Timothy C. Hain,
Main ITG/TTG page. Please read our disclaimer. Last edited: February 2, 2017
Our dizziness practice in Chicago has had a large experience and very good results using low dose gentamicin for intractable Meniere's disease. This is how we do it. When there are multiple options, the one in bold is the one we use.
The easiest and most straightforward approach is the simple needle injection method. There does not seem to be much of an advantage to the butterfly catheter. The idea behind the Micro-Wick and Microcatheter is to provide a lower level, steady exposure to gentamicin. While the logic is reasonable, practically at the present writing, it doesn't seem to matter what delivery system is used to administer gentamicin. Perhaps this is because gentamicin is very slow to act on the inner ear. The Silverstein "Micro-Wick" requires a tympanostomy tube. The Microcatheter pump also requires a more invasive approach.
Gentamicin for IV injection comes in vials that contain 40mg/cc in 2 cc. In other words, the vial contains 80 mg of gentamicin, which is a solution with a concentration of 40 mg/cc.
The simplest technique is to just use the stock solution (2 cc of 40 mg/ml) as is. Having the solution close to body temperature helps to prevent dizziness from the caloric effect.
A more refined technique is to buffer the gentamicin and dilute it for patients where one wants to be more cautious. A small amount of gentamicin solution is made up by the pharmacy or in the office, in concentrations of 20 to 40 mg/ml, diluted with saline and buffered with bicarbonate. The purpose of the buffer is to prevent irritation of the middle ear due to pH. A small amount of lidocaine may also be added to the mixture, to make the process less painful. The solution should be at body temperature, to avoid caloric stimulation. In other words, warm it up if the vials are stored in the fridge.
If one is worried that the patient might be very sensitive to gentamicin, such as perhaps an older individual with long-standing Meniere's, use a 20 mg/ml solution (i.e. dilute it with an equal volume of sterile saline). If one wants to use the standard strength, then use the 40 mg/ml solution. The buffer or lidocaine, if used, will make it slightly more dilute.
Because the middle ear will only hold about 0.5 cc of any solution, practically, the round window is exposed to a solution that contains a total of roughly 20 mg, for 30 minutes.
A TB syringe is combined with a small spinal needle, containing 1 cc of the diluted gentamicin solution, and the solution is injected so as to fill the middle ear. The middle ear only holds about 1/2 cc, so one does not need to use the entire 2 cc contents of the vial. The rest will just run out or down the eustachian tube.
The spinal needle is bent so that the doctor can place the needle without having the syringe obscuring his/her vision. The solution is left in the middle ear for 30 minutes, by having the subject lie flat. Typically they are simply left in the ENT chair, reclining at about a 45 degree angle, with the injected side turned about 45 degrees to its side. This is to make it less likely that drug just goes down the eustachian tube. Of course, patients are encouraged to avoid "clearing the ears" during this time, as this could defeat the whole purpose of letting the solution diffuse across the round window membrane into the inner ear.
After 30 minutes have passed, the patient is sat up, and an attempt is made to clear the solution from the middle ear through the eustachian tube by having the subject sit upright and "pop" their ears. Just yawning or swallowing is often good enough.
Sometimes (rarely) an attempt is made to provide a "vent" with a second small hole in the TM. The idea is to ensure that the drug leaves the middle ear after 30 minutes. Clearance of the solution can be ascertained by looking to see if the drug is in the middle ear (perhaps it would be a good idea to use a colored solution ?).
Whether or not clearing the drug from the middle ear is important in obtaining predictable results has not been studied, but we suspect that it is. We say this because we have (rarely) encountered patients with much stronger effects of the gentamicin treatment than was expected. We expect that in most individuals, standing up and talking is enough to get the watery solution to drain out of the middle ear.
Patients ordinarily feel exactly the same after the injection, and can go home under their own power (i.e. drive, if they were driving before the injection). Dizziness from the injection is not expected till roughly a week post-injection.
A single dose (in total), possibly repeated on one occasion, is presently the preferred dosing regimen for low dose gentamicin in Meniere's disease. One injection is given, and the patient is rechecked by a doctor who has video-frenzel goggles available in the office at one month (or 2 months). If symptoms are unchanged in the 2nd half of the preceding month, and there are no signs of drug effect on vestibular function and hearing, a second injection is given.
Checking for signs of a drug effect is called "titration". In our experience, checking for spontaneous nystagmus (beating away from the injected ear) and vibration induced nystagmus is very effective. These methods of checking require use of video-frenzel goggles however. Generally speaking, if there is no spontaneous nystagmus, and no vibration induced nystagmus -- the injection is not going to work. Another method is to use the "VHIT" test. This is not as well standardized however and it is more difficult to find a practice that has one of these devices.
For weekly injections, between one and 6 injections are administered, typically once/week. This regimen is little used because it may take a week or more before the effects of gentamicin are appreciated on the ear. We don't think this is generally a good idea, unless one is planning to completely eliminate inner ear function. This is generally not needed to control Meniere's disease.
Otologists (ENT doctors who specialize in ear disorders) are the doctors who generally do these injections.
At Chicago Dizziness and Hearing, we make arrangements with ENT doctors that are close to us to carry this out (i.e. some planning is needed).