Supplementary information regarding Intra/Transtympanic Gentamicin

Timothy C. Hain, MD
Main ITG/TTG page. Please read our disclaimer. Last edited: June 18, 2009 .

Local anesthesia for TTG (or myringotomy)

• A drop of phenol on the ear drum is the most commonly used method of numbing the ear drum. It burns for a few minutes but leaves an area on the drum that is numb for several weeks.
• Less long lasting anesthesia can be obtained with tetracaine or lidocaine, with or without iontophoresis.
• EMLA cream (lidocaine with prilocaine) can also be used.

Delivery systems for gentamicin

• Needle injection through ear drum (as above)
• Butterfly catheter T-tube assembly
• Micro Wick placed through a tympanostomy tube
• Microcatheter pump through tympanostomy tube

The cheapest approach appears to be 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. The Silverstein "Micro-Wick" requires a tympanostomy tube. The Microcatheter pump also requires a more invasive approach.

Drug preparation

A small amount of gentamicin solution is made up by the pharmacy, in concentrations of 20 to 40 mg/ml, 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.

Injection methodology

A TB syringe is used fit with a small spinal needle, containing 1 cc, and the solution is injected so as to fill the middle ear. 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.

After 30 minutes have passed, an attempt is made to clear it from the middle ear through the eustachian tube by having the subject sit upright and "pop" their ears. Sometimes 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. This seems to be a reasonable procedure to us, at least for people who are unable to clear the drug after 30 minutes. This 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.

Dosing Regimen -- monthly (low-dose)

This is presently the preferred dosing regimen. One injection is given, and the patient is rechecked 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.

Dosing Regimen -- weekly

For weekly injections, netween one and 6 injections are administered, typically once/week.