Clinical Application of the Digital Venogram

2007 - 18th Annual Bluegrass Laminitis Symposium Notes Written and presented January 2007 by Amy Rucker, DVM

(Watch the Digital Venogram video performed by Amy Rucker, DVM.)


Objectives Briefly stated, laminitis is the failure of attachment between the distal phalanx and the hoof wall. The classic layman's idea of a "foundered" horse is one that got into the grain in the past and now lives with extreme pain, standing in a saw-horse stance as it attempts to shift weight off the front feet, with distorted hooves growing in an "elf-shoe" curl. The reality of laminitis is that it is a complex disease that we still do not fully understand. Furthermore it manifests itself in many forms and clinical presentations. The purpose of this presentation is to explain some ideas of how laminitis develops, classify the various presentations of the disease, and discuss treatment options that I use in my practice.


Anatomy

The palmar digital vessels enter the palmar surface of P3 and join to form the terminal arch (A) within P3. The terminal arch gives off several branches which course through P3: they supply the dorsal lamellar vessels (B) which join proximally with the coronary plexus (C) and distally the circumflex vessels (D) of the sole. The circumflex vessels (D) are located a few millimeters peripheral to the rim of P3 and perfuse the sole corium via dermal papilla. The blood vessels anastomose, or join with one another; they do not perfuse an area and then terminate.


The dermis is the soft tissue containing connective tissue, vessels, and nerves. The dermal lamellae are oriented on the "face" of the distal phalanx, projecting outward towards the hoof wall. There are 550-600 primary lamellae within the foot, and each of those has 150-200 secondary lamellae projecting from its axis. These dermal lamellae interdigitate with the avascular epidermal lamellae of the hoof wall and bars, separated only by the basement membrane: a structure that unites the epidermal basal cells and the dermal connective tissue attachments to the distal phalanx.


Although it can withstand great stresses associated with galloping and jumping, the hoof anatomy is complex and even delicate. During the acute phase of laminitis, the hoof capsule epidermal laminae separate from the dermal laminae and P3 as the basement membrane is broken down. The loss of structural integrity renders the distal phalanx unstable within the hoof capsul. As the distal phalanx displaces, the dermal architecture is altered as tissue is stretched or torn. Vessels in the dorsal lamellae are compromised and may hemorrhage into the submural dermal space or be completely disrupted. There is abnormal loading of sole by the distal phalanx, initially reducing the perfusion within the sole papillae and progressing to eliminate the solar vasculature including the circumflex vessels. Coronary perfusion may be eliminated by tissue shearing or mechanical compression. Continued loading of the diseased foot by the weight-bearing limb perpetuates the mechanical damage. The sole may prolapse or bulge downward due to displacement of the distal phalanx. With "rotational" displacement of P3 away from the dorsal wall, the apex of P3 may penetrate the sole; if complete breakdown of the laminae occurs, the entire P3 will "sink" onto the sole.


Phases of Laminitis Laminitis can be initiated by multiple diseases, each causing an alteration in the normal mechanisms of an organ system. One theory is that during the developmental phase of laminitis, diseases somehow activate a laminitis trigger factor which arrives in the feet via the circulation. This factor activates enzymes which destroy the basement membrane and break down the attachments of the basal cells of the epidermis, beginning the destruction of the hoof architecture. (see Chris Pollitt's website.) The developmental phase lasts 24 to 30 hours and although pathology is occurring, these changes might not be evident to an owner.

The acute phase begins when the feet change clinically. Elevated digital pulses, foot pain and lameness may be noted. If the horse is lucky, it enters a sub-acute phase, with milder signs and minimal damage to the hoof architecture. The foot may remain stable, and the horse fully recovers.


Unfortunately most horses have changes within the hoof capsule, and enter the chronic phase. The chronic phase lasts an indefinite time period, and has many manifestations. During the early chronic phase, some horses are able to recover and become compensated with the amount of damage to their foot. The distal phalanx may be stable, the wall and sole are able to grow appropriately and the horse becomes sound. Other horses are not able to compensate, have a persistent level of pain associated with bone disease or continued loss of integrity of the dermal and epidermal structures. Many factors determine the course of the chronic phase of laminitis.


Key Points for Horse Owners: When your horse shows clinical signs of laminitis, damage to the hoof architecture has already occurred. How you respond to these signs will help determine the course of the disease. 1. Limit the movement of the horse!!! Ignore old thoughts of walking the horse to "increase the circulation." Movement will further damage the architecture of the hoof. (Pollitt) 2. Research suggests that standing a horse in an ice water bath up to its knees and hocks will reduce blood flow to the feet, and perhaps prevent the trigger factors from reaching the feet. Standing a horse in an ice bath PRIOR to the development of lameness may PREVENT laminitis from occurring. (See Pollitt's website.) Standing a horse in icy water AFTER the development of lameness reduces blood flow to tissues which need to repair and may be contraindicated. 3. Support the foot! Styrofoam insulation cut to fit the foot will help support the sole surface until help arrives. (See Ovnicek's website.) 4. Call your veterinarian and farrier. The successful outcome of a laminitis case requires a team effort be made by three individuals: the veterinarian, farrier and horse owner. If any one of the members of this team is not willing to give a 100% effort, success will be compromised.


Initial Medical Treatment Laminitis is a medical emergency, and should be treated as such. "Waiting to see" how the horse does over time only decreases you chances of having a functional horse. Aggressive, immediate treatment should be aimed at reducing inflammation and pain via anti-inflamatories such as Phenylbutazone, Flunixin Meglumine (Banamine), or DMSO. Depending on the case, vasodilators and other drugs may be used in an attempt to increase blood flow to the feet. Most importantly, an effort is made to identify and treat the initiating cause of the laminitis.


Part 2: Assessing the Laminitic Horse


Physical Exam A physical exam should begin with assessment of the demeanor of the horse. A level of comfort or pain should be established. Note the stance of the horse, degree of lameness, comparison of lameness between limbs, and conformation of body, legs and feet. Finally, the owner can provide vital information concerning the use, environment, feed, and care regime of the horse.


The digital pulses may be increased if the horse is painful or the foot inflamed. Increases in wall temperature may be noted. Distortion of the coronary band, hoof wall, frog, bars and sole is noted. If the sole appears thick, some may GENTLY place hoof testers on the sole to ascertain areas of sensitivity. Alternatively, the sole may be flat or prolapsed due to displacement. Separations or draining tracts may be evident. With chronic laminitis the white line may be widened at the toe due to long term changes within the laminae. Growth rings of the wall may be close together at the dorsal hoof wall, but diverge at the heels as the rate of heel growth exceeds toe growth.


Compare all the feet on the horse. Laminitic horses typically have one foot that is most painful on a given day. Be considerate of the horse. Don't attempt hoof testers if the animal is obviously painful. Don't expect the horse to stand with a foot up for long periods of time. QUICKLY perform your examination, diagnostics and therapeutics!


Radiographs It is essential that you perform quality radiographs, and consistently use the same techniques so that you may compare films!!! If proper techniques are not used, information obtained from radiographs (x-rays) may be distorted, leading to incorrect conclusions. Again, information gained from radiographs allows assessment of the location of the distal phalanx within the hoof capsul, and precise measurements allow a "blueprint" to be developed regarding treatment strategies to support the foot. Limitations of radiographs include a poor correlation between the amount of displacement of the distal phalanx and the eventual outcome of the case. For example, a horse may have only subtle or no rotation, yet the blood supply to the laminae and circumflex vessels is completely occluded.


Another limitation of single radiographs is the inability to assess changes in paramet ers of the foot with chronic laminitis. As the hoof tries to repair by epidermal proliferation within the dorsal laminae, the CE and HL zone numbers increase. It is sometimes difficult to distinguish between acute distal displacement of the phalanx (sinking) and a chronic "healing" by tissue proliferation/remodeling.


The Digital Venogram


One method of better assessing the internal changes and load within the foot is the digital venogram. The venogram is a radiograph that is taken with contrast dye in the blood vessels. The amount and patterns of displacement of the contrast suggests where the foot is heavily loaded, where architecture has collapsed, or areas of vascular impairment. Patterns are also identified as typical of acute or chronic changes, which helps determine the state of disease. Also, serial venograms allow assessment of progress within a case, or the lack thereof.