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This category includes alcohols and alkyl halides that act non- specifically as local anaesthetics for certain application. They do not constitute routinely used local anaesthetic agents, and most possess actions in addition to some local anaesthetic activities.
i. Ethanol:
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Ethanol applied to skin and allowed to evaporate acts as a refrigerant. Mucosal applications are irritating due to dehydration, and precipitation of surface proteins. It destroys nerves; un-myelinated thinner ones permanently and myelinated thicker ones temporarily.
Nerve function following damage may or may not recover depending upon extent of damage. It has been occasionally used to treat local foot lameness in equines, 1 ml injected near planter nerve. Ethanol or isopropyl alcohol have been to control excessive straining in cattle that accompanies rectal and/or vaginal prolapse; 1-2 ml 70-95% epidurally.
ii. Phenol:
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Phenol is caustic to tissues; it denatures proteins at lower concentration and precipitates proteins at higher concentrations leading to tissue necrosis and sloughing. A 0.5-% solution has been used for its local anaesthetic effect as powerful antipruritic. A 5% solution may be used to devitalize the pulp in deciduous teeth. Liquefied phenol (80% w/w phenol in water) has been used as analgesic for sensitive dentine.
Aqueous solutions of phenol (6%) have also been used for chemical sympathectomy. Phenol is rapidly absorbed from intact skin. Its use in cats is contraindicated; faster absorption and high toxicity due to deficient conjugation mechanism.
iii. Creosote:
A distillation liquid product from wood tar consisting of a mixture of guaiacol, cresol and other phenols. It has been used as a dental analgesic; a cotton plug soaked in creosote is put into the carious tooth for relief of pain. It has disinfectant and expectorant actions.
iv. Chlorbutol:
Chlorbutol has a mild sedative and local anaesthetic action. It has been used as antipruritic dusting powder or as a dental preparation for exposed or infected teeth pulps. It has antibacterial and antifungal potentials. It has been used a gastric anodyne (sedative) in canine gastritis. The drug has high abuse (addiction to sedative action) potential.
v. Eugenol:
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Eugenol is a volatile oil present in clove oil (82-87%). Applied externally eugenol or clove oil is an irritant with rubefacient and slight analgesic activity. A plug of cotton wool soaked in the oil is inserted in the cavity of carious tooth; repeated applications may damage the gingival tissues.
A mixture of zinc oxide and clove oil has been used to fill tooth sockets in dogs at the time of extraction; severe inflammation causes damage of sensory nerve endings, and delays healing.
vi. Menthol:
Menthol is a constituent of mint oils chiefly of peppermint oil. It is a rubefacient; dilates blood vessels and causes a sensation of coldness (stimulation of cold receptors) followed by an analgesic effect. It has been used as antipruritic and anti-urticaria as 0. 25 to 1% ointment, cream or lotion.
It can be rubbed topically (1: 16 in an oily base) to relieve pain associated with neuralgia. Mixture of equal parts of menthol and chloral hydras, phenol or camphor have been applied to carious teeth to relieve pain but they should not be applied repeatedly over short periods.
vii. Benzyl Alcohol:
Benzyl alcohol has weak local anesthetic activity. A few drops applied to cavity or exposed nerve has been used to relieve pain due to toothache. It can be used topically upto 10% as antipruritic agent. Strong solutions should not be injected as they may cause oedema and pain. It is used in injectable for subcutaneous and intramuscular uses (up to 1%) as antiseptic and local anaesthetic.
viii. Ethyl Chloride:
Ethyl chloride is safer to use than methyl chloride as a spray for skin anesthesia lasting about a minute. It causes freezing and hardening of tissue so not suitable for skin surgery.
It can be used for very simple cutaneous procedures such as puncturing for draining an abscess. It tends to damage the local tissue, and retard healing. It should not be used near mouth or nostrils; it produces rapid narcosis. Methyl chloride is equally effective but has higher systemic toxicity including damage to vital organs.
