3.1 Equine History, Physical Exam & Diagnostics

Equine History, Physical Exam & Diagnostics

Evaluating the equine patient requires a structured, logical approach that combines a comprehensive history, a detailed physical examination, and targeted diagnostic modalities. Due to the horse's size and athletic function, diagnostic examinations must be performed systematically to ensure both safety and accuracy.

The Equine Physical Examination

A standard equine physical examination begins with a visual assessment from a distance, noting the horse’s posture, behavior, body condition score, and respiratory pattern. Following this, hands-on clinical evaluation is performed to gather vital parameters, collectively known as temperature, pulse, and respiration (TPR).

Vital Parameters

Mucous Membranes and Perfusion

Evaluating the oral mucous membranes provides vital clues regarding hydration, tissue perfusion, and oxygenation. Normal mucous membranes are moist, light pink, and have a capillary refill time (CRT) of less than 2 seconds. Clinical variations include:

• Pale pink to white: Suggests anemia or severe vasoconstriction associated with shock.
• Congested or muddy (dark red/purple): Indicates poor tissue perfusion, venous pooling, or septic shock. A distinct purple line along the margin of the teeth, known as a toxic line, is highly suggestive of endotoxemia (e.g., due to a strangulating intestinal lesion or anterior enteritis).
• Icteric (yellow): Suggests hyperbilirubinemia, which in horses is most commonly caused by anorexia (fasting hyperbilirubinemia), hemolysis, or hepatic disease.
• Tacky or dry membranes: Indicates dehydration.

Gastrointestinal Auscultation

Gastrointestinal motility is assessed by auscultating borborygmi in four abdominal quadrants over a minimum of one minute per quadrant:

1. Right Upper Quadrant: Primarily represents the cecum. A characteristic mixing sound, the "cecal flush," should be heard 1 to 3 times per minute.
2. Right Lower Quadrant: Represents the large colon (ventral colon).
3. Left Upper Quadrant: Represents the small intestine and small colon.
4. Left Lower Quadrant: Represents the left ventral and dorsal colons, including the pelvic flexure.

Borborygmi are graded clinically as normal, hypermotile (increased frequency, often associated with spasmodic colic or early enteritis), hypomotile (decreased frequency, indicating ileus, impaction, or ischemia), or absent (indicating complete ileus or necrotic/strangulated bowel).

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Lameness Evaluation

Lameness is one of the most common reasons for veterinary evaluation in equine practice. A systematic workup includes observing the horse at a walk and trot, applying hoof testers, performing joint flexion tests, and utilizing perineural anesthesia (nerve blocks).

The AAEP Lameness Scale

The American Association of Equine Practitioners (AAEP Lameness Scale) is the industry standard for grading lameness on a scale of 0 to 5:

• Grade 0: Lameness not perceptible under any circumstances.
• Grade 1: Lameness is difficult to observe and is not consistently apparent, regardless of circumstances (e.g., under saddle, circling, inclines, hard surface).
• Grade 2: Lameness is difficult to observe at a walk or when trotting in a straight line but consistently apparent under certain circumstances (e.g., weight-carrying, circling, inclines, hard surface).
• Grade 3: Lameness is consistently observable at a trot under all circumstances.
• Grade 4: Lameness is obvious at a walk.
• Grade 5: Lameness is characterized by minimal weight-bearing in motion and/or at rest, or a complete inability to move.

Perineural Anesthesia (Diagnostic Nerve Blocks)

To localize the anatomical source of pain, veterinarians perform diagnostic nerve blocks, injecting local anesthetics (e.g., 2% mepivacaine) around specific nerves. The procedure is performed in a distal-to-proximal sequence to systematically rule out lower structures before moving higher up the limb.

1. Palmar Digital Nerve Block (PDIB / Heel Block):
   • Technique: Subcutaneous injection over the medial and lateral palmar digital nerves at the level of the collateral cartilages (palmar aspect of the pastern).
   • Desensitized Structures: Palmar one-third to one-half of the foot. This includes the solar corium, navicular bone, navicular bursa, digital cushion, distal deep digital flexor tendon (DDFT), and the palmar aspect of the distal interphalangeal (coffin) joint.
2. Abaxial Sesamoid Nerve Block:
   • Technique: Subcutaneous injection over the palmar digital nerves on the abaxial surface of the proximal sesamoid bones (base of the fetlock).
   • Desensitized Structures: The entire foot (including the toe), middle phalanx (P2), proximal interphalangeal (pastern) joint, distal sesamoidean ligaments, and the distal branches of the superficial and deep digital flexor tendons.
3. Low 4-Point (Low Palmar) Block:
   • Technique: Injections are placed around the medial and lateral palmar nerves (between the suspensory ligament and DDFT) and palmar metacarpal nerves (just distal to the buttons of the splint bones), proximal to the fetlock.
   • Desensitized Structures: The fetlock joint and all structures distal to it.
4. High 4-Point (High Palmar) Block:
   • Technique: Injections placed at the level of the proximal metacarpus (just distal to the carpus).
   • Desensitized Structures: The metacarpal region, including the suspensory ligament and the deep/superficial digital flexor tendons.

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Colic Evaluation

Colic is a clinical sign representing abdominal pain. A rapid, thorough diagnostic evaluation is essential to differentiate medical cases from surgical emergencies.

Nasogastric Intubation

Nasogastric intubation is both diagnostic and therapeutic. Because horses possess a highly developed one-way cardiac sphincter, they are physically incapable of vomiting. If fluid or gas accumulates in the stomach due to a distal obstruction (e.g., small intestinal strangulation) or ileus, the stomach can rupture.

• Technique: A lubricated tube is passed through the ventral nasal meatus, down the esophagus, and into the stomach.
• Interpretation: Fluid obtained by siphon is termed net gastric reflux. Retrieving more than 2 liters of fluid is considered abnormal. Large amounts of reflux (>4–6 L) suggest a small intestinal obstruction, strangulation, or duodenitis-jejunitis (proximal enteritis).

Rectal Palpation

Rectal palpation is a critical diagnostic step in colic evaluations, allowing direct palpation of caudal abdominal structures:

• Left Side: The spleen is palpated against the left body wall. Just dorsal and medial to it is the left kidney. The nephrosplenic space lies between the spleen and kidney; entrapment of the left colon here (nephrosplenic entrapment) is a common cause of colic. The pelvic flexure of the large colon is located in the left ventral quadrant; it is free-floating, smooth, and lacks bands (taenia) or sacculations.
• Right Side: The cecum is located in the right dorsal quadrant, secured to the body wall. It is characterized by its large size and a prominent medial band running dorsoventrally.
• Midline: The small colon is identified by its small diameter, the presence of formed fecal balls, and distinct sacculations (haustra) and bands.

Abdominocentesis (Belly Tap)

Abdominocentesis provides a sample of peritoneal fluid, reflecting the health of the intestinal walls and vasculature.

• Technique: Performed at the most dependent part of the abdomen, typically 3-5 cm to the right of the midline (to avoid the spleen) using a sterile teat cannula or needle.
• Interpretation:
  • Normal Peritoneal Fluid: Clear, straw-colored; total protein < 2.5 g/dL; nucleated cell count < 5,000 cells/µL; lactate < 2.0 mmol/L (and lower than blood lactate).
  • Serosanguinous (bloody/turbid) fluid: Indicates capillary leakage from ischemia, highly suggestive of strangulating colic.
  • Elevated Protein (>2.5 g/dL) & Cell Count (>10,000/µL): Indicates inflammation (peritonitis) or bowel wall compromise.
  • Peritoneal Lactate > Blood Lactate: Strongly correlates with intestinal strangulation and necrosis, indicating an immediate need for surgery.

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Laboratory Interpretation

Blood Lactate

Blood lactate is an excellent indicator of systemic perfusion and tissue oxygenation. Under normal aerobic metabolism, lactate is low (<1.5–2.0 mmol/L). In hypovolemic shock (e.g., severe dehydration from colic, hemorrhage), anaerobic metabolism increases, elevating systemic lactate. Serial measurements of blood lactate are used to monitor response to intravenous fluid therapy.

Fibrinogen

Fibrinogen is the primary acute-phase protein measured in equine medicine to assess systemic inflammation. Unlike in small animals where C-reactive protein rises rapidly, equine fibrinogen has a slow response time, taking 24 to 48 hours to peak after an inflammatory stimulus.

• Reference Range: Typically 100 – 400 mg/dL.
• Clinical Significance: Values exceeding 400 mg/dL indicate active, severe inflammatory processes, such as internal abscessation (e.g., strangles), peritonitis, pleuropneumonia, or enteritis. It remains elevated for several days, making it useful for tracking chronic inflammatory conditions.