This study demonstrates pharmacological hearing protection in an animal model of ELH. The results support the assertion that inflammatory (reactive oxygen species) injury, which is part of the excitotoxic pathway, contributes to the development of ELH-associated hearing loss.
Surgical induction of endolymphatic hydrops (ELH) in the guinea pig by obliteration and obstruction of the endolymphatic duct is a well-accepted animal model of the condition and an important correlate for human Meniere's disease. In 1965, Robert Kimura and Harold Schuknecht first described an intradural approach for obstruction of the endolymphatic duct (Kimura 1965). Although effective, this technique, which requires penetration of the brain's protective covering, incurred an undesirable level of morbidity and mortality in the animal subjects. Consequently, Andrews and Bohmer developed an extradural approach, which predictably produces fewer of the complications associated with central nervous system (CNS) penetration.(Andrews and Bohmer 1989)The extradural approach described here first requires a midline incision in the region of the occiput to expose the underlying muscular layer. We operate only on the right side. After appropriate retraction of the overlying tissue, a horizontal incision is made into the musculature of the right occiput to expose the right temporo-occipital suture line. The bone immediately inferio-lateral the suture line (Fig 1) is then drilled with an otologic drill until the sigmoid sinus becomes visible. Medial retraction of the sigmoid sinus reveals the operculum of the endolymphatic duct, which houses the endolymphatic sac. Drilling medial to the operculum into the area of the endolymphatic sac reveals the endolymphatic duct, which is then packed with bone wax to produce obstruction and ultimately ELH.In the following weeks, the animal will demonstrate the progressive, fluctuating hearing loss and histologic evidence of ELH. Petroleum lubricant is applied to the eyes to protect the animal from corneal injury due to drying. 3. The animal is shaved from the top of the head to the shoulders and between the ears. 4. The shaved area is prepped in a sterile fashion first with 7.5% povidone-iodine followed by 70% ethyl alcohol. 5. The animal is placed onto an operating table with warming blanket and onto a platform that elevates the head and flexes the neck. The head of the animal is directed away from the surgeon. Adequate flexion of the neck is required in order to adequately expose the occipital ridge. 6. The animal is draped in a sterile fashion.1. Bupivacaine plus epinephrine (Marcaine) 0.25% is applied along the prospective incision site (midline dorsal skin incision along the occiput).This causes local vasoconstriction, limiting bleeding, and provides lasting post-operative analgesia after the animal wakes. 2. Midline dorsal skin incision is made on the scalp along the occiput. 3. To gain adequate exposure, the skin is retracted laterally by placing a small curved hemostat on each side of the incision. 4. A horizontal incision is made along the neck muscle to expose the bony occiput. At this stage the animal may twitch despite adequate anesthesia due to transection of the greater occipital nerve. Spinal cord transection is a danger with horizontal incision into the suboccipit...
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