Abstract

To the Editor:

Cryotherapy using liquid nitrogen spray is a mainstay of office skin care. It is used by both dermatologists and primary care physicians to eradicate select skin lesions with minimal to no scarring. The spray is trigger released from a pressurized canister. Since the spray is released from varying (unmeasured) distances from the target; and since a variety of (unmeasured) canister trigger-release pressures are used; and since a variety of (usually undocumented) nozzle tips are selected, there is no way to tally the total dose administered. There is no dosing measuring tool. There is only the operator's experience to guide dosing. The best alternative for guidance is the counted number of seconds of spray administered after the white freeze ball or halo appears. Time to the moment of freeze appearance is expected to vary with skin thickness, with vascularity and with inherent qualities of the lesion type. Continued seconds of spraying after freeze ball appearance is presumed similar for all lesions of the same histology. Thus 10 seconds for warts, 5 seconds for skin tags, etc.¹

As liquid nitrogen is sprayed, the target lesion and encompassed adjacent tissues form a white halo or turn into a white ball called the freeze ball. The freeze ball is associated with a detectable therapeutic effect down 4 mm from the skin surface. For most benign skin lesions, it is deemed that effective destruction begins at −22 C. Interestingly, although lesions of the epidermis are quite vulnerable to destruction at that temperature, the dermis is immune and remains sturdily intact. Thus, while liquid nitrogen spray destroys unwanted epidermal lesions, the underlying dermis remains safely preserved. Of further interest is the fact that cellular destruction takes place during the thawing phase, not the freezing phase. Therefore, prolonged freezing may indeed lower the attained temperature and even extend the depth of penetration, but it is often less effective than a short freeze/thaw followed by a second freeze/thaw several minutes later. A repeat procedure rather than simply a protracted initial freeze phase seems the more effrective.^1,2

Most liquid nitrogen is used for annoying skin tags, warts, actinic keratoses, some seborrheic keratoses, prurigo nodularis, etc. Patients are alerted that the treated area may blister and to let the blister break on its own (so underlying tissues are more mature when the blister does break). Most treated lesions turn brown and fall away in 10 to 14 days. Occasionally, a lesion only gets smaller and a second dosing is then appropriate and effective. Some larger and more vascular lesions, however, may simply not respond at all.

The customary spray may unexpectedly extend beyond the intended target of a diagnosed lesion and its narrow margin of normal skin, ie, 1mm margin for benign lesions, 5 mm for malignancies and 2.5 mm for premalignant ones. Unplanned intrusion of the spray onto normal contiguous tissues risks unintended blistering and unintended scarring.

Otoscope speculums are available in various sizes, from 2.5 mm, 3.0, 4.0, 5.0, 6.0, to 9 mm. Selecting a speculum whose tip matches the target lesion plus a narrow margin of normal tissue and then boldly thrusting the speculum tip into the skin provides an effective protective barrier. Adjacent normal tissues remain untouched beyond the freeze.

Holding the speculum with one's hand is uncomfortably cold for the operator. A grasper was devised (Fig. 1) from two wooden spring-loaded clothespins. The initial clothespin was unaltered. Parts of the second clothespin were glued onto the initial one to make the final clothespin grasper longer and its mouth wider to accept the speculum. All six speculums have different-sized tips, but their tops are all one size. This permits the grasper to hold and push the speculum into the patient's skin to better provide a spray barrier (Fig. 2). The canister spout is also pushed into the speculum to reinforce the skin's circular compression. On withdrawing the speculum, canister, and grasper, there is a visible, temporary circular depression. The area beyond is protected, thus providing splatter-free cryotherapy. The circular compression also seems to enhance effectiveness and it is conjectured that this compression transiently diminishes the target's blood supply, making the target lesion more vulnerable to cryotherapy freeze/thaw destruction.¹