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Diagnostic and Imaging Techniques for the Lame Horse


2017/04/01


By Michael Caruso III, VMD, DACVS-LA

Associate Surgeon/Sports Medicine Clinician, Tennessee Equine Hospital

The Rutherford County Farmer’s Co-Op sponsored a Purina Horse Owners Workshop (HOW) at Meadowood Farm in Murfreesboro, TN on Tuesday evening, March 14, 2017. Speakers included Purina Animal Nutrition Equine Specialist Rusty Bane, Laura Wells from Purina Animal Nutrition and Rutherford Farmer’s Co-Op, and Dr. Mike Caruso, Associate Surgeon at Tennessee Equine Hospital.

Following is a transcript of Dr. Caruso’s presentation.

You observe your horse in the pasture moving awkwardly or while riding you notice that your horse is lame or not performing the way it has in the past… now what? Your veterinarian can help by localizing the source of pain or discomfort causing the lameness or gait abnormality. Your veterinarian will do this through a good physical examination and a thorough lameness evaluation that may include “nerve blocks” or “joint blocks.” Blocking the limb, or diagnostic analgesia, is performed by desensitizing large nerves in a specific region of the limb with a numbing agent, mepivacaine or lidocaine, to help localize the area from which the source of pain is originating. For example, if your horse is lame in the left front limb, blocking the medial and lateral palmar nerves at the level of the heel bulbs, and noticing that the lameness resolves, will tell your veterinarian that the source of pain causing lameness is likely in the foot. Conversely, blocking a joint and noticing that the lameness resolves, indicates the source of pain is likely within that joint.

Once the location of pain is diagnosed, most veterinarians will elect to image that area to attempt to ascertain what specific structure is injured, such as a bone/cartilage from arthritis, a tendon issue, a ligament issue, or potentially a combination of the above. The selection of imaging modality is important, as different modalities are better than others for imaging bone versus soft tissue. Radiography, or x-ray, is used to mainly image bones or joints.  Radiographs are best used for small “chip” fractures, larger fractures, bone/joint infections, OCD (osteochondritis dissecans) lesions, arthritis, and laminitis. Ultrasonography or ultrasound is mainly used to image soft tissue structures, such as the superficial or deep digital flexor tendons, suspensory ligaments, muscles, tumors/abscesses, and portions of the gastrointestinal tract.

More sophisticated and complicated imaging modalities include nuclear scintigraphy and magnetic resonance imaging (MRI). Nuclear scintigraphy, or bone scan, involves administering a radioactive drug intravenously through a catheter, waiting a period of time, and then using a gamma camera to capture the amount of radiation being emitted from the horse. The camera and special computer software takes the radiation being captured and forms a set of pictures on a computer screen. A bone scan can help a lameness clinician localize the source of inflammation and therefore, pain, causing a horse to appear lame. The radioactive drug travels to areas of the body that have increased blood flow and therefore, increased inflammation. This imaging modality is particularly helpful for lamenesses that do not “block out” or cannot be localized to a specific area; horses that are not overtly lame, but are exhibiting poor performance; horses that may be so subtlety lame that blocking is not an option; or horses that have multiple limbs that appear lame. Bone scans are also helpful for imaging areas in an adult horse that are difficult to x-ray, such as the upper portions of the limbs, the pelvis, and the spine.

MRI involves anesthetizing the horse and positioning a portion of the limb that was previously blocked out or the portion of a limb to which the lameness was isolated into a large magnet. Briefly, as the body contains mostly water (H2O), the MRI magnet interacts with hydrogen (H+) atoms/protons within tissues, aligns them in a certain orientation, and a computer software uses the orientation of the protons to produce an image based on their interaction with the magnet.

The images captured are cross-sectional images, or slices, of a selected area. Using an MRI, the lameness clinician can image multiple slices in different orientations or planes to obtain different information about a region of the body. The MRI evaluation provides profound detail that an ultrasound or x-ray cannot. MRI is the best imaging modality for injuries in the lower portion of the limb, such as the superficial and deep digital flexor tendons, suspensory ligaments, cartilage within a joint, and many of the important, but small ligaments in the pastern region and within the hoof capsule.

A thorough lameness evaluation and diagnosis utilizing one or a combination of techniques and modalities discussed in this article can give your horse the best opportunity for a long, sound life. Next month we will focus on treatment options for lesions diagnosed in the lame horse. Stay tuned!
 
 

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