Laser Surgery Primer
Three items are required for lasers to function: an appropriate molecular medium, a photoresinator, and a way to energize the medium. An appropriate molecular medium (i.e., Argon, carbon dioxide, Nd:YAG, Holmium:YAG) suitable for lasing action has electrons that can be excited to move from lower energy levels to higher energy levels (stimulated).
A photoresinator is an optical chamber having two opposing mirrors in parallel alignment. One mirror is totally reflective and the other mirror is partially reflective (or shuttered). Photons that strike the mirror parallel to the shutters are allowed to escape from the photoresinator and become part of the laser beam, otherwise the photons are reflected within the photoresinator.
In veterinary medicine, there are three common ways to energize the molecular medium: 1) by applying a voltage in a gas discharge tube; 2) by placing an electric current across a solid semiconductor; and 3) by applying a laser light from another medium.
The photoresinator receives external energy to excite the mo-lecular medium. The external energy causes the electrons from the molecular medium to move from lower to higher energy levels (stimulated). As the electrons drop back to the lower energy levels, the absorbed energy is released as light (photon). This released photon can shock another molecule to raise a second electron to a higher energy level, resulting in the release of the original photon and a second photon (light amplification). This reccurring interaction results in a photocascade of stimulated emission between the mirrors after increasing the number of excited molecules. The fraction of light that is emitted through the shuttered mirror is released as the laser beam.
Surgical lasers are inefficient (5-15% efficiency) because many emitted photons do not reflect between the mirrors and fail to join the final beam. Because this energy is lost as heat, high-powered lasers require substantial electrical supply and cooling capacity.
The effectiveness of a laser is determined by the tissue-laser interaction that occurs. Laser irradiation interacts with biological tissues through reflection, transmission, scattering, and absorption. Absorption is the most critical interaction of tissues and laser light because of the associated conversion of energy to heat. Absorption causes a number of biologically relevant chemical, thermal, or mechanical effects to occur. At 60° C to 70° C, denaturation of tissue proteins lead to coagulation necrosis. At 100° C, the water within biological tissue boils and causes the cells to explode. Absorption of laser energy by the tissues produces necrosis, hemostasis, lymphostasis, vaporization (cutting), and membrane destruction, and can be used to produce a number of surgical effects.
Because scattering and reflection of the surgical laser beam can be hazardous to patients and the operating team, safety precautions are required.
Laser beams can be reflected from shiny surgical instruments and cause skin burns. Knowing that the eye captures and focuses light, a major safety objective is to prevent ocular damage to the patient and operating team. Glasses or goggles designed to filter out damaging photons are worn to prevent irreversible eye damage. Because far infrared light from the CO2 laser is reflected from simple safety glasses, these are sufficient to prevent eye burns. Specific eye protection for the Nd:YAG laser is essential to prevent reflected irradiation from causing irreversible damage to the retina.
Since lasers can be used in conjunction with endoscopes, they offer a unique opportunity to perform surgery without general anesthesia in certain cases. However, additional safety measures include placing a filter cap over the viewing port of the endoscope, or attaching an endoscope to a video camera.
Laser surgery is based on heat production. This produces smoke that might be offensive to the operating team and irritating to the patient and bystanders. It is possible that the released smoke contains carcinogens, although this has never been documented.
Direct impact of laser light on an endotracheal tube can cause rapid melting. Melting in combination with surgical oxygen can cause a fire within the patient's trachea. Endotracheal tubes are melt resistant when covered with crinkled aluminum foil.
Faster Wound Healing
Steel blades induce less enzymatic and ultrastructural damage than laser (thermal) tissue cutting. With this increase in tissue damage, healing of primarily closed laser skin incisions is increased by five days when compared to cold-blade incisions. After the increased lag phase time has occurred, the ultimate tensile and bursting strength, dehiscence (separation of layers in the wound), and other aspects of wound healing are normal. There also reduced postoperative swelling, drainage, and pain from laser incisions. This is a result of thermal sealing of transected nerves, capillaries, and lymphatics.
Types of Lasers
Two types of lasers are commonly used in equine veterinary medicine: carbon dioxide (CO2) and neodymium:yttrium-aluminum-garnet (Nd:YAG).
The carbon dioxide laser produces a beam of electroirradiation in the far infrared range. Laser use is restricted to superficial lesions that are in the direct line of sight, because CO2 lasers cannot be transmitted through fiberoptics (i.e., an endoscope).
Water is the primary mechanism through which CO2 laser energy is absorbed. When using the CO2 laser, the temperature of water is raised to a superheated steam state within cells causing cell membranes to explode. This vaporizes the tissue, and cellular particles are converted into plumes of steam and smoke. Because of the high absorption of far infrared light by intracellular water, the thermal effects of carbon dioxide lasers are confined to the target tissue. In other words, the water in adjacent tissues serves as a heat sink that limits the thermal damage to adjacent tissues. Because of this principle, the CO2 laser has been referred to as the "what you see is what you get" laser.
The CO2 laser is an excellent tool for incisions and hemostasis (stopping blood flow) as it can coagulate vessels up to 0.5 mm in diameter. The CO2 laser is well suited for getting rid of infected, dysplastic lesions (abnormally developed structures), or neoplastic lesions (such as tumors) on skin and mucous membranes. Wound bed debridement by the CO2 laser encourages healing. Because CO2 lasers seal lymphatic tissues, which limit the spread of cancer cells, they have found wide application in surgical oncology.
Various skin tumors (i.e., sarcoids, squamous cell carcinoma) can be removed with this laser. Following local anesthesia of the affected area, the CO2 laser is used in the defocused mode to remove the base of the lesion. When lesions involve the ear or the eye, laser surgery is preferred to cryosurgery (freezing) because of increased precision when shaving tissue, less latent trauma to adjacent tissues, and reduced postoperative swelling. Ocular squamous cell carcinomas respond well to CO2 laser removal, and masses on the eyelids or nictitating membranes can be removed without general anesthesia.
A dry, clean, or sterile wound bed results from CO2 laser use, and the wound bed supports the immediate placement and reception of a skin or tissue graft. By sterilizing the dermis, the CO2 laser can be used to prepare granulating beds for pinch graft replacement.
The Nd:YAG laser produces a beam of electroirradiation in the near infrared range. In contrast to the CO2 laser, the Nd:YAG laser is not well absorbed by water; rather, tissue protein is the primary mechanism through which the irradiation of Nd:YAG laser energy is absorbed. Because Nd:YAG laser photons are absorbed by protein, radiation scatter occurs beyond the target tissue, which is known as latent thermal damage. The Nd:YAG laser has become known as the "what you see is not what you get" laser, because the latent thermal damage leads to thermal necrosis in surrounding tissue. Surgeons must anticipate the consequences that 4-6 mm of adjacent tissue will slough when using the Nd:YAG laser.
Unlike the CO2 laser, the wavelength of electromagnetic irradiation of the Nd:YAG laser can be transmitted through fiberoptics. A synthetic sapphire tip can be attached to the distal end of the bare fiber to concentrate laser energy for surgical incisions--a laser scalpel. The sapphire tip has found applications in both transendoscopic and hand-held techniques, although the distal tip of the sapphire must remain in constant contact with tissue when the laser is activated or the tip will be destroyed by the heat produced.
Transendoscopic use of the Nd:YAG laser allows the surgeon to perform minimally invasive surgery in any cavity where the laser light can be transmitted. Transendoscopic applications are often done in the standing, sedated patient, once the target tissue has been anesthetized with mepivacine or lidocaine spray.
The strong thermal effects of the Nd:YAG laser cause local tissue contraction, coagulation, and desiccation, and produce excellent hemostasis in both capillaries and larger vessels. The Nd:YAG laser can coagulate vessels up to 4 mm in diameter. Because the Nd:YAG laser can be transmitted through water, it is useful in fluid-filled cavities or in the presence of hemorrhage.
ENDOSCOPIC USES OF LASERS
Diseases of the nasal septum, although relatively rare, include abscesses, traumatic thickening, malformation, and neoplasms (tumors). All of these lesions can produce similar clinical signs, such as decreased air flow, complete unilateral nasal obstruction, stridor (respiratory noise), and discharge. Horses with smaller lesions of the nasal septum in rostral (forward) locations are best suited to treatment with the laser.
Ethmoid hematomas are slowly expanding angiomatous masses (tumors made up of blood or lymph vessels) that originate from the mucous lining of the ethmoid turbinate in the back part of the nasal cavity. They appear in middle-aged to older horses, and symptoms include an intermittent unilateral bloody nasal discharge. Historically, treatment has been directed at surgically removing the mass through a facial bone flap with the horse under general anesthesia. Now, lasers can be used in the standing, sedated animal.
Hematomas and other nasopharyngeal masses respond well to photovaporization with the laser. Vaporization is extended to the margins of the lesion, and multiple applications are often required on an every other day basis to obliterate these masses. If a component of the hematoma involves the nasal sinuses, it can be approached through a small hole made into the sinus in the standing, sedated patient.
Pharyngeal Lymphoid Hyperplasia
Pharyngeal lymphoid hyperplasia (PLH), an inflammation of the lymph tissue of the pharynx, is common in young horses (less than two years old). PLH is classified from Grades I-IV. Grade I has been defined as a few small, inactive, whitish follicles over the dorsal
pharyngeal surface. Grade IV has been defined as large edematous follicles that frequently blend into broad-based, out-growing (polypoid) structures. Grades III and IV have been associated with abnormal respiratory noise and possibly with exercise intolerance.
When clinically indicated, a persistent case of a Grade III or IV pharyngeal lymphoid hyperplasia can be treated with the laser. A possible complication associated with this therapy is scar formation in the caudal pharynx.
Cystic structures like dorsal pharyngeal cysts or subepiglottic cysts are easily treated with the Nd:YAG laser. The presenting complaint for horses with pharyngeal cysts include an abnormal respiratory noise and a variable amount of exercise intolerance. In young foals and weanlings, subepiglottic cysts can cause dysphagia, coughing, and inhalation pneumonia. Occasionally, these cysts are diagnosed in older horses and are associated with problems of coughing and respiratory noise.
In the mare, uterine cysts can be inadvertently mistaken for a pregnancy during ultrasound examination. Because of this, the cysts can cause misdiagnoses regarding conception or multiple pregnancies. Uterine cysts can impede the spermatozoa as they ascend to the oviduct, the normal movements of the early conceptus through the uterine horns, or implantation. These fluid-filled structures are easily treatable with Nd:YAG infrared irradiation. As the heat and energy of the laser are absorbed by the fluid in the cyst, the surface of the cyst glows white and results in hardening of the inner cyst membrane.
Guttural Pouch Tympany
Guttural pouch tympany is usually observed in the young foal. The affected guttural pouch becomes distended with air and is noted as a non-painful swelling caudal (toward the rear) to the angle of the jaw. The cause of this condition is unknown and often there is no gross abnormality associated with the opening to the guttural pouch. It has been suggested that the problem is due to a mucosal flap acting as a one-way valve that traps air and fluid in the pouch.
In some foals, guttural pouch tympany does not interfere with respiration, while in others it causes dyspnea and aspiration pneumonia. The Nd:YAG laser can be used to create a permanent opening in the combined septum that divides the guttural pouches into left and right halves. After a special instrument is inserted into the affected guttural pouch, the medial septum is directed toward the unaffected pouch, and the laser creates an opening of an appropriate size. This technique is less invasive than conventional surgical approaches, and therefore reduces the complication rate and convalescent period.
Soft Palate Displacement
The most common palatal abnormality in the racehorse is intermittent dorsal displacement of the soft palate (DDSP). Initially, these horses appear to have normal racing ability. Once the palate becomes displaced, the horse might produce an abnormal gurgling sound and a marked decrease in racing performance. Although the cause of DDSP is still not known, most surgeries are directed at improving the seal between the larynx and the opening in the soft palate.
Currently, four surgeries have been used to treat horses with DDSP, including sternothyrohyoid myectomy, sternothyrohyoid tenectomy, epiglottic augmentation, and partial staphylectomy. An Nd:YAG laser staphylectomy (resecting a portion of the caudal free margin of the soft palate) can be performed in the standing patient if the palate can be easily displaced from the epiglottis. The scar contracture that results from the surgery is theorized to produce a smaller, tighter opening for the larynx.
Epiglottic entrapment (EE) occurs when the mucous membrane ventral to (below) the epiglottis envelopes the dorsally positioned epiglottis. Epiglottic entrapment is a common cause of abnormal respiratory noise and exercise intolerance in the racehorse, although occasionally it has been identified coincidentally in clinically normal horses. Because treatment of EE through a laryngotomy (opening through the throat) has been associated with several postoperative complications, a laryngotomy should be reserved for cases where the entrapping mucosa is excessively thickened or scarred.
In the standing patient, correction of epiglottic entrapment can be performed using the bare fiber, or by direct contact with the sapphire tip. Care is exercised when using the Nd:YAG laser to correct EE because the latent thermal effect can damage the ventrally positioned epiglottis.
Historically, ventriculectomy (sacculectomy) has been performed to abduct (pull back) and stabilize the arytenoid cartilage and vocal cord in order to prevent dynamic collapse of these structures during exercise. When used alone, there is evidence to suggest that this procedure has no beneficial effect on improving air flow. As an adjunct treatment for laryngeal hemiplegia, ventriculectomy is often used in combination with prosthetic laryngoplasty. Given that the lateral ventricle is thought to be the major source of noise production, patients with noise production as the presenting complaint can be treated by this procedure alone. For horses with laryngeal hemiplegia, the Nd:YAG laser can be used to cut the mucous lining of the lateral ventricle.
For horses with a presenting complaint of noise production during exercise, the technique is less invasive, horses do not require general anesthesia, and the recovery period is shorter.
Although the exact cause of arytenoid chondropathy (chondritis) is unknown, theories of trauma from foreign bodies, aggressive use of the nasogastric tube, and abnormal contact from the opposite arytenoid have been proposed. These horses usually have a history of abnormal noise during respiration, exercise intolerance and dyspnea (difficulty breathing), or even the need of an emergency tracheotomy.
Chondritis, a slowly progressive enlargement of the arytenoid cartilage, is usually a unilateral disease, although horses can be bilaterally affected. The disease is characterized by thickening, dystrophic mineralization, and protuberances of granulating tissue into the laryngeal opening. Classically, the disease has been treated under general anesthesia by removing the affected arytenoid cartilage through a laryngotomy incision.
Standing arytenoidectomies have been performed using the Nd:YAG laser, although some lasers do not produce enough energy to ablate the body of the arytenoid cartilage. The larynx heals by second intention, and horses require from eight to 12 weeks to recover from surgery. Repeated applications might be necessary for added cartilage obliteration or for control of granulation tissue formation.
Occasionally, abnormal growths or masses are identified on the pharynx, larynx, or trachea. These can be necrotic ulcers, abscesses associated with foreign bodies, or benign lesions. These abnormalities can often be treated transendoscopically with the Nd:YAG laser.
Any cavity that can be evaluated transendoscopically can potentially be treated with the Nd:YAG laser. Possible examples include gastric squamous cell carcinoma, urethra papillomas and squamous cell carcinoma, and transitional cell carcinomas of the bladder.
So, if your veterinarian offers the services of laser surgery for a problem diagnosed in your horse, this high-tech solution might be a way to minimize the trauma of surgery, and get your horse back on its feet in less time.
About the Author
John G. Peloso, DVM, MS, Dipl. ACVS, is owner and surgeon of Equine Medical Center of Ocala in Fla.
POLL: Horse Height