Bluegrass Laminitis Symposium Notes
Interpreting Venograms: Normal or Abnormal And Artifacts That May Be Misinterpreted.
Written and presented January 2004 by Amy Rucker, DVM
(Watch the Digital Venogram video performed by Amy Rucker, DVM.)
Objective Becoming familiar with the range of normal venograms is difficult, but it is more difficult to become accustomed to artifacts that are induced by incorrect techniques. This presentation will attempt to discuss interpreting changes present on venograms. We will also review common findings of the pathologic foot.
Consistency in procedure is the key to quality venograms! Changes in procedure
usually result in artifacts or poor quality venograms. If you do every venogram the same way every time, you will gain a large amount of information. More importantly, following a consistent procedure allows you to compare venograms in order to assess progress or lack of progress resulting from your treatment.
Normal Variations Because heel perfusion results from multiple sources and is palmar in origin, pathologic conditions of the foot seldom affect the appearance of heel vasculature. However, lateral radiographs will show compromised blood flow to a normal heel loaded by wedge pads. Raising the heel ten degrees significantly reduces the vascular fill in the palmar vessels and compresses the frog and sole papillae. The circumflex vessels, dorsal lamellae, coronary plexus, and terminal arch appear normal when a normal foot is elevated with heel wedge pads.
Medial/lateral imbalance of the foot is apparent on the DP view. Horses with a jammed (proximally displaced) medial coronary band often have reduced fill on the medial coronary plexus when compared to the lateral plexus. This effect is exaggerated when a foot is wedged medially or laterally.
Papillae extending into the solar corium are often not evident on a thin-soled horse.
Although this horse may not be lame, the lack of sole depth and blood supply is far from ideal. The same phenomena is occasionally seen when a foot is bruised or compressed internally; Saddlebreds or Tennessee Walking Horses may have inches of sole and pad between P3 and the ground surface yet still be sore footed.
Artifacts Perivascular contrast medium is the most common artifact we induce. Contrast can escape into the extravascular space if the vein is punctured multiple times during catheterization, the horse moves during the process, or the catheter comes out of the vein. A large pool of contrast media is evident on both the lateral and DP views at the level of the catheter in the palmar digital vein. The amount of leaked contrast must be taken into consideration as it may result in inadequate fill of the foot.
Inadequate volume of contrast medium results from perivascular leakage, syringes coming loose from catheters, loosened catheter clamps or incorrect calculations in volume needed for the venogram. A four-inch foot will fill with 20ml of contrast; 5.5 inch feet require 24ml. A seven-inch foot will hold 40ml of contrast. Inadequate volume can be confused with poor perfusion. A characteristic narrowing of the blood vessels and lack of perfusion of the heel indicate that volume is the problem. Vessels have a "tree limb" appearance, becoming narrow in the distal aspect of the foot.
A loose tourniquet will also cause an inadequate volume appearance. Radiographs will reveal contrast proximal to the tourniquet. Keys to placing a tourniquet include protecting the skin at the fetlock with Elastikon®, yet not using too much Elastikon® to pad the vessels beneath the tourniquet.
Excess time to inject the contrast media and take radiographs will result in contrast media "leaking" into the corium and dermis. A fuzzy appearance at the margins of the circumflex vessels and dorsal laminae results if it takes more than 30 seconds to radiograph the foot. This artifact can be confused with leakage into chronic scar tissue. It is imperative that you time your venogram to distinguish between the two.
A fully loaded foot with a long toe may not have fill in the dorsal laminae if the knee is not bumped to disengage the deep flexor tendon while injecting the second syringe of contrast. This artifact is difficult to induce in the healthy foot, but easy to induce in the laminitic foot.
Venograms of the Pathologic Foot Reduction in vessel fill on venograms results from various conditions: I believe compression of vasculature by mechanical forces is the most important cause. When you review the vascular supply of the foot, it is apparent that all areas receive perfusion from at least two sources. However, rotation of the coffin bone within the hoof capsule will routinely cause compression of the circumflex vessels as the palmar surface of P3 moves distally. If the rotation is more significant, the margin of P3 moves past the circumflex vessels, compressing them between the wall and dorsal surface of P3. The coronary plexus will also be compressed by the extensor process dorsally and the ungual cartilages medially and laterally. Decreased fill in the dorsal laminae may be evident. The solar venous plexus may also be mechanically compressed, however the heels rarely appear affected unless the entire coffin bone sinks distally. When the coffin bone sinks, perfusion is reduced by mechanical compression, mechanical tearing of the tissue and vasculature, increased interstitial fluid pressure and edema, and vascular stasis. Increased sympathetic tone, vascular spasm, and arterial embolization will also reduce fill on a venogram.
The Clubfoot Acute "Clubfeet" caused by deep digital flexural contracture may not have associated abnormalities of perfusion. If the condition were untreated, decreased perfusion at the tip of P3 would result.
Clubfeet that develop over time have a dished dorsal wall and remodeling or lysis of the tip of P3. The dorsal laminae at the distal aspect of P3 will have reduced fill, as will the circumflex vessels.
Acute Laminitis with Mild Rotation of P3 Initially you will see mild compression of the circumflex vessels, or rotation of the tip of P3 past the circumflex vessels. This is a common presentation among the chronic mild grass founders of Missouri. These horses often respond well to conservative treatment, only to repeatedly have flare-ups, because the initial damage never healed. If changes are mild enough, the circumflex vessels will remodel around the apex of P3 as the laminitis becomes chronic.
Acute Laminitis With Moderate Rotation of P3 Compression may be evident in the circumflex vessels and the coronary plexus with reduction in fill of the dorsal laminae. If the coffin bone rotates quickly, a space is created between the dorsal wall and the dorsal aspect of P3, where the laminae are torn from the epidermis. The space is wedge-shaped, narrow at the top, and does not penetrate the sole. Contrast media will pool in this space.
Chronic Laminitis With Rotation of P3 There are many variations of chronic laminitis. Remodeling of P3 may include mild lipping, or lysis of the tip until the bone is eroded through the terminal arch. Vascular changes also vary from mild to severe depending on the case. Characteristic of chronic scar tissue is a "feathering" appearance of contrast media into the dorsal lamellar scar tissue. The feathering occurs in the same area as the space seen with acute rotation, however the perimeters of the area are not defined and the contrast appears to feather into the tissue instead of pooling into the space. The circumflex vessels may be flipped up proximal to P3, or may attempt to remodel around the tip of P3. The dermal papillae may be irregular in orientation at the tip of P3, and may be exaggerated at the coronet where the dorsal hoof wall is thickened as the capsule is distorted.
Rotation of P3 with Penetration of the Sole Any of the above mentioned changes might be evident. In addition, no vasculature is present distal to the tip of P3. When performing venograms on this foot, it is common to have a sticky, serous fluid leak from the penetrated area onto the radiograph block.
Sinkers Sinkers may be difficult to identify. On lateral soft tissue radiographs, evaluate the distance of the extensor process from the level of the coronary band; compare front and hind feet. A foot that is sinking will have a Horn-Lamellar zone that is 20mm or greater, and the sole depth will be less than that of the other feet. A "halo" may be evident at the coronary band as the proximal and distal borders of the coronet become apparent. The DP view may reveal a foot that is sinking uniformly, or listing to one side.
Venograms must be taken at light techniques to catch all the detail of the compressed vessels. The coffin bone of a sinker has fallen past the circumflex vessels, which are trapped at the periphery of the coffin bone and may not be evident. The palmar angle may approach zero, and the solar vessels will be crushed. The dorsal lamellar vessels will be compromised, and a rectangular pooling may be evident. The coronary plexus at the extensor process is not perfused, and may have reduced fill on the DP view at the medial and lateral aspects of the coronary band. Mild sinkers will have reduced distal perfusion on the DP view, but proximal compromise does not occur until several weeks of loading the coronet has compressed these vessels. Severe sinkers have reduced perfusion in the terminal arch and even the heel.
Unstable Feet Previously I would take four views with my venograms: a standing lateral, elevated heel lateral, elevated heel DP, and a standing DP. If a foot is unstable, movement of P3 within the hoof capsule is evident: the HL zone may change, and a reduction in perfusion of the dorsal laminae may be evident. Most cases of laminitis have an increase in perfusion of the circumflex vessels and the solar and frog corium when the heel is elevated. Redden cautions against standing an acute laminitic, unstable foot flat on the ground, even for the few minutes it takes to perform radiographs or venograms. During that time the horse usually appears uncomfortable; if the horse is blocked and then stood flat, the horse is usually very lame when the block wears off. Enlightened podiatrists disengage the deep flexor tendon during the initial exam, and never let it pull at the laminae until the foot has healed. I now routinely radiograph horses in Modified Ultimate shoes, or remove their platinos and place them in the Modified Ultimates for radiographs and venograms. Note that the Ultimates will displace the radiograph cassette slightly from the hoof wall, and cause a small amount of magnification. Consider this when comparing previous radiographs without the Ultimates.
If you have a laminitic horse that has recovered nicely and you are unsure if the feet have healed, you may try comparing flat and elevated heel venograms. Do not try this unless the horse has been sound for several weeks and all radiographic parameters have returned to normal. The flat venogram should be comparable to the elevated heel venogram with the only exception being a decrease in fill in the digital cushion area. Any other changes would indicate instability in the foot.
Information in these presentations has been gathered from the "Valiant Project", which has received generous support from the following: Kim Abernathy, Betsy Arthur, Sarah Bailey, Shannon Baker, Mike Balke, Stacie Boes, Josh Bolte, Manda Boos, Kristin Campbell, Bobby Colley, Nick Coston, Kyle Creech, David Cross, Kellie Daly, Jenn Demko, Treena DeVault, Chris Downs, Elaine Dziuban, Tom Goss, Kelly Grabeel, Marcy Hammerle, Kevin Hatten, Brad Hill, Mark Hope, Laura Koenigsdorf, Raelynn Kemp, John Kreeger, Therese Kreutzberg, Kurt Kreutzer, Alison LaCarrubba, Jimmy Lattimer, Dean Morgan, Mark Mosbacher, Chris Nord, Shannon Reed, Margret Rogers, Beth Smith, Lisa Stephens, Keith Taraba, and David Wilson. Thank you!
References
Lyle, B.E. The Digital Venogram: Interpretation and Suggested Implications for Therapy in the Laminitic Horse. Proceedings Bluegrass Laminitis Symposium 2001.
Redden, R.F. Classifying Laminitic Damage: How Using a Simple Scale Can Help All Concerned Understand and Project the Aggressiveness Needed, the Length, Cost and Future Outcome for What Lies Ahead. Proceedings Bluegrass Laminitis Symposium 2001.
Redden, R.F. Equine Podiatry 101 Short Course. 2001.