Cryotherapy in Dermatology

Cryosurgery was first described in the 1800s and has since evolved into a mainstay of therapy within dermatology. Cryosurgery is an effective alternative to more invasive techniques and can be delivered quickly and cost-effectively in an outpatient setting. Because this is a noninvasive technique, cryosurgery can produce excellent cosmetic outcomes. Cryosurgery is performed using a cyrogen, typically liquid nitrogen, to cool the targeted tissue to subzero temperatures. This effect induces tissue damage in 2 mechanisms. The first mechanism is the induction of tissue ischemia by damaging blood vessels and capillaries within the target area which leads to ischemic necrosis of the tissue. The second mechanism damages the cells more complexly by forming ice crystals and inducing osmotic cell injury and cellular membrane disruption. As the tissue is cooled, ice crystals form between cells which creates an osmotic gradient rapidly drawing water out of the cells. As cooling continues, crystals form within the cell which can lead to rupture of the cell. The thawing process also damages cells. As the tissue thaws, crystals outside the cells melt which creates a gradient that rapidly draws water back into the cell which can cause cells to swell and burst.

There is an advantage to cryosurgery compared to excisional techniques for malignancies. When a malignant lesion is excised, the host is no longer exposed to the antigens present in the malignant cells. In the case of cryosurgery, however, the antigens present on the dead malignant cells are retained, allowing for a host immune response that may lead to a systemic response to the malignant cells.

The amount of tissue damage increases with each free thaw cycle. The target temperature for the destruction of benign cells is -20 C. Cancerous cells can be more resistant to cell death and require -50 C to be destroyed. Unfortunately, melanocytes are very susceptible to thermal injury and may die at temperatures below -5 C.

Zones of spherical freezing advance are known as isotherms. The radius for each isotherm is the same in all directions. For example, a -5 C degree isotherm may have a 10 mm distance away from the center of freezing in both depths and along the surface of the skin. A -10-C isotherm may be found at 5 mm away from the center and represent a temperature of -10 C at 5 mm in depth. Knowing the target temperature and understanding the concept of isotherms, a cyrosurgeon can achieve a targeted temperature for a certain depth by measuring the temperature at the periphery of his zone of freezing.

It is also important to understand that conduction will play a role in the temperature of a lesion. Air is the least conductive material for heat, so spraying a lesion with liquid nitrogen from a distance will be less effective compared to holding the spray tip close to the lesion. Keratin is also a poor conductor so for hyperkeratotic lesion it may be prudent to debulk the lesion before performing cryosurgery.