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 an