Warts are among the most common dermatoses to be seen by doctors. They affect around 10 % of the general population, with a higher prevalence among children and adolescents, immunocompromised people, and those professionally exposed to shared humid environments - swimming pools, changing rooms, sports halls. Benign in the vast majority of cases, warts can nevertheless be painful - especially plantar warts - contagious, unsightly and a source of social anxiety for sufferers. They also tend to multiply and spread in the absence of treatment.
Liquid nitrogen cryotherapy is the reference medical treatment for warts - it is recommended by dermatological societies and represents the best-documented therapeutic option in terms of efficacy. Performed in a few seconds to a few minutes per session, it is available in many of our Quebec branches without undue delay. This guide explains in detail how the treatment works, the types of warts involved, what you'll feel during and after each session, and the factors that influence the number of sessions needed to achieve complete disappearance.
What are warts and what causes them?
Warts are benign skin lesions caused by the infection of cells in the epidermis - the superficial layer of the skin - by the human papillomavirus, better known by the acronym HPV. This group of viruses comprises over 200 different genotypes, of which around 20 are implicated in common skin warts. The genotypes most frequently responsible for skin warts - as distinct from genital warts linked to other genotypes - are mainly HPV-1, HPV-2, HPV-4 and HPV-7. The virus enters the body via small, invisible skin lesions - scratches, cracks, areas of maceration - and infects basal keratinocytes, causing their uncontrolled proliferation and the formation of the wart-like lesion.
Transmission occurs through direct contact with a wart - on one's own skin or on the skin of another person - or indirect contact with contaminated surfaces in damp environments - swimming pool floors, changing rooms. Incubation is long - from one to twelve months between infection and the appearance of a visible wart. The immune system plays a crucial role in the natural evolution of warts: in immunocompetent individuals, around two-thirds of warts disappear spontaneously within two years without treatment. However, this natural resolution may take much longer, the lesion may multiply and spread in the meantime, and the unpredictable nature of the evolution justifies active treatment in many situations.
The different types of warts and their characteristics
Vulgar warts - verruca vulgaris - are the most common. They take the form of raised keratotic papules with a rough, irregular, flesh-colored to grayish surface, ranging in size from a few millimeters to over a centimeter. They preferentially occur on fingers, hands, elbows and knees - areas frequently exposed to microtrauma. Their surface typically shows small black dots corresponding to thrombosed capillaries - a useful clinical sign for distinguishing them from corns and calluses.
Plantar warts - myrmecia or mosaic warts - develop on the sole of the foot. A myrmecia is a solitary, deep plantar wart, often very painful to pressure as it is pushed inwards by weight-bearing - unlike a horn, whose pain is greatest on direct pressure. The mosaic plantar wart is a confluence of multiple small superficial warts, less painful but more extensive and more difficult to treat. Flat warts - verruca plana - are small, raised, smooth, flesh-colored or slightly pink lesions, often found on the face, forehead, back of hands and forearms, particularly in children and adolescents. Filiform warts are thin, elongated, pedicelled lesions, located preferentially on the face - around the eyes, nose and mouth - and neck.
How does liquid nitrogen cryotherapy work?
Liquid nitrogen is a cryogenic gas maintained at a temperature of -196°C. When applied to a wart, it causes rapid, deep freezing of the treated tissue. This sudden drop in temperature leads to the formation of intracellular ice crystals that mechanically destroy HPV-infected cells. Freezing also causes local vasoconstriction, followed by vasodilation on reheating, inducing further vascular damage in the treated area. The local inflammatory reaction triggered by cryotherapy also stimulates the local immune response against the virus - an additional mechanism that contributes to the elimination of HPV-infected keratinocytes beyond mere mechanical destruction.
Liquid nitrogen is applied to the wart either by direct spraying using a cryogun fitted with a probe adapted to the size of the lesion, or by direct contact using a soaked cotton swab. The spray cryogun technique is more precise and allows optimum control of freezing time and the diameter of the treated area. The physician determines the duration of application - from a few seconds to twenty to thirty seconds, depending on the thickness and type of wart - and observes the appearance of a white freezing halo around the lesion, a clinical sign that the target tissue has reached the desired therapeutic temperature.
Session sequence and expected outcomes
The cryotherapy session requires no special preparation. The doctor or nurse practitioner examines the lesion, confirms the clinical diagnosis of a wart, and applies liquid nitrogen using the appropriate technique. For thick, hyperkeratotic warts, prior debridement with a curette or keratolytic emollient can be performed to improve cold penetration. The sensation experienced on application is intense, transient urent pain - comparable to a cold burn - which gradually subsides in the minutes following cessation of application.
In the hours following the session, redness, swelling and sometimes phlyctenes - blisters filled with clear or serum-like fluid - form on the treated area. This reaction is normal and expected - it reflects the local inflammatory response. The phlyctenes should not be intentionally pierced - they will reabsorb naturally within a few days. A protective dressing can be applied. Local sensitivity and sometimes moderate pain persist for two to three days after the session, but are generally well managed with paracetamol. The scab that forms on the treated area will fall off spontaneously within one to two weeks, giving way to new underlying skin. Depending on the response, a new session is scheduled two to four weeks after the previous one.
Frequently asked questions about wart treatment at Clinique Omicron
How many cryotherapy sessions are needed to remove a wart?
The number of sessions varies considerably according to the size, thickness and age of the wart, its location - deep plantar warts are among the most resistant - and individual immune response. On average, two to five sessions spaced two to four weeks apart are required to achieve complete disappearance. Some warts respond as early as the first or second session, while others may require more. Extensive mosaic warts and periungual warts generally take the longest to treat. Combining cryotherapy with a local keratolytic treatment - salicylic acid applied between sessions - improves remission rates and often reduces the number of sessions required.
Is the treatment of warts with liquid nitrogen covered by the RAMQ?
Cryotherapy for warts performed by a participating physician is covered by the RAMQ when it is performed for medical purposes. The initial medical consultation and treatment sessions are covered upon presentation of the health insurance card. Certain situations - treatment of exclusively cosmetic warts in adults with no symptoms or risk of complications - may be considered differently depending on the clinical context. The doctor documents the medical indication in the patient's file. For people without a family doctor, Clinique Omicron's physicians in several Quebec locations can assess and treat warts directly during a consultation.
Can warts reappear after cryotherapy?
Yes - recurrences are possible. Cryotherapy destroys the visible wart and infected keratinocytes in the treated area, but does not necessarily eliminate all HPV viral particles present in the perilesional skin. If infected cells remain at the periphery of the treated area, a new wart may reappear at or near the same spot. In addition, new contamination is possible if risk factors persist - contact with contaminated surfaces, wearing bare feet in communal wet areas, direct contact with active warts. Hygiene measures - such as not walking barefoot in changing rooms and swimming pools, and not sharing pedicure instruments - reduce the risk of re-infection after treatment.
Are there alternative treatments to cryotherapy for warts?
Several alternatives exist, depending on the situation. Salicylic acid-based keratolytic preparations - available over-the-counter from pharmacies - are effective on superficial, thin warts, but require rigorous daily application for several weeks to months. Electrocautery - destruction by electric current - is an option for warts resistant to cryotherapy, performed under local anaesthetic. Cantharidin - a vesicant extracted from an insect - is used by some dermatologists, particularly in children, to avoid the pain of cryotherapy. Local immunotherapy - application of diphenylcyclopropenone or intralesional candidin injections - is reserved for multiple or highly resistant warts. Surgical excision is rarely indicated for common warts, due to the risk of scarring. The doctor determines the optimal therapeutic strategy according to the wart's profile and the patient's preferences.
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