The overall goal of this surgical procedure is to evaluate the effects of specific drugs of interest on the hearing function of non-human primate common marmoset through their safe administration into the inner ear without inducing hearing loss. This method can help answer key questions about inner ear drug delivery, including how to open the master cavity and where to deliver the drug. The main advantage of this technique is that the inner ear can be a plus without complications, which is critical for evaluating the efficiency of inner ear treatments in primates.
The implications of this technique extend towards the study of potential therapies for human deafness, as the auditory system of the marmoset is quite similar to that of humans. The masteration of this method is critical and the posture and anatomy is difficult to learn and require that small window to be created in the narrow space without damaging the facial knob or the ossicular chain. Confirm the appropriate level of sedation by toe pinch and place the animal in the supine position on a heating mat with the head and neck extended.
Fix maxilla to the table with nylon thread and pull out the tongue. Insert an endotracheal tube along the laryngeal surface of the epiglottis. Check the entitle carbon dioxide level in the endotracheal tube to confirm that the tube has been properly placed within the trachea and secure the tube to the animal's face.
Incline the head to the 30 to 45 degree angle toward the non-operative side and shave the postauricular region of the operative side of the animal Then disinfect the postauricular region and the external auditory canal with povidone iodine and cover the animal with a surgical drape sheet with a round opening over the surgical area. Before opening the skin inject 1%of lidocaine hydrochloride subcutaneously to the incision site. Then, using a surgical microscope and a number 15 round shaped blade, make an incision into the postauricular skin and apply pressure to the bleeding area with a gauze soaked in one to 5, 000 diluted epinephrine.
Open the soft tissue of the postauricular region so that the auricular cartilage and the postauricular muscle can be identified and expose the external auditory canal bone deep within the cartilage. To confirm the depth of the tympanic membrane, expose the bony part of the tympanic rim and peel the postauricular muscles toward the caudal aspect to separate the muscles from the temporal bone. Expose the lateral surface of the temporal bone and use a one millimeter diamond burr to make a hole approximately five millimeters behind the external auditory canal.
Drill further to enlarge the hole as appropriate and locate the horizontal semi-circular canal, which bulges toward the mastoid cavity. Then drill and remove any bone septa as necessary to obtain a clear view of the facial recess, located at the center of the triangular area between the facial nerve and the corded tympany. Using a 0.6 millimeter diameter diamond burr at 1, 000 rpm, drill a small hole between the vertical section of the facial nerve, which appears as a pink line through the thinned bone and the tympanic rim.
Stop the drilling just before the nerve is exposed as a white line structure and confirm that the round window niche can be visualized through the hold. Using a 25 gauge or thinner needle, deliver the drugs of interest into the niche and cover the niche with an appropriate miscues solution to ensure the drug is retained. Confirm that hemostasis is securely controlled and use six o absorbent threads to close the epidermal and dermal layers with simple, interrupted sutures.
Then place a piece of gauze onto the surgical wound, fix the gauze in place with a bandage, and allow the marmoset to recover in an intensive care unit at 37 degrees Celsius in 30%oxygen until complete arousal. In these graphs, the auditory brain stem responses of a seven year old common marmoset before and two months after the delivery of one microliter of PBS to the round window niche, as just demonstrated, are shown. Comparison of these wave forms and thresholds to a previous study demonstrated no changes in these parameters before or after the surgery, confirming that posterior tympanotomy is a safe and useful procedure for drug administration to the round window niche of the common marmoset.
Once mastered, this technique can be completed in an hour, including the anesthesia and induction, if it's performed properly. While attempting this procedure, it's important to remember not to damage the ossicle or the facial knob which can result in conductive hearing loss or facial paralysis respectively. After its development, this technique paved the way for researchers in the field of the translation research to test potential new therapies for hearing loss.
After watching this video, you should have a good understanding of how to approach the inner ear of the common marmoset safely and to administer the drug into round window niche properly.
