Fine structure of the rat tympanic membrane after treatment with local anaesthetics

Peptides actively transported across the tympanic membrane: Functional and structural properties

Otitis media (OM) is the most common infectious disease of children under six, causing more antibiotic prescriptions and surgical procedures than any other pediatric condition. By screening a bacteriophage (phage) library genetically engineered to express random peptides on their surfaces, we discovered unique peptides that actively transport phage particles across the intact tympanic membrane (TM) and into the middle ear (ME). Herein our goals were to characterize the physiochemical peptide features that may underlie trans-TM phage transport; assess morphological and functional effects of phage peptides on the ME and inner ear (IE); and determine whether peptide-bearing phage transmigrate from the ME into the IE. Incubation of five peptide-bearing phage on the TM for over 4hrs resulted in demonstrably superior transport of one peptide, in level and in exponential increase over time. This suggests a preferred peptide motif for TM active transport. Functional and structural comparisons revealed unique features of this peptide: These include a central lysine residue, isoelectric point of 0.0 at physiological pH and a hydrophobic C-terminus. When the optimal peptide was applied to the TM independent of phage, similar transport was observed, indicating that integration into phage is not required. When 10⁹ particles of the four different trans-TM phage were applied directly into the ME, no morphological effects were detected in the ME or IE when compared to saline or wild-type (WT) phage controls. Comparable, reversible hearing loss was observed for saline controls, WT phage and trans-TM peptide phage, suggesting a mild conductive hearing loss due to ME fluid. Perilymph titers after ME incubation established that few copies of trans-TM peptide phage crossed into the IE. The results suggest that, within the parameters tested, trans-TM peptides are safe and could be used as potential agents for noninvasive delivery of drugs, particles and gene therapy vectors to the ME.

Phenol anesthesia of the tympanic membrane in purulent otitis media: a structural analysis in the rat

In the present study, local anesthetics were applied to the tympanic membrane (TM) of rats following experimentally evoked purulent otitis media (POM). The structure of the TM was evaluated 24 h, 2 weeks and 3 months after application of phenol to a discrete are of the right TM. The left ear was used as a control. POM induced swelling of all tissue layers of the pears tensa. The tissues were invaded by inflammatory cells and disintegrated. The fibrous layer also dissolved, leaving spontaneous perforations. However, phenol on inflamed TMs did not further alter the TM structure. These findings infer that the injurious effect of topical anesthesia on the intact TM is more or less negligible when applied to diseased TMs.

Short-term effects of iontophoresis on the structure of the guinea pig tympanic membrane

Guinea pig tympanic membrane was anesthetized iontophoretically. The animals were sacrificed between 2 h and 4 days after the anesthesia and the tympanic membrane was studied histologically by light microscopy and transmission electron microscopy. Edema in the subepidermal connective tissue layer and detachment of the basement membrane from the basal cell layer were observed 2 h after iontophoresis. Eight and 12 h later, the epidermal layer between the handle of the malleus and the bony annulus peeled and retracted towards the bony annulus, leaving only hornified outer epidermis on the fibrous layer. Four days after iontophoresis, the retracted margin of the epidermal layer became greatly thickened and evidence of hyperplasia and migration was observed. This study has demonstrated that iontophoresis of the tympanic membrane produces remarkable histological changes. However, no perforation was observed.

Structural effects of the topical lidocaine-prilocaine anesthetic Emla on the tympanic membrane

A eutectic mixture of two anesthetic solutions is that of lidocaine and prilocaine, which is available commercially as Emla (eutectic mixture of local anesthetics). This solution is commonly used in clinics in superficial skin surgery and when inserting intravenous catheters, and we also wanted to determine its surface effects when applied onto the tympanic membranes (TM) of rats and guinea pigs as animal models. The animals were observed by otomicroscopy for 2 weeks up to 5 months after administrations of the anesthetic solution. Tissue specimens were then obtained for light microscopy and fine structural analysis. The application of Emla caused minor structural changes to the rat TM, but hardly any alterations to that of the guinea pig. The changes displayed were an overall increased thickness, with typical submucosal edema but only slight epithelial reaction. In comparison with previous results with lidocaine, phenol and Bonain's solution, Emla seemed to be harmless to the structure of the TM. However, the efficacy of Emla as a topical TM anesthetic remains to be proved in future clinical trials.

Late effects of local anesthetics on tympanic membrane structure

With the rat tympanic membrane used as a model, various topical anesthetics in current clinical use (phenol, Bonain's liquid, and Xylocaine spray) were tested. Structural changes were observed for up to five months from the time of anesthetic application. The most protracted changes affect the lamina propria, which exhibits a pronounced thickening. The extent of the changes, however, differs according to the agent used; for example, phenol applied to discrete areas exhibited the least widespread changes. In the tympanic portion of the lamina propria, the connective tissue was loosely packed with less regularly arranged collagen fibers, while a tightly packed fiber bundle existed on the meatal side. It is inferred that the submucosal connective tissue layer is the portion of the connective tissue layer where the remodeling and reconstruction of the fibrous layer occur.