What is the biochemical changes found in active phase of COM? Why does not a local anaesthetic agent(Lignocaine) work properly during this phase of inflammation?

• There are many biochemical events as a result of tissue injury induced by inflammation. Trigger factors such as infection, inflammation, pressure and keratin lead to elaboration of variety of chemical factor.
• Inflammatory mediators in initial inflammatory phase includes

1. Proteins (glycoproteins): peptides, cytokines such as IL-1 and IL-6, TNF  α, arachidonic acid metabolites, macrophages, T lymphocytes, monocytes and many other cells at at the site of infection.
2. Non-protein mediators: nitric oxide, prostaglandins, neurotransmitter and free radicals.
3. Abnormal distribution of epidermal differentiation marker such as involucrin, c-jun and p53 protein and increase epidermal growth factor receptor are seen in the middle ear cholesteatoma matrix.

• Increased level of proteins cytokeratin 13 and cytokeratin 16 are found.
• Erb B-2 protein was found to be over expressed and cell proliferation and apoptosis of keratinocytes are accelerated.
• Caspases play a key role in apoptosis. Caspases-8 which is activated by the introduction of tumor necrosis factor α leads to the activation of caspase-3,  which activates apoptotic nucleases in cholesteatoma tissue. (Miyao and coworker)
• Fibroblast in the subepithelium of cholesteatoma did not exhibit the invasive phenotype characteristics of true neoplastic cells (Chole and colleagues)
• Immune response is also involved: Langerhans cell may initiate immune reaction and may promote proliferation of keratinizing epithelium via interleukin and transforming growth factor (TGF)-β mechanism.
• Vascularization within the peri-matrix of cholesteatoma shows five fold as compared to middle ear mucosa and twofold as compared to skin.
• Area of localized bone resorption exhibits mononuclear inflammatory cells attached to the resorption margin. These cell contain abundant acid phosphatase, as did isolated fibroblast within the zone of inflammation and the resorption margin. Extra cellular acid phosphatase are seen in the areas. Within the bone , osteocytes adjacent to resorption display increased cytoplasmic organelle patterns and contained the acid phosphatase reaction. The localization of destructive enzymes in mononuclear inflammatory cells and oteocytes explains the bony destructive changes observed in human chronic otitis media in the absence of multi-nucleated osteoclasts.

However, the factors participating in the regulation of molecular and cellular processes leading to the chronic inflammation remain to be further investigated.

Although a variety of hypotheses were proposed for explaining the decreased effects of local anaesthetic  in the presence of inflammation, the most cited is the theory that the acidosis of inflamed tissues reduces the anaesthetic potencies of local anaesthesia. Inflammatory cells produce peroxynitrite which may affect local anaesthesia.

• Local anaesthetics are structurally the tertiary amines with aromatic rings
• Local anaesthetics diffuse in uncharged form through nerve sheaths and penetrate into cell membranes to reach the cytoplasmic binding site or receptors or transmembrane channels.Lower the pH from 7.4 to 6.4, lesser the relative concentrations of uncharged molecules. The mechanism of action depend on their medium pH and pKa values.
• The mode of action of anaesthesia includes

1. The blockade of voltage-gated sodium channels.
2. The drug and membrane lipid interaction which include the changes in membrane physiochemical property, fluidization or disodering. These pharmocological features, together with the correlation between anaesthetic potency and lipid solubility, strongly suggest that the drugs interact with membrane lipid bilayers to induce local anaesthesia.

• Lactic acid and acidic by-products are increasingly produced and concentrated in and near inflamed tissues, causing the acidosis which lowers the tissue pH. The pKa values almost all local anaesthetics in clinical use are larger than 7.5 . Therefore, local anaesthetic effects would be decreased in the presence of inflammation, leading to the hypothetical mechanism based on tissue acidosis.
• An alternative hypothesis which was speculated from the potential reactivity of peroxynitite pathologically relevant to inflammation.

Inflammatory cells produce nitric oxide and superoxide anion which react to form peroxynitrite. Several local anaesthetics, including lidocaine and bupivacaine, react with peroxynitrite. Peroxynitrite also inhibits Na+/K+ – ATPase activity through the modification of membrane fluidity. Therefore peroxynitrite may interact with local anesthetics to affect their membrane inter activities and modify their pharmacological activities.