Things to try when routine diagnostics are not enough (Proceedings)

Often times a thorough physical examination complemented with a blood count or chemistry profile provides enough information to establish a tentative diagnosis in ill horses and consequently guide appropriate therapy.  However, all equine clinicians are faced with the occasional horse that has an abnormality (i.e. weight loss) but yet, the physical examination and basic clinicopathologic data is not providing any clues to the cause. 

In these cases, ancillary diagnostics may help provide further information to establish a diagnosis.  The purpose of this lecture is to provide information on some less commonly used diagnostic tests as well as therapies that can possibly help in your diagnostic or treatment plan. 

Oral glucose absorption test (ogat)

The OGAT is used to test the ability of the small intestine to absorb glucose, hepatic glucose uptake and endocrine function of the pancreas.  All glucose is absorbed within the small intestine, thus OGAT results reflect small intestinal function.  The primary indications for the OGAT are chronic weight loss.  The test is simple and can be accomplished in a short period of time.  The patient is fasted for 12 to 18 hours; subsequently, resting blood glucose is measured and then 1 gram/kg BWT of dextrose, as a 20% solution, is administered via nasogastric tube.  Blood is then collected every 30 minutes, for 3-4 hours and the blood glucose is measured and recorded. 

In healthy horses with normal small intestinal absorptive function, the blood glucose should peak (doubling in value) within 120 minutes of dextrose administration.  Subsequently, the blood glucose should return to base-line values within 3-4 hours of dextrose administration.  In horses with abnormal small intestinal absorption the glucose curve will appear flat or demonstrate only a modest rise in blood glucose over time; total malabsorption is defined as a failure of blood glucose to increase >15% above baseline within 120 minutes whereas partial malabroption is defined as an increase in blood glucose >15% but <85% of baseline within 120 minutes. 

The accuracy of the OGAT was good when normal small intestinal function or total malabsorption was present, based on one study evaluating OGAT results compared to histopathologic findings.1  However, horses classified as partial malabsorption were not as reliable and correctly identified 75% of horses with structural abnormalities of the small intestine.1  Diseases to consider when a lowered or delayed blood glucose concentration is observed include infiltrative lymphosarcoma, inflammatory bowel disease, cyathostomiasis, chronic colitis (Salmonella), multisystemic eosinophilc epitheliotropic disease and food allergies.2         

Biopsy

Biopsy samples of various tissues is a common diagnostic tool utilized by equine clinicians for histologic examination of masses (i.e. neoplasia) and muscle (i.e. PSSM, EMND).  However, clinicians can use similar techniques to biopsy structures that are, perhaps, less accessible such as the liver, kidney, spleen, rectal mucosa and lung.  The Tru-Cut Biopsy Needle (Cardinal Health) is an affordable disposable biopsy device available as an 18- or 14-gauge needle with variable lengths that typically provides adequate samples.  The Biopty-Cut Biopsy Gun (Bard) is a reusable automated device that acquires the sample for the clinician once the disposable needle is inserted into the appropriate sampling area.  When collecting biopsy samples from vital organs such as the liver, kidney or spleen, ultrasound guidance is highly recommended. 

·         Liver Biopsy is indicated to better define elevated liver enzymes and may provide a definitive diagnosis (i.e. chronic active hepatitis, acute serum sickness).  Biopsy is typically performed on the right side of the horse where there is a larger window to visualize the liver ultrasonographically.  Lung and diaphragm have been inadvertently biopsied when trying to attain a liver sample but puncture of the colon, which is ventral and deep to the liver, should be avoided.  Although bleeding is a major complication, a recent report documented that even though at least one coagulation abnormality was identified in 58% of the horses with liver disease, complications secondary to liver biopsy of those horses were extremely low.   

·         Kidney Biopsy is not commonly performed but can help better define the cause and severity of acute or chronic renal failure as well as identify renal neoplasia.  The right kidney is more superficial whereas the left kidney is typically deep to the spleen.  Thus, if given an option, the right kidney is more accessible.  In a recent retrospective study of 151 renal biopsies, the authors reported an 11% complication rate, mainly associated with signs of colic or hemorrhage (hematuria or intra-abdominal hemorrhage). Biopsy specimens provided sufficient tissue for histopathologic diagnosis in 94% of cases but diagnoses had only fair (72%) agreement with postmortem findings.

·         Rectal Biopsy allows collection of the mucosa/submucosa of the caudal intestinal tract and is indicated in horses suspected of having infiltrative, inflammatory or neoplastic intestinal disease such as those presented for chronic weight loss or diarrhea.  An accurate diagnosis can be obtained in approximately 1/3 of horses with of some form of inflammatory bowel disease.5  Various techniques have been used but uterine biopsy forceps are readily available to most clinicians.  Samples are collected approximately 30 cm proximal to the anal sphincter at the 11 or 1 o'clock positions.  In this procedure, the well-lubricated gloved hand guides the biopsy forceps to the appropriate location and isolates a small section of the mucosa/submucosa; the instrument is advanced, the jaws are opened to capture the fold of tissue and the biopsy is then collected.

·         Miscellaneous Tissues such as the Spleen and Lung can be biopsied in the case of splenic masses or thoracic masses or interstitial pneumonia.  Samples of the spleen are easily collected from the left paralumbar area with minimal complications.  Lung tissue is a bit harder to collect as the tissue is typically air-filled and collected a large sample of lung parenchyma can be challenging.  Hemothorax, epistaxis, tachypnea and respiratory distress are legitimate complications that should be monitored.   

Transfusions

Transfusions of various blood products is often needed in equine practice and include products such as commercially available equine plasma.  However, other products such as whole blood, packed red cells, and fresh plasma may need to be harvested; often times, the need for the later blood products can be an emergency situation with limited time to harvest blood from a donor.  Therefore, it is important for the equine clinician to have the necessary equipment ready as well as have potential donors identified. 

Commercial frozen plasma (frozen for > 1 year) is readily available and is a good source for albumin and stable clotting factors (FII, VII, IX, X).  If the patient needs both stable and labile (FV, VIII, vWF) clotting factors, fresh frozen plasma (frozen w/in 6 hours; stored < 1 year) must be used.  Fresh plasma (separated from the RBCs and used within 6 hours) is necessary to gain functional platelets.  In patients with severe hemorrhage or anemia, whole blood and/or packed red cells may be necessary. 

If possible a major (donor RBCs + recipient plasma) and minor (donor plasma + recipient RBCs) should be performed.  However, in the field setting, this may not be possible.  In this instance a gelding of the same breed should be used if possible.  The clinician can perform a stall-side test for agglutination between the donor and recipient as a screen for major incompatibilities.  In order to estimate the amount of blood necessary to increase the patients PCV one can use the following formula: 

(PCV desired – PCV patient)/PCV donor x 0.08 x BWT (kg)

A horse can safely donate 15-18 mLs/kg of blood (7.5-9L in a 500 kg horse).  The minimum necessary equipment for blood collection includes a large bore catheter (10-12 gauge), extension set, collection bag or bottle (glass inactivates platelets) and anti-coagulant (acid-citrate-dextrose or citrate phosphate dextrose).  Anti-coagulant should be placed in the collection bag in a 9:1 ratio (blood:anticoagulant). 

During collection of blood, the collection bag should be continuously agitated to allow mixing of the blood with anticoagulant.  If packed red cells are necessary (i.e. to avoid volume overload) the red cells can be allowed to sediment to the bottom of the bag and then remove the plasma.  The red cells can them be re-suspended with saline if necessary to achieve a solution of ~ 70% RBCs.  The blood or blood product should be administered through a filter set and the patient should be monitored for signs of transfusion reaction (fever, tachycardia, colic, urticaria).    

Intracavitary Drains

Placement of thoracic or abdominal drains is sometimes necessary to facilitate drainage of accumulated pleural or abdominal fluid or evacuate air from a pneumothorax.  When drains are placed correctly and in a sterile fashion, they may greatly improve patient outcome.

Thoracic drain

Treatment of pleuropneumonia is the main indication for placement of a thoracic drain.  Not all horses with pneumonia require a drain but when an obvious accumulation of fluid is noted ultrasonographically, removal is indicated.  Once the most ventral limit of the thoracic fluid is identified, the hair is clipped and the area is scrubbed with disinfectant.  Lidocaine is then administered in the skin, SC and muscle in an area cranial to the rib; a small stab incision with a blade is necessary to help introduce the trocar into the thoracic cavity.  A 20 to 32 French thoracic catheter should be held near the tip of the catheter with one hand and steady pressure placed on the trocar from the end of the device with the other hand as it is gently guided into the thoracic cavity. 

This will help stabilize the instrument as well as avoid jamming the entire length of the catheter into the thoracic cavity and potentially piercing the lung or heart.  Fluid will typically fill the catheter as you enter the pleural fluid.  At this point, advance the catheter so that all the fenstrations are within the thoracic cavity and gently remove the trocar.  A one-way valve, such as Heimlich valve, should be placed to avoid pneumothorax while allowing continual fluid drainage.  The catheter is then secured with a finger-trap suture pattern.  The drain may remain in place for several days as long as fluid is draining from the thorax and no evidence of local infection is present at the catheter insertion point. 

General principles of treatment of pleuropneumonia, such as broad-spectrum antimicrobial therapy, NSAIDs, and supportive therapy should be followed.  Placement of a thoracic drain to treat pneumothorax is accomplished in a similar fashion but the thoracic catheter is placed in a dorsal position to allow removal of air.  Smaller size catheters can be used as obstruction of the catheter is not as common as with treatment or pleuropneumonia, which may obstruct the catheter with large sheets of fibrin secondary to pleural inflammation.  Constant or intermittent suction should be applied to the catheter to evacuate the air from the thorax.           

Abdominal drain

Treatment of peritonitis and prevention of post-operative adhesion formation are the main indications for abdominal drains.  Some clinicians utilize abdominal drains for peritoneal dialysis (see below).  The site of insertion can be variable but a ventral location is necessary to allow gravity to facilitate fluid removal.  In addition, the spleen should be avoided, thus placement to the right of midline is common.  Ultrasound examination may also help identify a fluid pocket that can be targeted for catheter insertion.  The ventral colon rests of the floor of the abdomen and must be avoided. 

Once a location is identified, the same steps of clipping, preparing and administering local anesthetic at the site are followed.  The stab incision through the skin and ventral musculature should be large enough to accommodate the size of the drain as the clinician should avoid placing excessive amounts of upward pressure during placement and the possibility of inserting the drain into the colon.  Once inserted, the catheter is sutured in with a finger-trap pattern and a one-way valve is placed.  Some clinicians wrap the catheter in place with adhesive tap around the abdomen to avoid contamination or inadvertent removal of the catheter.  Other steps to treat peritonitis or post-operative adhesions should be followed.       

Peritoneal dialysis

The need to perform peritoneal dialysis in horses is rare but yet, the clinician may be faced with treatment of refractory acute renal failure that may benefit from peritoneal dialysis.  The peritoneum serves as a large surface are to exchange metabolic waste products such as urea and creatinine along with electrolytes down a concentration gradient and into the dialysate.  Peritoneal dialysis can be intermittent or continuous and is accomplished by placing a large (28-32 French) thoracic catheter into the ventral abdomen, as described above. 

This catheter can be used as both an ingress and ingress port for dialysate administration and removal.  Typically, approximately 10 liters of sterile fluid, such as normosol-R, is infused into the abdomen and kept in place for 30-120 minutes (dwell time).  The fluid is then removed via the abdominal drain and the amount of fluid recovered is recorded.  This can be repeated several times throughout the day; azotemia and electrolyte concentrations should be measured from the patient's blood every 6-24 hours to ensure that major derangements in electrolytes are not occurring and that azotemia is declining. 

In one report, a horse with acute renal failure that was refractory to conventional therapy was subsequently treated with intermittent peritoneal dialysis, facilitating a positive outcome.7  The horse was a 1-year-old 290-kg Thoroughbred colt with aminoglycoside-induced acute renal failure.  Azotemia was present on initial clinicopathologic evaluation with a serum creatinine concentration of 6.2 mg/dL (reference range 1-1.8 mg/dL) and BUN of 72.2 mg/dL (reference range 11-24.1 mg/dL) along with moderate proteinuria and the presence of renal epithelial cells and cellular casts observed cytologically.  

In this report, a 28-French thoracic catheter was placed percutaneously into the ventral abdomen and used as an ingress/egress catheter.  Peritoneal dialysis was performed twice daily by infusing 10 L of warmed polyionic fluids (Normosol-R) with a dwell time of 30-60 minutes; the fluid was then allowed to drain into a sterile collection bag.  The serum creatinine concentration decreased to 4.5 mg/dL 24-hours after initiation of peritoneal dialysis and was 2.7 mg/dL on day 7, at which point peritoneal dialysis was discontinued (Figure 3).  By day 14 of hospitalization, the serum creatinine concentration had returned to normal (1.4 mg/dL) and the horse with discharged. 

Cystoscopy

Many equine clinicians have endoscopy units available for examination of the upper airway.  Although not needed on a daily basis, endoscopic examination of the urinary tract can provide valuable information for such diseases as cystolithiasis, uretheral hemorrhage, urinary bladder neoplasia and anatomic defects, among other diseases of the urinary system.  Cystoscopic examination requires sterilization of the endoscope which is typically accomplished by soaking the endoscope in disinfectant while running the solution through the biopsy channels as well.  Once the endoscope is sterilized, the horse is sedated and the penis or perineal region is washed.  Sterile lubrication is applied to the endoscope and it is then guided into the urethra.  Visualization of the urethra, sex glands (male), urinary bladder and ureter openings is possible.  

There are numerous other diagnostic and therapeutic techniques not discussed here that the equine clinician should become familiar with to help provide the best care to patients.  This would include other procedures such as regional limb perfusion, interosseous limb perfusion, fecal analysis, abdominocentesis, transtracheal wash, bronchoalveolar lavage and enteral/parenteral nutrition, among others. 

References

Mair et al.  Small intestinal malabsorption in the horse: an assessment of the specificity of the oral glucose tolerance test. Equine Vet J 1991;23:344-46. 

Roberts MC. Small intestinal malabsorption in horses. Equine Vet Educ 2000;12:214-219. 

Johns IC et al.  Coagulation abnormalities and complications after percutaneous liver biopsy in horses. J Vet Intern Med 2008;22:185-9. 

Tyner GA et al. A multicenter retrospective study of 151 renal biopsies in horses. J Vet Intern Med 2011;25:532-539.

Kalck KA. Inflammatory bowel disease in horses. Vet Clin No American Equine 2009:25:303-315. 

Han JH et al.  Intermittent peritoneal dialysis for the treatment of acute renal failure in two horses.  Eq Vet Educ2008;20:256-264.