The Tipping Point: How Venograms Make the Difference Between Success and Failure When Treating Lamin
R.F. (Ric) Redden, DVM. Venogram technique, indication and interpretation, in Proceedings. Bain Fallon Memorial Conference October 2006; 28-35.
(Watch the Digital Venogram video performed by Amy Rucker, DVM.)
Since 1992 venograms have been used at the International Equine Podiatry Clinic to evaluate horses with lameness problems localized to the foot. This retrograde contrast study can demonstrate structural alterations of the venous as well as arterial supply within the soft tissue and bone early in the course of the disease before the effects of vascular compromise are detected radiographically.1
Venograms have proven to be a valuable tool for making specific diagnosis in horses with foot disease, injuries and diminished horn production. They have been particularly advantageous for treating laminitis as they allow us to identify structural alterations of the solar papillae, lamellar vessels, coronary plexus and terminal arch that can not be seen with other imaging techniques. This has allowed us to follow progressive deterioration of the disease in cases that otherwise might have been considered clinically stable.
Before venogram imaging, radiographs were the only images used routinely to evaluate laminitis and other foot disease syndromes. As a result, veterinarians have been limited to evaluating the significance and severity of the disease during the acute phase, meaning the greatest window of response cannot be effectively utilized. A large majority of cases with significant insult deteriorate, ultimately leaving the horse crippled or resulting in euthanasia. Venograms have added to our knowledge of laminitis and other common foot problems, in large part because for the first time we can see the structural components of the vascular supply to the horse's foot. This procedure is relatively easy, uses basic radiographic equipment and can be performed on the standing horse.
Equipment and Technique
The equipment used at IEPC is a 100/30 MinXray portable unit, asymmetric 6/12 screens, ultra detail film and a 6:1, 106 line/in grid. Several contrast mediums have been used in the past, however we have found that higher concentration produces slightly more information. Reno-60® has been the contrast of choice for the past several years. A variety of catheters have been used. The 21-gauge butterfly is easy to use, easily procured and provides a consistent means of delivering contrast.
Sedation of the horse, local analgesia of the foot and a tourniquet placed over the fetlock is required. The original venogram technique developed at IEPC was a collaboration with Dr. Chris Pollitt using his previous in vitro study model and has since been published with minor modifications.2 The imaging sequence that has been routinely used is soft tissue lateral, lateral with grid, DP with grid, DP with soft tissue detail, followed by a soft tissue lateral. The procedure is technique sensitive in large part because the sequence of film must be taken within a time frame of 45 seconds following contrast injection. Tissue contrast injected retrograde into the palmar vein is quickly absorbed into the interstitial space, significantly reducing the value of the information obtained.
The soft tissue lateral images have proven to be especially valuable because they allow us to see coronary papillae, circumflex vessels, solar papillae and acute and chronic lamellar leakage. The grid used with the higher MAS has proven to be a reliable means of imaging the terminal arch and its tributaries. The soft tissue and hard penetration grid views were implemented to allow us to evaluate normal vascular anatomy and structural alterations within the soft tissues and bone. The soft DP view allows us to evaluate the medial and lateral coronary supply and the medial and lateral circumflex network. The grid view offers further information concerning the terminal arch and tributaries. The soft tissue lateral view taken at the end of the sequence was implemented to allow for adequate filling time required to consistently image lamellar leakage, decreasing the possibility of missing significant structural alterations. More specific beam selection is required to image vascular lesions associated with White Line Disease and Keratomas. Techniques for imaging the equine foot will continue to evolve with increasing experience performing venograms.
Tourniquet failure and perivascular injections are frequent complications encountered in the infancy of procedural skill development. Both complications result in underperfusion and can lead to grossly inaccurate interpretation and misuse of the information. Technique underperfusion should not be confused with stark loss of contrast caused by pathological vascular compromise. The distinction should be well understood before deciding the fate of a laminitic horse based on the information gathered from this valuable tool.
1) Sedate the horse. 2) Block the feet just above the fetlock. Use only 4-6cc of blocking agent to prevent transient edema. 3) Place the horse's feet on proper positioning blocks. This will insure a pure lateral projection. 4) Set the x-ray machine in place, and have all the necessary cassettes and grids within arm's reach. 5) Take a scout film with soft-tissue detail, using a barium paste marker on the face of the hoof wall. 6) Wrap 4 inch Elastikon around the fetlock. This will provide an anchor point for the tourniquet and prevent twisting of the skin while applying the tourniquet. 7) Place a tourniquet over the fetlock. Avoid a mid-cannon tourniquet. 8) Catheterize the palmar vein using a 5/8 inch, 21-gauge butterfly catheter. Be careful not to thread the needle to far in to the vein. You risk making a second hole in the vein. 9) Inject 20cc of Reno-60. I recommend using two 12cc syringes instead of one 20cc syringe. A 20cc syringe builds too much back pressure and complicates injection. Injection needs to be completed in less than one minute, as the contrast will leak from the vessels quickly and skew your interpretation. 10) Pull the knee forward slightly while injecting the second syringe. The heel should remain flat on the block. This rocking assures lamellar perfusion by unloading the deep digital flexor tendon (DDF) 11) Clamp a hemostat on the catheter. Quickly tape the hemostat to the leg using the loose end of the tape that secures the tourniquet. 12) Take your series of film. All exposures should be taken within 45 seconds. Move quickly. Radiographic Views i) Lateral, soft exposure ii) Lateral, hard exposure with grid iii) DP, hard exposure with grid iv) DP, soft exposure v) Lateral, soft exposure Soft exposures are needed when there are significantly compromised vessels. Hard exposures offer a diagnostic image of the terminal arch and deeper vessels. The last DP view offers a look at vascular leakage that may not be apparent in the first couple films. Acute and chronic cases will exhibit similar patterns, yet there are distinct differences between the two. 13) Remove the tourniquet and place cotton or gauze over the medial and lateral vessels. Tape in place for 5-10 minutes.
Indications A venogram is a discovery experience, offering a means to track the disease syndrome as it alters the vascular supply. Therefore, it offers unlimited options concerning the medical, surgical and therapeutic regimes necessary to revive the compromised areas. Venograms can be used diagnostically for:
White Line Disease
Sore feet with thin soles and poor quality walls
Having performed hundreds of venograms, I have discovered a pattern of a sequence of events that appears to be repeatable in most pathological conditions. Laminitis can progress from mild onset to high-scale case within hours to weeks. Venograms can provide consistent evidence of each stage of the syndrome.
Likewise, the venogram offers a reliable means of monitoring the progress of reperfusion in compromised areas. They also help explain why some cases fail to progress in a favorable fashion. Being able to correlate the altered vascular pattern with the clinical picture, growth pattern and tissue response greatly enhances your insight for the planning and treatment stage.
Before this unique discovery experience can be used to reveal how badly areas are compromised, we must first learn the range of norm for specific breeds and age groups. The lateral and DP views are the most valuable views when dealing with laminitis. The 65 degree, DP reveals another perspective when looking at the circumflex vessel and papillae, but this view is not vital for assessing the damage caused by displacement.
Interpretation of the Normal Foot
Lateral View The normal foot will have a dense, uniform contrast pattern over the extensor process. I refer to this as the "waterfall". These vessels continue down the face of PIII in a relatively parallel plane to the bone. Approximately 8-10mm proximal to the apex of PIII, these vessels meet and join the branches of the circumflex network. This network supplies blood to the palmar surface and the rim of the bone. The normal, healthy foot has 10mm of vascular corium ventral to the palmar surface of PIII. The papillae are clearly seen penetrating the sole proper. The papillae are of great interest, as they are the first vessels to be compressed or crushed as PIII descends, whether it occurs from rotation or sinking. Conversely, the thin soled horse has a compressed palmar vascular supply, though it appears quite different from compression caused by medial displacement.