Hypokalaemic Polymyopathy in Burmese cats – Retrospective analysis of cases, new clinical observations and a call for new cases for genomic studies 

Fran Musca, Steve Metcalf, Marcus Gunew, Rhett Marshall, Vic Menrath, Richard Malik and Leslie Lyons

Clinical signs referable to hypokalaemia can be seen in a variety of feline diseases, viz. (i) chronic renal disease, (ii) primary hyperaldosteronism and (iii) hypokalaemic polymyopathy. This study focuses on the latter disease, an inherited condition most commonly encountered in young Burmese cats (2-6 months-of-age) which present with signs of myopathic weakness, classically passive ventroflexion of the head and neck, and sometimes also dorsal protrusion of the scapulae, muscle tremor and head bobbing (Jones et al 1988; Lantinga et al 1998). The disease in many respects resembles the human disorder hypokalaemic periodic paralysis, which is characterised by episodic attacks of muscle weakness associated with a transient decrease in blood potassium levels.  Reported on for the first time by Eger et al (1983), the condition was shown by Mason et al (1988) to have an autosomal recessive mode of inheritance. The disease has been reviewed by Gruffydd Jones (1997) and more recently by Jones (CVT 14).

In a feline-only practice in Brisbane (Australia), a recent cluster of cases of hypokalaemic polymyopathy in Burmese kittens prompted a review of the clinical manifestations of this condition in comparison of what has been reported in peer reviewed publications and in small animal texts. Additional anecdotal material from the period preceding computerised archiving of case material was provided by Drs Menrath, Metcalf and Malik.

Unlike textbook accounts, it has been our experience that (i) cats can develop genralised weakness without prominent cervical ventroflexion, but with alternative signs e.g. crouching hind limb gait, shifting lameness, stiff/stilted gait; these alternative signs may be present with or without classic signs such as cervical ventroflexion (ii) myalgia can be a prominent (iii) cats can present as adults (iv) the potassium concentration is not invariably low at the time the cat is weak, although serial potassium determinations will eventually detect moderate to severe hypokalaemia (v) marked elevations in creatine kinase activity can provide supportive evidence that hypokalaemia is the underlying problem if the potassium concentration is normal (vi) potassium chloride in large doses (1/2 to 1 Span K [7 mmol KCl] once or twice daily with food) is generally more convenient, less expensive and more effective than potassium gluconate for managing these cases (vii) some cats need on-going therapy, while in others KCl can be discontinued (viii) rarely, cases are refractory to KCl therapy and require low dose prednisolone or acetazolamide to control the clinical signs.

Recognition of this condition relies on a high index of suspicion, although consistent signs in a cat of the Burmese breed is strongly suggestive of this diagnosis.  The purpose of this communication is to highlight this condition to small animal clinicians, in an attempt to obtain blood for DNA retrieval from as many cases as possible, with a view to identifying the underlying genetic problem using a whole genome scan and the new feline single nucleotide polymorphism (SNP) microarray, in collaboration with Leslie Lyons and colleagues at UC Davis.

1. 1.        Eger et al., 1983 C. Eger, W. Robinson and C. Huxtable, Primary aldosteronism (Conn's syndrome) in a cat; a case report and review of comparative aspects, ... – if you read this, at the end they talk about the Burmese cats
2. 2.        Mason K V (1988) Hereditary potassium depletion in Burmese cats.  Journal of the American Animal Hospital Association 24, 481
3. 3.        Gruffydd-Jones et al (1997) Proceedings of the 14th American College of Veterinary Internal Medicine (ACVIM), San Antonio, Texas p 757
4. 4.        Lantinga E et al (1998) Periodic muscle weakness and cervical ventroflexion caused by hypokalemia in a Burmese cat [Dutch], Tijdschrift voor Diergeneeskunde 123, 435-437
5. 5.        Gashen F et al (2004) Congenital diseases of feline muscle and neuromuscular junction. Journal of Feline Medicine and Surgery 6, 355 – 366.
6. 6.        Jones BR, Swinney GW, Alley MR. 1988 Hypokalaemic myopathy in Burmese kittens. N Z Vet J. 36(3):150-1.
7. 7.        Lantinga E, Kooistra HS, van Nes JJ. 1998 Periodic muscle weakness and cervical ventroflexion caused by hypokalemia in a Burmese cat. Tijdschr Diergeneeskd. 123(14-15):435-7.

Figure 1 Affected cat being supported against gravity. Note prominent ventroflexion of the head and neck.

Ureteral obstruction has been previously considered an uncommon occurrence in cats.   With wider availability of advanced imaging, particularly ultrasonography, the diagnosis of ureteral obstruction appears to be increasing in incidence (Kyles et al, 2005).

Ureteral obstruction occurs secondary to an intraluminal obstruction, a mural lesion, or extraluminal compression.  Mural lesions include neoplasia, fibrosis, congential and acquired stricture and polyps.  Extraluminal compression most often occurs secondary to neoplasia arising from the ureter, the bladder, or the retroperitoneal space however there is also a report of an occurrence secondary to retroperitoneal infarction and fibrosis (Ragni and Fews, 2008).  Examples of intraluminal obstruction include calculi, debris, and spasm of the ureter.  Intraluminal obstruction may occur unilaterally or bilaterally.  Ureteral obstruction regardless of the underlying cause leads to restriction of urine flow, and if left untreated, obliteration of the renal parenchyma secondary to pressure.  The physiology of ureteral obstruction has been studied in dogs and revealed that the prognosis for recovery of renal function is dependent on both the degree of obstruction and the time that obstruction is in place. After four days of obstruction the prognosis for return of renal function is excellent, after 14 days there will only be a recovery of approximately half of the normal glomerular filtration rate, and after 40 days there is little or no recovery if the obstruction is corrected (Kyles, 2006).  This highlights the need for prompt diagnosis and intervention.  Unilateral obstructions are difficult to diagnose as renal function remains normal due to the efforts of the contralateral kidney. Unfortunately if the first obstruction is unilateral the cat is often left undiagnosed until a second obstruction occurs in the contralateral kidney resulting in renal failure (Evans et al 2007).

Clinical signs of cats presenting with ureteral obstruction may be vague.  Presenting complaints include lethargy, vomiting, inappetance, and weight loss.  The most common finding on physical examination is abdominal/spinal pain.  In cases where there has been previous undiagnosed obstruction one large kidney may be palpated and the contralateral kidney may be unable to be detected (“big kidney-little kidney” presentation).  In the author’s opinion, any cat that upon abdominal palpation one large kidney and one small kidney are palpated should have renal imaging performed.  Pyrexia and dehydration may also be clinical examination findings in cases where ureteral obstruction is occurring as a consequence of debris generated by pyelonephritis. In cats presenting with ureteral obstruction for the first time, physical examination findings may be very subtle and a high index of clinical suspicion needed for diagnosis.

Biochemical changes of ureteral obstruction can range from unremarkable through to severe azotemia (from acute renal failure). Bilateral obstructions usually have significant azotemia as neither kidney can function. Unilateral obstructions usually have unremarkable biochemical changes if the contralateral kidney is unaffected where as they will have severe azotemia if the contralateral kidney is non-functional. Clinical pathology may demonstrate severe renal dysfunction but cannot differentiate ureteral obstruction from other forms of acute renal failure. Common laboratory findings include severe azotemia, hyperphosphatemia, and hyperkalaemia.  Any cat presenting with these clinical pathology changes should be considered an emergency.  While there are many causes of acute renal failure it is the authors opinion that imaging of the urinary tract should be mandatory for any cat with acute renal failure. Urine specific gravity (USG) is often suboptimal in these cats (< 1.035).  Urine sediment should be examined for concurrent bacterial infection and urine should be sent for culture and sensitivity.

Definitive diagnosis of ureteral obstruction may be made via several different imaging modalities.  Ureteral calculi are often visible on plain abdominal radiographs.  A retrospective study revealed that sonography and survey radiography used in combination was able to diagnose 90% of ureteral calculi (Kyles et al, 2005).  Sonography will reveal dilation of the renal pelvis greater than 3mm.  Occasionally the cause of obstruction may be visualised sonographically, for example the visualization of a bladder mass or calculus within the proximal ureter.  In the absence of visualisation of the cause of suspected obstruction, an antegrade positive contrast pyelogram should be performed.  Intravenous urography does not usually provide suitable images.  A three-way tap is connected to: a 3ml syringe filled with suitable contrast material (eg Urograffin or Omnipaque) an empty 3ml syringe (for fluid collection) and a long 22 guage needle (2 ¼ inch).  Under ultrasound guidance the needle is inserted through the greater curvature of the kidney into the renal pelvis.  Urine is aspirated from the renal pelvis into the empty syringe and retained for sediment analysis and culture and sensitivity.  Once urine is collected, the 3-way tap is turned and contrast is injected into the renal pelvis.  Lateral and dorsoventral abdominal radiographs are then performed.  Where ureteral obstruction exists there will be abrupt termination of the contrast column.  In some cases, particularly those where obstruction is secondary to pyelonephritic debris the obstruction may be flushed under pressure into the bladder.  Other methods of diagnosis include advanced imaging such as CT, MRI and nuclear scintigraphy.

Treatment of ureteral obstruction varies depending on the underlying cause. The following notes discuss treatment of treatment of intraluminal obstruction secondary to calculi or debris only.

Intraluminal ureteral obstruction may be treated medically or surgically.  First and foremost pain relief is MANDATORY for cats with ureteral obstruction.  It is amazing how many angry aggressive cats seemingly change personality overnight with the administration of pain relief.  The most common analgesics used for ureteral obstruction at The Cat Clinic are the opiates methadone (used in intermittent bolus dosing) and fentanyl patches (Durogesic patch, 12.5microgram or 25 microgram depending upon the size of the cat).

The mainstay of medical therapy involves judicious use of intravenous fluids and agents to dilate the ureter to promote passage of the obstruction.  There are two distinct groups of cats that are medically treated.  The first group are those presenting with their first episode of obstruction, or those cats who have compromise of the contralateral kidney and are azotemic but not hyperkalaemic on presentation.  The second group of cats are those who present in acute renal failure with hyperkalaemia.  These cats have either bilateral obstruction or unilateral obstruction with little or no function of the contralateral kidney.

At The Cat Clinic non-hyperkalaemic cats are treated with intravenous fluids and ureteral dilating agents initially.  Cats that do have compromise of the contralateral kidney are at risk for oliguric or anuric renal failure.  As such body weight should be closely monitored (the author recommends every 4-6 hours).  Obviously another method of monitoring urine output is to place an indwelling urinary catheter and monitor urine output.  The authors do not routinely place urinary catheters in these cats for the following reasons; these cats are usually relatively bright and sedation/anaesthesia is often required for catheter placement, and the placement of a urinary catheter opens up a potential site of infection. Cats are maintained on intravenous fluids for a maximum of three days.  Agents used to stop ureteral spasm, dilate the ureter or increase urine flow include amlodipine, amyltriptaline, glucagon and diuretics.  Further study into the clinical efficacy of these drugs is needed.

Daily monitoring of packed cell volume (PCV), total protein (TP), creatinine and serum electrolytes (sodium, potassium, and chloride) should be performed.  It is vital to track changes in electrolytes in these cats.  The majority of cats are re-imaged via sonography on day three.  Our criteria for successful medical therapy are 1.  Reduction in creatinine to normal or previously determined baseline level and 2.  Reduction in size of renal pelvis.  If the patient has not responded to therapy within three days then surgical intervention should occur.  Cats that progress to hyperkalaemic acute renal failure within this three day period should be considered candidates for surgery post stabilisation.

Cats presenting with hyperkalemic acute renal failure are candidates for surgery following stabilisation.  Intravenous fluid therapy should be commenced with non-potassium containing fluids.  Close attention should be paid to the body weight and hydration status of the cat as it is very easy to fluid overload these cats.  Depending on the level of hyperkalaemia and the state of the patient, additional therapy for hyperkalaemia may need to be instituted.  Therapies include the administration of regular insulin and glucose intravenously and sodium bicarbonate.  Calcium gluconate may utilised for its cardioprotective qualities where appropriate.

After the initial period of stabilisation the decision must be made whether the patient is stable enough for a long surgical period, or if peritoneal dialysis or percutaneous nephrostomy tube placement should be attempted. This decision is based upon individual patient factors.
There are several different surgical techniques for relief of ureterliths.  The choice of technique is dependent upon the location of the obstruction.  In humans there are three specific sites where ureteroliths tend to lodge.  As yet it is not yet determined if such sites exist in cats (Kyles, 2006).  Obstructions in the proximal ureter are usually removed via uretotomy.  Given the size of the feline ureter magnification is essential during surgery.  At The Cat Clinic, an operating microscope is routinely used for this surgery.  Ureteroliths in the distal ureter may be treated via uretotomy or ureteroneocystostomy (transection and then reimplantation of the ureter into the bladder).  The most common post operative complications seen at The Cat Clinic is uroabdomen (Evans et al, 2007).  This is also the most common surgical complication seen at the University of California, Davis (Kyles et al, 2005).  Other reported complications include pulmonary oedema, septic peritonitis, and persistant ureteral obstruction.  Nephrostomy tubes divert urine away from ureterotomy site and allow for rapid reduction in azotemia. At The Cat Clinic, experience with uroabdomen has led us to routinely place nephrostomy tubes however surgeons at UC Davis feel this is not necessary with improved surgical technique. Nephrostomy tubes are associated with complications including obstruction of the tube, dislodgment of the tube, urine leakage, and infection.

Other therapies for resolution of ureteral obstruction include the use of extracorporeal shock wave lithotripsy.  The University of California, Davis had been using this treatment method with informed consent of the owner of the patient.  However it is associated with high rates of complications including sudden death, pancreatitis, diarrhea, and cardiac arrythmias.  The researchers also found that feline ureteroliths were generally “harder to fracture” than canine ureteroliths and thus this treatment methodology is not favoured (Hardie and Kyles, 2004).  There has also been a case report of endoscopic retrieval of a ureteral calculus (Kuntz, 2005).

Kyles et al 2005 reported twelve month survival rates of 66% for those cats treated medically, and 91% for those cats treated surgically. This study also reported that of 35 cats monitored post obstruction 14 had a recurrence of ureterolithiasis.  This occurred a median of 12 months post first obstructive episode.  At The Cat Clinic approximately 1 in 3 cats treated for an episode of ureteral obstruction will have a recurrence of clinical signs.  Data is not yet available with regard to long term survival.

Ureteral obstructions can be treated medically and surgically with good outcomes.  However recurrence is very common (Kyles et al, 2005, Evans et al, 2007).  With non specific clinical signs a high index of suspicion may be needed for diagnosis. Prompt diagnosis and intervention minimizes long term damage to the affected kidney.  Diagnosis is aided by imaging techniques such as radiography and sonography.  It is important that any cat with signs of an acute abdomen be imaged.

References:

Evans, N.A. Gunew, M. Marshall, R.  Ureteral obstructions can be treated medically and surgically with good outcomes.  Proceedings of the Australian College of Veterinary Scienctists 2007.

Hardie, E.M. and Kyles, A.E.  2004.  Management of ureteral obstruction
Vet Clin North Am Small Anim Pract. 34(4):989-1010.

Kuntz, C.A.  2005.  Retrieval of ureteral calculus using a new method of endoscopic assistance in a cat.  Aust Vet J.  83(8):480-2.

Kyles, A.E., Hardie, E.M., Wooden B.G., Adin C.A., Stone E.A., Gregory C.R., Mathews K.G., Cowgill L.D., Vaden S., Nyland T.G., Ling, G.V.  2005.  Clinical, clinicopathologic, radiographic, and ultrasonographic abnormalities in cats with ureteral calculi: 163 cases (1984-2002).  J Am Vet Med Assoc. 226(6):932-6.

Kyles, A.E., Hardie, K.M., Wooden, B.G., ., Adin C.A., Stone E.A., Gregory C.R., Mathews K.G., Cowgill L.D., Vaden S., Nyland T.G., Ling, G.V.  2005.  Management and outcome of cats with ureteral calculi: 153 cases (1984-2002).  J Am Vet Med Assoc. 226(6):937-44.

Kyles, A. E.  2006.  Renal and Ureteral Obstruction.  Proceedings if the British Small Animal Veterinary Congress 2006.

Ragni, R.A., Fews, D. 2008.   Ureteral obstruction and hydronephrosis in a cat associated with retroperitoneal infarction.  J Feline Med Surg. In press.

Minimally Invasive Surgery In Cats

Rhett Marshall BVSc MACVSc
The Cat Clinic
189 Creek Road,
Mt Gravatt, 4122.

Minimally invasive surgery (MIS) is a collective term for surgical techniques designed to minimise the extent of an anatomic approach while maintaining precision and efficiency. MIS is a new and rapidly advancing field in veterinary medicine, with new techniques and applications being published in nearly every monthly veterinary journal. The benefits of MIS in decreasing peri-operative pain and decreasing morbidity has well been accepted in human medicine and now also confirmed in animals. Reducing or minimising pain in animals has been a priority for modern veterinary clients and has fueled the pursuit of these veterinary laparoscopic and thoracoscopic techniques that cause minimal invasion.
Laparoscopy
Viewing the internal structures of the abdominal cavity via laparoscopy involves "insufflation’ of the abdominal cavity with carbon dioxide gas. This gas is used because it in non-flammable (when cautery is used), non toxic and inexpensive. Modern CO2 insufflating devices have a pressure limit (12-15mm Hg) that cannot be exceeded so that overdistention of the abdomen is prevented. Insufflation allows displacement of the abdominal wall from the underlying viscera so that instruments may be manipulated without trauma to the abdominal organs. A cannula is placed in the abdominal wall and a rigid telescope (laparoscope) inserted through and into the peritoneal cavity. Once the telescope is in place, biopsy forceps or surgical instruments can be introduced into the abdomen through adjacent cannulas to perform various diagnostic or surgical procedures.
Structures that can be visualized include the liver, gallbladder, kidneys, bladder, stomach, pancreas, spleen, small intestine, cecum, colon, uterus, ovaries, vas deferens, adrenal glands and diaphragm.
The advantages of laparoscopy compared with conventional open surgical exploratory laparotomy include improved patient recovery because of smaller surgical sites and lower postoperative morbidity with a lower infection rate and less postoperative pain. The complication rate of laparoscopy is low. A review of 360 consecutive cases of diagnostic laparoscopy performed by Eric Monnet at Colorado State University found a complication rate of less than 2%. Serious complications include anesthetic- or cardiovascular-related death, bleeding, or air embolism .

Few contraindications exist due to the minimal invasiveness of laparoscopy. Ascites, abnormal clotting times, poor patient condition or obesity are the only relative contraindications. Absolute contraindications to laparoscopy include septic peritonitis or conditions where obvious conventional surgical intervention is indicated. Patients who are a poor anesthetic or surgical risk are not suitable candidates and are even less suitable for conventional surgery.

The basic equipment required for diagnostic laparoscopy include

1. telescope (0 or 30°)
2. videocamera and monitor
3. 2 cannulae (access into abdomen)
4. veress needle (for initial safe insufflation)
5. light source
6. light guide cable
7. carbon dioxide insufflator (to continuously inflate the abdomen)
8. palpation probe
9. oval biopsy forceps
10. punch biopsy forceps
11. grasping forceps
12. scissors
13. device for photographic documentation (optional)

Most major video endoscopy equipment required for laparoscopy, thoracoscopy, and arthroscopy may be used interchangeably. Things such as the light source, light guide, monitor, telescope (arthroscope), camera and printer may be interchanged and significantly reduce set up costs if you already have these large ticket items. Purchased new, this basic equipment could cost more than $60,000. Good quality second hand units are readily available from endoscopic suppliers such as Austvet or on e-bay and may cost as little as $10,000.

All types of modern MIS systems involve a video camera, light source and monitor. The video camera will be the most important determinant of your image quality on the monitor and often represents the largest investment. Camera image quality and is determined by how many chips they have. The more chips, the better the image and unfortunately the more expensive. The least expensive single chip cameras are adequate for routine procedures while modern 3 chip cameras provide increased image quality. Xenon fiber optic light sources are most commonly used for MIS as they are brighter and are necessary to provide adequate lighting in body cavities. A single chip camera with a Xenon light source would serve most practitioners well as it can be used for both soft tissue or arthroscopic procedures. The light guide cable connects the telescope to the light source so must also be compatible at both ends.

Telescopes and other minimally invasive surgical instruments (needle holders, curettes, forceps, etc) are more specifically designed for laparoscopy, thoracoscopy or arthroscopy. Telescopes are classified by their diameter and their length; the larger the diameter of the telescope, the more light and the better the image. Telescopes also come in zero-degree (looking straight ahead) to thirty-degree field of view. Angled scopes allow the operator an increased field of view, but can be confusing to beginners. Telescopes cost about $4000-6000 new and $1800-2500 second hand . The diameter of the telescope and instruments must correspond with the trocar-cannula units.

The procedure
The animal is clipped, prepared and draped for abdominal surgery. A Veress needle is used to safely penetrate the abdominal wall, then attached to insufflation tubing which connects to the automatic carbon dioxide insufflator. Carbon dioxide is considered to be the gas of choice for insufflation because of safety in preventing air emboli and spark ignition during cauterisation. After insufflation of the abdominal cavity, the trocar-cannula unit is placed through the abdominal wall and the trocar removed. Most cannulae contain an internal one-way valve that prevents loss of insufflated gas once the trocar is removed after abdominal entry. The cannula remains in place traversing the abdominal wall and creates a portal for the introduction of the telescope or instruments into the abdominal cavity.

The telescope and instruments can then be moved between cannulas as required. When finished, the telescope and instruments are removed, the insufflation tube detached so abdomen decompresses, cannulas removed and sutures placed in the muscle and skin as required.

Whats different in cats?
Their abdominal wall is very thin making it difficult and somewhat dangerous to normally insert a trocar-cannula. The thin muscle wall provides little grip to the cannulas making them easy to accidentally pull out while working. Cats require less intra-abdominal pressure to maintain a good surgical field (4-6mmHg compared to 8-15mmHg in the dog) and more readily develop complications such as reduced cardiac output from pneumo-peritoneum. Maximal insufflation allowed is hence lower for cats (10mHg for cat vs 15mmHg for dogs). The small dimensions of a cats abdomen and thorax requires telescopes and instruments with shorter working lengths (14-18cm compared to 20-30cm for dogs)

To optimise results, we use

1. 3.5mm Endotip cannulas – the cannula screws through abdominal wall rather than blindly stabbing a sharp trocar to gain access. These threaded cannulas also resist falling out of muscle wall during use and are simply unscrewed after use leaving undamaged muscle fibres.
2. 2.7mm telescope with 14cm working length
3. 3mm instruments with uni-polar coagulation
4. maximal insufflation of 6mmHg

Is there a place for laparoscopy in cats?
Client acceptance is excellent, the biopsy specimens are high quality and cats clearly benefit from reduced trauma. While the applications of laparoscopic surgical procedures continues to grow, it is likely that the main application of laparoscopy in cats will be diagnostic sampling. Initial set up costs is the major deterrent in veterinary medicine and this can be reduced by purchasing second hand equipment.

Thoracoscopy

Visualisation of lungs, pericardium, heart and major blood vessels, thoracic duct, mediastinum, lymph nodes and the pleural and peritoneal surfaces of the thoracic cavity can be performed with minimal invasiveness by placing a cannula in the chest wall and inserting a rigid telescope.
Thoracoscopy has been used in diagnosing spontaneous pneumothorax, pericardial effusions, pulmonary disease, pleural diseases, neoplasia (including determining margins and respectability), and determining the etiology of refractory pleural effusions. The most common minimally invasive thoracic surgical procedure currently being performed in small animals is creation of a pericardial window. Other thoracic procedures being performed include partial and total lung lobectomies, thoracic duct occlusion, PRAA transection, ligation of PDA, mediastinal mass removal and debridement for pyothorax.

The thorax can be entered trans-diaphragmatically (under xyphoid) or intercostally using a screw in Endo-tip cannula. The trans-diaphragmatic approach allows a long axis view of the thoracic cavity and is best for exploration and biopsy. Once a cannula is introduced, a pneumothorax developes causing the lungs to partially collapse and allows visualisation. Mechanical ventilation is therefore required. Additional cannulas are inserted under thoracoscopic visualisation and instruments inserted as required. Once the telescope is in place, biopsy forceps or surgical instruments can be introduced through adjacent cannulas to perform various diagnostic or surgical procedures within the thorax. Once examination and operative procedures have been completed the instruments and cannulas are removed, air removed from the pleural space and the lungs re-expanded.

Thoracoscopy offers many significant advantages over a conventional open thoracotomy.  As such, it will likely become the gold standard for both diagnostic investigations and many surgical procedures performed in the thoracic cavity.

Summary
Minimally invasive thoracic and abdominal surgery is far less invasive than open surgical procedures and has much lower morbidity and mortality yet allows greater visual exploration of the chest and abdomen than can be done with a laparotomy or thoracotomy. General anesthesia is required for performing minimally invasive surgery but the duration and depth of anesthesia can be much less than for open surgery. An owners reluctance for surgery may play an important role in the decision to select a minimally invasive technique.