HOW TO PERFORM A VENOGRAM OF THE EQUINE DIGIT
Written November 2003 by R.F. (Ric) Redden, DVM
Take Home Message:
A venogram of the digit offers invaluable information concerning venous as well as arterial perfusion. It has value as a diagnostic aid, prognostic indicator and possible therapeutic application. The technique is a low risk procedure, easily performed on the standing horse.
(Watch the Digital Venogram video performed by Amy Rucker, DVM.)
Circulatory contrast studies are commonly used as diagnostic aids for a multitude of maladies in the human field but a reliable means of assessing circulation of the equine foot has not been reported. Hertsch's1 work with angiograms and Pollitt's2 experimentation with venograms became the stimulus for the author to develop a safe, consistent means of assessing the circulation of the equine foot. Dr. Hertsch indicated that general anesthesia was required for angiogram studies as vasospasm was a major problem in the standing horse. Using invitro models, Pollitt was able to fill the arterial supply retrograde via the common digital vein using a tourniquet proximal to the injection site. Redden and Pollitt collaborated in 1992 developing a reliable technique for the standing horse as a great need existed to better understand the parody of circulatory maladies involving the equine digit, especially with laminitis.
As an intermediate hypothesis it has been thought that shearing and compressive forces of the third phalanx, once displaced within the capsule, contributes to mechanical circulatory collapse, directly influencing varying degrees of osteitis of PIII3, associated with this observation was the fact that the circulation of the digit could be assessed with in vivo models and the influence of various therapeutic devices assessed4.
The etiology and pathogenesis of laminitis continues to be controversial and not well understood. A venogram offers a means for assessing the circulatory system of the digit at various stages of the syndrome. his information helps the veterinarian and farrier develop a protocol designed to mechanically and therapeutically address the forces at play that directly restrict perfusion of the digit. It also can be used as a diagnostic tool for soft tissue and space occupying lesions such askeratomas, tumors and scars. Comparative venograms can become valuable prognostic indicators and clinical observations suggest they may have therapeutic value as well. This paper describes how to perform a venogram in the standing horse and indications and results of specific cases.
Materials & Methods:
Initial studies were designed to develop a reliable and consistent technique using sound animals in an effort to establish a base line for normal contrast patterns.A variety of pathological foot problems were then examined, including acute and chronic laminitis with varying degrees of damage; club feet in young and mature animals, chronic quarter cracks, keratomas, white line disease and feet with severe imbalance.
The standing horse is sedated with 25 mcg Dormosedana I.V., and the foot desensitized with Carbocaineb , 6cc., placed subcutaneously over each common digital nerve just proximal to the fetlock.With bilateral problems the opposite foot is desensitized as well to facilitate quick manipulation of the studied limb.Shoes are pulled, and the feet thoroughly cleaned of all foreign material, taking care to remove all debris, especially along the deep sulcus of the frog and buttress area.The pastern is clipped and aseptically prepped.Each front foot is placed on a wooden positioning block (3x5x7), with a wire embedded along the long axis of the block to delineate the ground surface. Elastikonc is placed over the fetlock to prevent tourniquet slippage. A 21 gauge butterfly catheterd is inserted in the digital vein followed by placement of a tourniquet over the fetlock.To minimize back pressure, two separate 12cc syringes are used to deliver a total of 15 to 20cc of contrast medium. Renografin 76e is preferred over other products used. Slight thumb pressure over the vein distal to the catheter prevents ballooning prior to valve opening. The foot is unloaded while injecting the second syringe to assure lamellar perfusion which is compromised when the foot is loaded.A lateral, an A-P and 65 degree D-P views are taken with all cases. To avoid image distortion perpendicular beam and film projection is used for all views.The preferred x-ray equipment is a portable MinXray 80+f and 3M 2/6 screens with Ultra Detail filmg . For optimum contrast image all views within 45 seconds following injection of contrast medium.
Perivascular injection inadvertently occurs if the catheter penetrates the vein more than once. A temporary coolness of the distal limb occurs but with no apparent lasting side effects. Tourniquet failure causes varying degrees of filling and can lead to a false interpretation.
Developing the technique and range of normal, ten feet were examined. One Standardbred adult, two Arabian adults, One Thoroughbred adult and one Quarter Horse weanling.The basic contrast image did not vary significantly among this small group. To adequately perfuse the lamellar vessels, the foot is unloaded while injecting the last 10 cc. Further studies of the micro circulation are indicated to better understand the range of normal. Three suspected keratomas were examined, revealing a distinct space occupying lesion. One persistent quarter crack revealed a space occupying lesion that was surgically removed, eliminating further quarter cracks for a period of three years. Five white line disease cases revealed stark loss of contrast on the laminae, adjacent to the lesion. One hundred three laminitic feet were studied that were suffering from mid to high scale damage assessed clinically and radiographically, (500-1000). Several cases had periodic follow-up, comparative studies. A wide range pattern was found, apparently directly influenced by the initial insult and subsequent compartmental syndrome created by displacement of P3. The lamellar vessels, circumflex zone and terminal arch were assessed as normal, poor perfusion or stark loss of contrast. Treating dozens of high scale (750 to 1000) laminitis cases in a similar fashion, the author concluded that those with stark loss of contrast along the lamellar vessels, circumflex area and terminal arch had irreversible circulatory damage and a very grave prognosis.
Variables influencing adequate digit perfusion is multifaceted, not well understood and needs further studies. 29 bilateral cases classified as sinkers in the mid scale range were examined, 750-1000. Unilateral venograms were performed on the foot with the most damage assessed, significant new sole and horn growth was observed within two weeks following the venogram where the opposing foot showed very little, if any, new growth over the same period of time. The same feet grew approximately 1/2 to 3/4 inch of horn and sole respectively within six weeks, whereas the opposing foot had significant less growth. All cases were derotated, shod with a four point aluminum rail shoe with Advance Cushion Support h and had a deep flexor tenotomy. Three cases had stark loss of contrast in all three zones and were euthanized, as they remained non responsive, in spite of aggressive therapy.
Two specific cases classified with high scale damage (800-1000) stand out to suggest the validity of the venogram as a therapeutic aid and several others in this group revealed a similar response.
Case 1: Quarter Horse show filly, had a venogram right front at the time of admittance. This foot continued to show deteriorating signs and appeared to suffer more damage than the left, but responded favorably, growing 1/2 to 3/4 inch of sole in six weeks when the left front showed no growth and sloughed the hoof capsule,revealing dark gray laminae. A venogram revealed stark loss of contrast along the lamellar vessels and circumflex zone, with moderate filling of the terminal arch. Euthanasia was recommended due to overall grim picture. The client requested two days to absorb the shock of losing her mare. The mare was led out to be euthanized, and found to be quite sound, relative to her previous days of being down the majority of the time. The cast was removed, revealing several areas of pink, healthy granulation tissue. Reconsidering the circumstances, treatment resumed. A venogram was performed every three weeks for three consecutive studies. The clinical picture was improved within days following each venogram. The mare became increasingly sounder, and the hoof capsule steadily regrew over the next five months, protecting the sensitive structures. The mare returned to pasture soundness and continues to have a favorable outcome, nine months after being dismissed.
Case 2: American Saddlebred gelding with 500 to 700 range damage. Venograms were done on both feet at initial examination, revealing poor perfusion within the lamellar vessels and compromised areas in the circumflex vessels. Nine months later, the right front medial quarter showed increased sensitivity, the horse was lame, grade 2/5 and the coronary band bulged out over the horn wall. A venogram revealed a loss of contrast in this area. Both feet were reset with a four point aluminum rail with Advance Cushion Support following derotation. Six week follow-up, the horse was quite sound, 1/2 inch of new growth was seen at the coronary band, and a comparative venogram revealed good perfusion of the compromised areas.The following six week period there was approximately one inch of new horn along the medial quarter and the horse was given two additional months turn out and put back into training.
Case 3: Thoroughbred race filly with bilateral acute laminitis. A previous bout of laminitis one year ago had caused significant permanent damage in the left front foot, which showed significant radiographic damage and was more painful than the right at the time of onset. The left front was examined by venogram, revealing a pattern consistent with a chronic lamellar scar and reasonably good perfusion in the other zones. She was shod using the above described technique. Six weeks later the left front showed a favorable clinical response, right front, very little if any noticeable new horn growth and exhibited grade 2/5 lameness. Both feet were examined approximately eight weeks from onset via a venogram, followed by shoeing and bilateral deep flexor tenotomy. Within two weeks following the venogram and surgery there was a noticeable horn growth and reduced sensitivity along the medial coronary band right front. The left front continued to show a more favorable response over the following six months. This case continues to be in the recovery stage of the syndrome.
It was noted that the lamellar vessels had poor to stark loss of contrast when the foot was fully loaded while injecting the contrast medium. This finding, coupled with clinical observation leads to the hypothesis that counter limb laminitis is triggered by inadequate lamellar perfusion. The clinical case that is content to stand for long periods of time without shifting weight to the injured limb is more likely to develop laminitis within three to six weeks from injury than the case with protected laminae that constantly shifts weight or moves about the stall, in spite of being grade 4/5 lame. A Venogram of the digit reveals arterial and venous perfusion, to include micro circulation. It provides a means to visually assess numerous pathological conditions that alter the circulation of the digit and is potentially a helpful diagnostic, as well as prognostic tool. Severely compromised digits suffering from laminitis can show a favorable clinical response following this technique.
Further studies are needed to determine the mode of action, and how the response is related to this procedure.
References & Footnote:
1Hertsch, Professor Dr. B., Klinik Fur Pferde der FU Berlin, Germany-personal communication.