monochloroacetic acid
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monochloroacetic acid
; CH3ClCOOH; monochlor strongly caustic and keratolytic topical agent used to destroy skin lesions, e.g. verrucae (see Table 1 and Table 2); application typically causes pain (pain develops typically ~48 hours after application, and subsides 4–5 days after application); it may be applied as a saturated solution directly to an unmasked lesion, or as a tiny crystal (approximate size of salt grain) strapped directly to a masked lesion (see mask (2)), or as a tiny crystal embedded in 40–70% salicylic acid contained in a cavitied felt pad applied over a masked lesion; has a deeply penetrating action and must be used with caution (i.e. not suitable for lesions overlying bony prominences, involving the nail apparatus, or in patients with reduced fibrofatty padding, peripheral vascular disease or diabetes); patients must be advised of likely effects (e.g. pain, deep action) and appreciate that the dressing must be kept dry and in situ, and reattend in 3–7 days; see Table 3| Factor | Comment |
| Lesion site | Superficial lesion, non-weight-bearing skin – use liquid caustics 20% salicylic acid in collodion Trichloroacetic acid, saturated solution (+75% sliver nitrate) Deeper lesion, weight-bearing skin, good fibrofatty padding – either liquid or solid caustics Monochloroacetic acid, saturated solution 40–70% salicylic acid ointment |
| Number and size of lesions | Large lesions: ointment-based caustics 40–70% salicylic acid ointment Smaller and satellite lesions: caustic solutions Alternating layers of trichloroacetic acid, saturated solution and 75% silver nitrate |
| Skin texture | Sweaty or hyperhidrotic skin Padding cannot be retained in situ Fair skin or atopic individuals; atrophic or dry skin Tend to overreact/undergo tissue breakdown, to applied caustics |
| Circulation | Reduced arterial supply (diabetes, atherosclerosis) Caustics may cause ulceration or predispose to infection as healing response is depressed (use astringents or mild keratolytics) Impaired venous or lymphatic drainage (oedematous tissues) Avoid strong caustics (use astringent agents or mild keratolytics) |
| Neuropathy | Impaired pain awareness (as in diabetic neuropathy) Do not use caustics (use astringents or mild keratolytics) |
| Availability | Strong acids should not be used unless both practitioner and patient are available for emergency appointments Caustics may not be treatment of choice if patient cannot return weekly for ongoing treatments (consider a ‘one-off’ treatment, e.g. cryotherapy) Opt for self-applied milder, topical ongoing treatments, if in patient's best interests |
| Age | Strong caustics should be avoided in young patients with a low pain threshold Caustics that require padding to be retained in situ between treatments may be contraindicated in patients who cannot keep foot dry (e.g. swimmers) |
| Previous treatments | It is pointless continuing with a treatment that has already proved to be ineffective, or has caused an adverse reaction |
| Single treatments | Verrucae pedis do not often respond to a single treatment, but methods include: • Cryotherapy (application of liquid nitrogen, optimally every 3 weeks; ice ball must extend beyond lesion edge; contraindicated in patients with peripheral vascular disease) • Electrosurgery (peripheral tissues must also be removed in order to clear all virally infected cells; requires local anaesthesia; contraindicated in patients with peripheral vascular disease or those with an indwelling pacemaker) |
| Alternative treatments | Alternative treatments may be indicated for cases that have not responded to other forms of treatment: many of these therapies have not been tested by formal research Thuja tincture: painted on lesion once or twice a day Kalanchoe leaves: fleshy leaves split open and fleshy pulp left in situ on lesion; changed every 24–48 hours Tea tree oil: painted on lesion daily, and covered Banana skin: small piece of banana skin cut to size of lesion and strapped in place, pith side against lesion; changed every 24–48 hours |
| Keratolytic/caustic agent | Indicated use |
| Whitfield's ointment (3% salicylic acid and 6% benzoic acid in white soft paraffin) | Treatment of mild tinea pedis |
| 5% salicylic acid ointment | Applied daily for 7 days to soften hyperkeratosis and facilitate its removal |
| 12% salicylic acid in collodion | Macerating agent; painted over callosity and left in situ for 7 days, to assist removal of heavy callosity |
| 20–40% salicylic acid plaster | Applied topically and left in situ for 1–2 days to aid removal of corns |
| 40–70% salicylic acid ointment | Applied topically in a cavitied pad and left in situ for 7 days for verruca treatment; the lesion should be masked |
| Calmurid cream (10% urea) | To hydrate anhidrotic skin; applied daily to treat dyskeratosis |
| 40% urea cream | A strongly keratolytic agent, applied under an occlusive dressing and left in situ for 7 days, to soften, macerate and aid the reduction and removal of hypertrophied nails in patients who are unsuitable for nail avulsion |
| Monochloroacetic acid | A deeply penetrating caustic that is painful in use Applied, retained in situ and reviewed within 3–7 days, for the treatment of verrucae: 1. as a saturated solution to the lesion 2. as a tiny crystal strapped over a masked lesion 3. as a tiny crystal embedded in 40–70% salicylic acid retained by a cavitied pad |
| Trichloroacetic acid | A self-limiting caustic with a superficial action Applied directly to the verruca, after initial scalpel debridement of the lesion May be used in conjunction with 75–95% silver nitrate (see below) as a diagnostic or a final treatment of verrucae |
| 75–95% Silver nitrate | A self-limiting caustic with a superficial action, causing a dark brown discoloration of the skin to which it is applied; it is used for the treatment of shallow or mosaic verrucae. Note: Some patients show an idiosyncratic local sensitivity or inflammatory reaction to applied silver nitrate 1. Applied directly to the lesion, after overlying callosity has been debrided off 2. Applied directly to the lesion in alternating layers with trichloroacetic acid 3. As a diagnostic aid to identify verrucous tissue; viral-infected skin cells show up as bright white dots within a few moments of the application of the layers of silver nitrate and trichloroacetic acid |
| Potassium hydroxide (KOH) | A powerful keratolytic caustic with a great affinity for water that penetrates deeply dissolving precipitated protein; used to destroy soft tissues 1. Overlying hyperkeratosis is debrided off the verruca and the foot is immersed in water for 5 minutes to hydrate the skin, then dried; a KOH pellet is rubbed into the lesion; the foot is reimmersed in water and the jelly-like material formed by the KOH on the lesion surface is debrided off. The process is repeated once or twice until the lesion appears to have gone, then glacial acetic acid is applied to neutralize the KOH 2. A similar protocol may be used to ablate nail matrix after removal of the overlying section of nail plate 3. KOH 5% liquid applied to heavy callosity and left in situ for 5 minutes softens heavy callosity to ease its removal |
| Pyrogallol (pyrogallic acid) | A powerful analgesic, non-self-limiting, caustic reducing agent. It may be used in the treatment of recalcitrant verrucae or neurovascular corns in areas of skin overlying a healthy layer of fibrofatty padding. It should only be used with great caution as its action continues after application has ceased and can lead to severe tissue breakdown that is slow to heal. It is incompatible with alkalis, iron salts, oxidizing agents and ammonium salts 1. 20% pyrogallol ointment for the treatment of neurovascular corns 2. 40% pyrogallol ointment for the treatment of VP 3. WP ointment (20% pyrogallol, 20% wheat germ oil) for the treatment of tough, fibrous, hyperkeratotic plantar lesions |
| Phenol | An analgesic, corrosive caustic. It is used as an 80% solution (liquefied phenol) to destroy soft-tissue lesions such as VP, or nail matrices (three applications, each of 1 minute duration). Its action is quenched by dilution with IMS or isopropyl alcohol. Healing is delayed for several weeks after its application |
| Glacial acetic acid | A weakly acidic mild caustic that is crystalline at 14°C 1. As a paint to hard or vascular corns, or VP (return period 14–21 days) 2. As a paint to VP, alternating with silver nitrate 75% (in a similar manner to trichloroacetic acid) 3. To neutralize KOH (see above: KOH, point 1) |
| Nitric acid | A powerful analgesic oxidizing caustic agent with a superficial action that offers a ‘one-off’ VP treatment 1. Applied to VP with a glass rod and left in situ for 5 minutes, followed by phenol solution 10%; the skin stains bright yellow 2. The lesion is saturated with phenol solution 5% for 5 minutes, then with nitric acid for 20–30 seconds, then once again with phenol solution 5% |
| Strong iodine solution (iodine solution 10%; iodine fortis) | A strong astringent and vesicant agent. It is incompatible with many topical medicaments, and can cause sensitivity reactions in some patients 1. to shrink nail tufts 2. to shrink hypergranulation tissue |
| Formaldehyde | A strongly astringent and antiseptic agent used in the treatment of VPs (the skin surrounding the lesion should be protected with petroleum jelly; sensitivity is likely) 1. 10% formaldehyde in collodion, painted on daily 2. 36% formaldehyde solution, painted on daily |
| Thermal caustics | |
| Cryosurgery | The topical application of liquid nitrogen (at -196°C) or nitrous oxide (at -88.5°C) to destroy small soft-tissue lesions; the cell cytoplasm must be reduced to and maintained at -24°C or lower for at least 1 minute, and repeated for two further freezing episodes between which the area has been allowed to thaw. Cryosurgery is more effective when any overlying hyperkeratosis is removed before freezing |
| Hyfrecation | Tissue destruction by initial fulguration (outlining and superficial charring) of the lesion by the application of high-frequency electrical energy), then electrodesiccation (electrocautery) of the lesion by the release of electrical energy whilst the probe is inserted into the lesion |
| Electrosurgery | Tissue removal using high-frequency energy waves to incise through tissue |
Note: Please also refer to the text entries for each listed agent. IMS, industrial methylated spirit; VP, verrucae pedis. | |
| Agent | Action | Special precautions | Contraindications |
| Monochloroacetic acid (crystals or saturated solution) e.g. single VP | Keratolytic; hydrolysing agent Non-self-limiting | Deep penetration Painful Do not use mask if applying solution Use a mask if applying crystals Review in 5–7 days Neutralize with foot bath ± NaHCO3 or NaCl | Soft-tissue atrophy Peripheral vascular disease Sensory neuropathy |
| Salicylic acid paste (40–70%) e.g. single VP; plantar hard corn | Keratolytic; hydrolysing agent | Macerates tissues Review in 7–14 days May be used in conjunction with monochloroacetic acid crystals Neutralize with foot bath ± NaHCO3 or NaCl | Soft-tissue atrophy Peripheral vascular disease Sensory neuropathy |
| Pyrogallic acid e.g. single VP | Keratolytic; oxidizing agent | Deep penetration Review in 3–5 days Prolonged caustic action Do not apply more than 3 times sequentially Stains skin black/brown | Use with great care: may cause deep tissue breakdown Soft-tissue atrophy Peripheral vascular disease Sensory neuropathy |
| Trichloroacetic acid (saturated solution; 10% solution) e.g. mosaic VP | Mild keratolytic Protein precipitant | Shallow penetration Neutralize with foot bath ± NaHCO3 or NaCl Review in 3 weeks | Peripheral vascular disease Sensory neuropathy |
| Silver nitrate (70% solution; 75–95% stick) e.g. mosaic VP; as a protective skin application below a mask | Protein precipitant Self-limiting Stains skin black/brown Maximum effect occurs within 24 hours | Some patients show hypersensitivity to silver nitrate (or experience acute pain) Neutralize with NaCl foot bath May be applied in alternate layers with trichloroacetic acid | Peripheral vascular disease Known sensitivity |
| Potassium hydroxide (KOH; 85% pellets) | Strong keratolytic | Potentially deep penetration Action of KOH stopped by application of 5% acetic acid after macerated coagulum has been removed Single treatment | Soft-tissue atrophy Peripheral vascular disease Sensory neuropathy |
| Phenol (80% solution or 100% crystal) | Protein precipitate | Action retarded by flooding with industrial methylated spirit Skin overspill flooded with glycerine Review as per postoperative protocol | Peripheral vascular disease (phenol suppresses inflammatory response) |
VP, verruca pedis. | |||