Laminitis - Horse

Introduction

Laminits is a common and debilitating condition of horses cause by separation of the laminae of the hoof. It can be acute or chronic and can result in the horse being put down. To revise the anatomy of the hoof see Equine Phalanges - Anatomy & Physiology

Etiology

Acute degeneration of the sensitive primary and secondary Laminae. The cause for this is unknown.

Epidemiology

Increased Risk Factors

Obesity
Access to lush pastures
Grain overload (Ingestion of large quantities of soluble carbohydrates)
Retained Placenta
Colic and Diarrhoea
Systemic Illness
Animals having little exercise
Pituitary Pars Intermedia Dysfunction / Equine Cushing's
Trauma/excessive work on hard surfaces
Increased weight bearing on one limb

Pathogenesis

The separation of the sensitive laminae (originated from the third phalanx/pedal bone) from the laminae lining the inside surface of the hoof.

This allows the pedal bone to rotate within the hoof capsule and come to rest on the sole. This causes the sole to be pushed downwards and at the toe the pedal bone mat penetrate the sole.

Rotation occurs due to torque from the deep digital flexor tendon.

Also due to the weight of the animal the pedal bone can sink (displace ventrally). The bone may rotate, sink or both.

Serum accumulates in the space between the laminae and can breakdown the white line.

Mechanism of Separation of the Laminae

The mechanism is unknown but theories are:

Ischaemia and necrosis. Ischaemia may be caused by:
- Vasoconsriction
- Arterio-venous shunts
- Interstitial Oedema
- A mixture of the above

Inflammation and then degeneration of the sensitive laminae

Enzymatic digestion of laminae by Matrix Metalloproteins (MMPs)

Abnormalities in the hoof metabolism or corticosteriods (Endogenous cortisol or iatrogenic) which results in increased glucocorticoid activity.

Subsequent to separation of the laminae a pain-hypertension-vasoconstiction cycle will occur in acute cases

Pain causes:
- Release of vasoconstictors (catecholamines, angiotensin II, vasopressin)
Vasoconstriction causes a reduced blood flow to the foot and systemic hypertension.

Clinical Signs

Acute Disease

Develops Rapidly
Most commonly in the front feet
Mild or early disease can be seen as reluctance to move and frequent shifting of weight
Characteristic gait
If more severe the horse may be:
- Unwilling to move or pick up it's feet
- Show signs of pain (Anxiety, sweating, increased heart and respiration rate)
- Characteristic posture which looks like the horse is standing on its heels, all feet are placed forward of their normal position, the head is low and the back is arched
- Occasionally there may be serum like exudate at the coronary band and the hoof may become detached and shed. Although this indicated a very severe case with good and dedicated owners, vets and farriers who are prepared for the time and cost of treatment the horse can survive sloughing of the hoof and go back to normal work.

Chronic Disease

Separation of the laminae and sinking and/or rotation of the pedal bone have occurred. The sole has dropped. The hoof wall spreads which results in marked transverse ridges on the hoof. Also the angle of the hoof as viewed laterally decreases. Degeneration of the white line may allow for infection to enter. The pedal bone may become septic. The Animal may become lame with exercise and have repeated bouts of mild laminitis.

Diagnosis

Pain on palpation around the coronet
Marked withdrawal in response to hoof testers
Increased height and strength of pulse in the palmer digital artery
If the pedal bone has sunk a concavity may be palpable at the junction of the coronet

Radiography

Radiographs should be taken of front feet or all feet if all legs are affected. A metal strip should be placed on the dorsal wall of the hoof and the sole should be maked with a similar metal strip or a pin can be used. They the angle of the pedal bone in relation to the hoof can then be analysed and the radiograph should be shown to the farrier who is working with you on the case for corrective farriery. Radiographs are essential for prognosis. You should measure:

Distance between the top of the hoof wall the top of the extensor process of pedal bone
Distance between dorsal hoof wall and dorsal cortex of pedal bone

Radiographic changes:

If mild or acute may show no changes
May show rotation of the pedal bone as a tilting of the distal aspect towards the sole
Serum build up may be evident as a radiolucent line between the dorsal hoof wall and the pedal bone

Prognosis

The worse the rotation of the pedal bone the less chance of returning to athletic function or even to becoming pain free.

Differential Diagnosis

Symptoms of the following diseases can be similar but there is no pain in the feet:

Treatment

Acute laminitis is an emergency. Box rest is an important part of treatment and return to work should be very gradual.

Aims:

Removing inciting cause
Relieve pain and reduce inflammation
Dilate blood vessels in the foot
Prevent microthrombi formation
Prevent rotation or sinking of the pedal bone
Promote hoof growth

Analgesics, mainly NSAIDs (Phenylbutazone) are the main treatment. It may also help break the theorised pain-hypertension-vasoconstriction cycle. Other available NSAIDs are Flunixin Meglumine, Ketoprofen and Dimethyl Sulfoxide. Vasodilators and anticoagulants are also optional add on drugs, depending on the case.

Mechanical support is important and may provide some pain relief and help prevent rotation or sinking of the pedal bone. It can be done with polystyrene or other packing materials or keeping the animal on soft, deep bedding. Elevating the heel with a wedge or wedge shoe may be useful to take off some of the strain on the deep digital flexor tendon and help to reduce rotation. The farrier will be able to help here with the use of egg bar, heart bar and plastic shoes.

Treatment should be monitored physically and radiographically.