined 25 cadaveric human temporal bones and measured the distance from the umbo to the round window in each sample. We found that the round window was an average of 3.44 mm ([+ or -]0.68) from the umbo and was situated at an average angle of ll3.2[degrees] ([+ or -]9.8)from the long process of the malleus. Our simple and reliable determination of the relationship between the malleus handle and the round window niche allows for the accurate placement of the Silverstein Micro Wick an d other devices.

Originally introduced by Schuknecht in 1956, the practice of delivering aminoglycoside perfusion to the middle ear for the treatment of Meniere's disease has become increasingly popular." [2] It has been established that the primary route of aminoglycoside entry into the inner ear is through the round window membrane. [3] The problem with perfusing the entire middle ear space is that it is unclear exactly how much medication is being applied directly to the round window membrane. The Silverstein Micro Wick inner ear medication delivery technique allows for the direct and precise delivery of medication to this site. [4,5]

Placement of a myringotomy incision or laser-assisted tympanostomy that allows for line-of-sight visualization of the round window niche should greatly increase the chance of a correct placement. The round window niche can often be seen through a normal tympanic membrane. However, previous disease, membrane thickening, and histologic variations can sometimes obscure the view. [6] In order to facilitate localization of the round window niche through an opaque tympanic membrane, we measured 25 cadaveric temporal bones to determine the relationship between the round window and an easily visible landmark, the malleus.

Materials and methods

We examined 25 randomly selected human cadaveric temporal bone specimens under an operating microscope. Often, the round window could be detected as a shadow through the intact tympanic membrane (figure 1). To make our measurements, a tympanomeatal flap was elevated in the standard fashion. A millimeter ruler was used to measure the distance between the umbo and the closest edge of the round window niche. These measurements were supplemented with photo documentation. We also measured the angle between this vector and a straight line drawn down the handle of the malleus in each temporal bone (figure 2).

Results

We calculated the angles and distances between the malleus and round window in each of the 25 temporal bones examined (table). We found that the mean distance from the umbo to the round window niche was 3.44mm ([+ or -]0.68) and that the mean angle from the malleus was 113.2[degrees] ([+ or -]9.8) (table).

Discussion

One of the methods used to treat Meniere's disease is the transtympanic perfusion of gentamicin. However, this medication is often injected blindly through the tympanic membrane in the area of the round window niche. As a result, an unknown quantity of medication is actually delivered to the round window; much of the medication becomes lost down the eustachian tube.

To overcome this obstacle, we previously advocated placing Gelfoam over the round window niche. [7] However, although this method provides a more direct delivery of drug than does blind perfusion, the dose must be administered under a microscope by a surgeon. Another method of delivering transtympanic perfusion is through a middle ear microcatheter, which must be placed in a hospital setting. [8] Both the catheter and the surgical procedure are expensive.

We recently developed the MicroWick technique for inner ear perfusion. With this method, the round window can be localized by using the malleus as a landmark (figure 3). After creating a laser-assisted tympanostomy, the MicroWick is advanced through a ventilation tube down to the round window membrane. Thereafter, patients can self-administer their medications to the inner ear.

The advantages of using the MicroWick are that it is easy to insert in the office, it is well tolerated and inexpensive, and patients can easily administer their medication themselves in a relatively continuous fashion.

We have performed this procedure in more than 100 patients, and we have experienced no difficulty locating the round window niche using the measurements and relationships we have described. Our examination of human cadaveric temporal bones also demonstrated that a reliable relationship exists between the malleus handle and the round window niche. Putting to use our knowledge of this relationship will allow for the safe and accurate placement of standard instruments and materials into the round window niche and will simplify inner ear perfusion techniques.

References

(1.) Schuknecht HF. Ablation therapy for the relief of Meniere's disease. Laryngoscope 1956;66:859-70.

(2.) Silverstein H. Use of a new device, the MicroWick, to deliver medication to the inner ear. Ear Nose Throat J 1999;78:595-S, 600.

(3.) Smith BM, Myers MG. The penetration of gentamicin and neomycin into perilymph across the round window membrane, Otolaryngol Head Neck Surg 1979;87:888-91.

(4.) Silverstein H, Jackson LE, Rosenberg SI. Silverstein Microwick for the treatment of the inner ear. Operative Techniques in Otolaryngology-Head and Neck Surgery. In press.

(5.) Silverstein H. Self-treatment of inner ear disease using otic medication delivered by the Silverstein MicroWick. In: Sterkers O, Ferrary E, Dauman R, et a], eds. Meniere's Disease 1999: Update. The Hague: Kugler Publications, 2000:627-3 6.

(6.) Silverstein H, Rowan PT, Olds MJ, Rosenberg SI. Inner ear perfusion and the role of round window patency, Am J Otol 1997;1S:586-9.

(7.) 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.

(8.) DeCicco MJ, Hoffer ME, Kopke RD, et al. Round-window microcatheter-administered micmdose gentasnicin: Results from treatment of tinnitus associated with Meniere's disease. Int Tinitus J 1998;4:141-3.

Table. Measurements of the distance
and the angles between the malleus
and round window in the 25 cases
                   Angle from        Distance from
   Case      the malleus ([degree])  the umbo (mm)
     1                120                 3.0
     2                110                 4.0
     3                110                 3.5
     4                130                 3.0
     5                110                 4.5
     6                120                 4.5
     7                100                 4.0
     8                120                 3.5
     9                110                 3.5
    10                110                 2.5
    11                110                 3.5
    12                130                 2.5
    13                130                 2.5
    14                120                 4.0
    15                100                 3.0
    16                120                 4.0
    17                100                 3.0
    18                120                 3.0
    19                120                 4.0
    20                110                 3.5
    21                100                 2.5
    22                120                 3.5
    23                100                 5.0
    24                110                 3.0
    25                100                 3.0
Mean (SD *)       113.2 (9.8)         3.44 (0.68)
(*)Standard deviation.