VN February 2023

Vetnuus | Februarie 2023 36 Regulars I Ophthalmology Column In general, removing an eyelid or other periocular tumours in cats and dogs is achieved by an excisional biopsy and the wound closed with fine suture material. This surgical option has an extremely high success rate with negligible recurrence of the masses. However, some masses may not lend themselves to such a treatment option, and alternative methods could be considered. Other modalities do exist for specific types of lesions and tumours based on their location and degree of infiltration around the eye. These include electrochemotherapy, intralesional injection of chemotherapeutic drugs like 5-fluoreouricil [5FU] and cryosurgery. The latter is the selective destruction of lesions by causing cryogenic cell death, killing cells with extreme cold temperatures. Overview of cryosurgery : Effective cryosurgery involves attempting to get tissue temperature changes to occur extremely fast (50-100°C/min), i.e. the heat is extracted from the desired tissue at a rapid rate and an ice ball forms. The final temperature of the deep tissue cells must be colder than -20°C. Generally, two cycles of rapid freezing and slow [3-5 minutes] spontaneous thawing are used. The tissues are frozen to at least −25 °C but nomore than −30 °C (excessive freezingmay cause severe necrosis). The chosen cryogen must achieve a surface tissue temperature range of -50°C to -80°C in order to result in a deep tissue temperature of -25 0 C, and this would achieve effective deep tissue destruction. The destructive effect of freezing the intracellular water results in an osmotic gradient, causing a rupture of membranes and organelles in unwanted tissues. When the liquids inside the cells freeze, [frozen water expands and causes cell destruction] an increased concentration of solutes outside the cell wall causes a transmembrane osmotic gradient. Membranes rupture from the ice crystals, and osmotic pressure changes. Vascular changes include an initial decrease in flow due to the cold, with flushing after thawing. Cryosurgery preserves the tissue matrix, which is relatively cold-resistant, and therefore allows for proper cosmetic healing with minimal scarring. The thawing rate should be slow and continued for a long time period; rapid thaw rates allow cell survival, so the slow thaw is more important to allow the maximal growth of ice crystals and increase the osmotic pressure. Repetitive freeze-thaw cycles, rapid freezing and slow thawing, do not guarantee effective cell destruction but certainly increase the success rate. Therefore, the cryosurgery process should be managed in such a way as to produce an appropriate lethal effect on a certain volume of tissue. In some cases, this may require more than 2-3 freeze/thaw cycles. Getting started : At the Jhb Animal Eye Hospital, we make use of the CryoPen, which provides the application of the nitrous oxide [N2O, -88°C] cryogen through small focal tips which are ideal for small lesions and allow the application of the cryogen to be very well controlled so as not to interfere with surrounding delicate ocular structure. Various cryogen products are available, and each has different freezing characteristics. Always check the cryogen you will use and its ability to maintain a sustained cold temperature when the application is done. The availability and transport of liquid nitrogen, as used previously by veterinarians, is a thing of the past due to health and safety rules. These are some general considerations: 1. Examine the lesion and decide if it would be best removed by excisional biopsy or if it is of such a size and location that this surgical option would be inappropriate. Is cryo your next best option? 2. Have an idea of the duration of freeze required? 3. Have an indication of the depth and surface area that needs to be frozen. 4. Choose the CryoPen tip size correctly. The tip size will greatly affect the depth and surface area of the ice ball as well as dramatically change the rate at which you use the cryogen in the cylinder. There is a cost associated with this. Cryosurgery in veterinary ophthalmology Dr Antony Goodhead, Specialist Veterinary Ophthalmologist, Johannesburg and Cape Animal Eye Hospitals, www.animaleyehospital.co.za [Image from Google Images]