Using this procedure, endolymphatic hydrops can be induced relatively quickly in guinea pigs with an adequate success rate. The main advantage of this technique is that endolymphatic hydrops can be induced with minimal risk for CSF leak or injury to the posterior semicircular canal. After confirming a lack of response to pedal reflex, inject a 12 milliliter subcutaneous fluid bolus of lactated Ringer's solution into the back of the anesthetized guinea pig and place the guinea pig in the supine position on a warming pad.
Place a 27.5 gauge butterfly needle intraperitoneally confirming a correct placement by the presence of air only within the needle tip and place the guinea pig in the prone position. Secure the head in a stereotactic holder and secure a pulse oximeter onto an unpigmented foot. Use a rectal probe and a warming blanket to maintain the body temperature at 38 degrees Celsius and apply ointment to the animal's eyes.
Place a piece of masking tape over the back to provide adequate tension along the skin overlying the occiput and fix the ends of the tape to the stereotactic holder. Then liberally prep the skin overlying the occiput and posterior neck with three consecutive iodine scrubs and 70%alcohol and place a sterile drape over the animal. Begin the surgical procedure by using a number 15 blade to make a small midline incision along the posterior occiput extending down to the posterior neck.
Once under the skin, use iris scissors to detach the right posterior cervical muscles from the occipital bone, controlling any bleeding with pressure from a sterile cotton ball. Using a dissecting microscope and a combination of a number three, two, and one millimeter diamond burrs with a 5-0 suction and sterile irrigation, perform a craniectomy that is bounded as far as the external occipital crest laterally, lambdoidal ridge superiorly, the occipitomastoid suture line medially, and the dorsal margin of the foramen magnum inferiorly. And use a 0.5 millimeter diamond burr to skeletonize the sigmoid sinus.
Gently place a small piece of saline moistened cotton under the bone while separating the occipital bone from the dura. Carefully remove the bone overlying the sinus and use a cotton ball to gently retract the sigmoid sinus medially. Carefully retract the sigmoid sinus and identify the operculum as a slit-like structure located within the petrous temporal bone.
The extraosseous portion of the endolymphatic sac can be visualized as a clear sac-like connection between the operculum and the dura overlying the sigmoid sinus. Apply gentle medial retraction to the sigmoid sinus to clearly visualize the extraosseous portion of the endolymphatic sac and increase the tension between the extraosseous and intraosseous portions of the sac. Using a fine angled pick, gently expunge the intermediate portion of the endolymphatic sac and broadly scrape a fine pick inside the operculum along the inside of the bone.
It's critical to the success of this procedure that no visible connection between the dura and the operculum remain. Use a small curette to scrape along the temporal bone to obtain bone chips and generously pack the operculum with the bone chips. Seal the operculum with bone wax taking care that there is no excess wax dislodged into the skull and use bone wax to cover the craniectomy defect.
Then approximate the posterior cervical muscles with 4-0 braided absorbable sutures in an interrupted fashion and perform a two-layer closure by approximating the deep layer with simple interrupted sutures and the epidermal layer using a subcuticular closure with a 4-0 braided absorbable suture. In this representative experiment, histologic analysis of the temporal bones revealed endolymphatic hydrops throughout the right cochlea compared to the left cochlea in six out of seven guinea pigs. Quantification of the cross-sectional area of the scala media across each turn indicated that the cross-sectional area was generally larger in the ears of animals that survived 30 days after obliteration of the endolymphatic sac compared to that observed in control animals.
Auditory nerve overlapped waveform thresholds were increased in animals with endolymphatic hydrops demonstrating the presence of low-frequency hearing loss while most high-frequency cochlear compound action potential thresholds between eight and 20 kilohertz were within the normal range. If the area between the dura and the operculum is dry after expunging the sac, try placing additional retraction on the sigmoid sinus to make sure there's no visible connection between it and the operculum. Following this procedure, additional studies on cochlear anatomy and physiology can be performed to address important research questions about endolymphatic hydrops.
