Dr Redden's Equine Podiatry Series In-Depth Laminitis
Venogram Technique, Indication and Interpretation
Written and presented November 2005 by R.F. (Ric) Redden, DVM
(Watch the Digital Venogram video performed by Amy Rucker, DVM.)
I developed this clinical protocol out of the need to better understand the degree of vascular damage during different stages of the syndrome. Working with Dr. Chris Pollitt at my clinic, we used his in vitro study to perform the first in-vivo venogram procedure in 1992. It was conducted on a standardbred filly that was owned by the International Equine Podiatry Center. Since that time, I have modified the procedure to meet the requirements of specific breeds, as well as a variety of foot problems that involve the circulatory system.
Sedate the horse.Block the feet just above the fetlock. Use only 4-6cc of blocking agent to prevent transient edema.Place the horses feet on approved positioning blocks. This will insure a pure lateral projection.Set the x-ray machine in place, and have all the necessary cassettes and grids within arms reach.Take a scout film with soft-tissue detail, using a barium paste marker on the face of the hoof wall.Wrap 4 Elastikon around the fetlock. This will provide an anchor point for the tourniquet and will prevent twisting the skin while applying the tourniquet.Place a tourniquet over the fetlock. Avoid a mid-cannon tourniquet.Catheterize the palmar vein using a 5/8, 21-gauge butterfly catheter. Be careful not to thread the needle too far into the vein. You risk making a second hole in the vein.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.Pull the knee forward slightly while injecting the second syringe. The heel should remain flat on the block. This rocking assures lamellar perfusion and unloads the deep digital flexor tendon (DDF).Clamp a hemostat on the catheter, or apply a stopcock. Quickly tape the hemostat to the leg so it will not be in your way.Take your series of film. This series of film should be completed within 45 seconds.Lateral, soft exposureLateral, hard exposure with gridDP, hard exposure with gridDP, soft exposureLateral, soft exposureSoft 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 film. Acute and chronic cases will exhibit similar patterns, yet there are distinct differences in the two.Remove the tourniquet and place cotton or gauze over the vessels. Tape in place for 5-10 minutes.Indications
A venogram is a discovery experience, as it offers 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:
LaminitisWhite Line DiseaseKeratomasPuncture Wounds
Having performed a few thousand venograms over the years, I have discovered a pattern that appears to be repeatable as laminitis progresses from a mild onset to high-scale cases; whether it is acute or chronic.
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 65º, DP reveals another perspective when looking at the circumflex vessel and fimbriae, but this view is not vital for assessing the damage caused by displacement.
Lateral View Points of Interest
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 fimbriae are clearly seen penetrating the sole proper. The fimbriae 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.
Sport horses that become foot sore, have thin soles and poor quality horn walls. They often have no more than 6-10mm of soft-tissue space between the palmar rim and the foot side of the shoe. In this case, the fimbriae are not visible on a soft-detail venogram, and the major vessels are compressed tightly between the bone and hoof wall. This may be one explanation why short feet stop growing at a normal rate.
Therefore, I conclude that a healthy sole requires a minimum depth of 15mm; 10mm for the vascular network and a minimum of 5 mm for the non-sensitive, protective sole. This information should be of particular interest to farriers who strive to maintain a healthy, sound foot.
When a foot is trimmed short, small red dots appear along the freshly trimmed sole. These dots are the ends of the individual fimbria. A healthy sole (20mm) will have longer fimbriae than a sole that is only 15mm in depth. This may explain why a long foot will bleed much easier than a short foot that is trimmed to within 15mm of the palmar rim.
The lamellar vessels can be seen superimposed over bone as they coarse downward from the coronary band to the palmar surface.
The hard penetration radiograph clearly reveals the terminal arch as it supplies blood to the bone. If you examine the coffin bone, you will find small nutrient openings along the face of PIII. These openings allow interior vessels to emerge from the bone, linking with the lamellar network. The more upright, sturdy and apparently healthier hoof has fewer and smaller holes than that found in a low-heel, thin-sole horse.
The terminal arch appears to be a major vascular reserve for the bone and laminae, as it can be seen filling even when the dorsal vessels and circumflex network have collapsed. Once the nutrient supply to the bone is gone, the bone no longer has a chance for survival.
DP View Points of Interest
This view helps evaluate the flow pattern over the medial and lateral aspects of the coronary plexis and the circumflex vessels dorsal to the palmar rim. Often the sound, healthy foot that toes outward will have a diminished flow medially over the coronary plexis. Likewise, if the foot toes inward, there will be a diminished flow laterally. This leads me to believe that this is a load-induced deficit.
Many sinkers will list to the medial side, which compresses the coronary supply and the circumflex zone. When they recover from the episode, a very distinctive, deprived growth pattern can be seen along the medial coronary band. In addition, the palmar rim often loses considerable bone due to pressure necrosis. This zone of necrotic bone typically becomes septic and is manifested as a full-blown abscess that migrates to the coronary groove. This focal coronary band separation is often mistaken as sloughing of the hoof.
Overall Points of Interest
Placing the tourniquet over the sesamoid is more dependable than placing it mid-cannon, as it is difficult to shut down the vascular supply that is well protected between the tendons and bone. Performing digital hyperfusion with the tourniquet placed mid-cannon has questionable efficiency simply because it is susceptible to tourniquet failure. It is a misconception that mid-cannon tourniquet placement adequately and consistently prevents blood flow to the digit. Without the advantage of contrast media to reveal what actually happens, we would have no reason to question this thought process.
In my early studies, I found that 50% of my venograms failed to be diagnostic due to mid-cannon tourniquet leak. Trainers also dislike having the tourniquet placed tightly over tendons, and justifiably so. Therefore, the mid-cannon placement has been abandoned.
It is relatively easy to determine when tourniquet leak is present, as the contrast will be visible up to the tourniquet. A properly applied tourniquet will completely preclude the arteries and veins. Radiographs will reveal 2-3cm of non-filled vascular space just below the tourniquet.
Apparently the contrast compresses the blood, pushing it to the limit of the tourniquet. A healthy foot will have the arterial supply filled retrograde. The arteries will appear as a long strand of beads. The contrast apparently affects the perineum of the vessel wall, causing spasms along the entirety of the vessels.
Reasons for Technique FailureTourniquet leakPerivascular injectionFailure to inject the contrast in a timely fashionFailure to take all necessary views in 45 secondsHorse steps off of the blocksForgetting to rock the leg to allow for total perfusion of the dorsal vesselsRemoving the catheter prior to taking radiographsInadequate nerve blockVenogram Interpretation
The range of norm is quite varied, as the vascular network is influenced by load, anatomical variations and pathology. Pathology also has a broad definition as sound horses are often found to have compromised blood flow in areas where other horses have a more prominent, well-filled network. Strong, healthy feet have unique characteristic differences, but all seem to have common anatomical, vascular similarities. Being able to identify a healthy foot requires good experience and an eye for minute details.
Hoof angles are reflections of coffin bone angles, which are influenced by breed as well as congenital and acquired foot problems. Therefore, to speak of our ideal hoof angle we must consider the bone angle, as all healthy feet will have a parallel relationship between the face of the wall and the face of PIII. The efficiency of horn growth centers also influences the mass of the heel.
The digital cushion and sensitive frog rely upon the protection of the horny, heel mass. As a complex unit, all major zones are dependent on the nutrient blood flow. The rate of wear and growth influences, and often alters, the mass of horn protection thereby challenging the integrity of the vascular supply. Trimming and shoeing also greatly influences the perfusion to the digit as the mechanics of the shoes can alter mass-load distribution and nutrient supply to vital growth centers.