THE DIAGNOSIS OF SAID

SAID can be diagnosed at an early stage before horses experience the scourge of persistent coughing and labored breathing, and worse, permanent lung scarring. At this early stage, no single test is perfect for diagnosis, so tests are employed together.  

In many cases, physical examination (lung auscultation, etc), endoscopy, and chest X-rays are normal in horses with early SAID, yet the horse isn't peforming well. That's why more sensitive tests are necessary, such as bronchoalveolar lavage (BAL) and lung function tests. More complete descriptions of BAL and lung function tests can be found in this website.

Diagnosis of SAID in the context of the 'poor performance' syndrome:

SAID can be diagnosed during the evaluation of horses for poor performance. In the workup of such a vague complaint, a number of tests can be used, but only a few help with the diagnosis of SAID.  BAL and lung function testing, including a histamine challenge are the most sensitive measures currently available for diagnosing SAID.

• Endoscopy:  SAID horses may have mucus flecks or blobs in the trachea, especially obvious after exercise. However, not all horses with mucus have SAID.
• Endoscopy during exercise: SAID horses fade early during intense exercise on the treadmill.
• X-rays of the chest:  SAID horses may have a bronchointerstitial pattern. In many cases X-rays are normal or indistinguishable from normal.
• Lung function tests: Baseline lung function tests are often normal, but one can see increased lung resistance, or frequency dependence of resistance (see Lung Function Testing).
• Histamine Challenge:  Airways are hyper-reactive to histamine when administered as an aerosol. The reason for this is unclear, but persistent inflammation and airway wall thickening are likely causes. Below is shown a horse with a facemask, receiving histamine by nebulizer, during a test of airway reacivity. See Lung Function Testing.

Histamine nebulization during test for airway reactivity

• Bronchoalveolar lavage: Saline pushed into the lung is recovered by suction, and the cells within this fluid are evaluated: increased numbers of mast cells, neutrophils, or eosinophils (rarely) are seen in SAID.
• Exercise testing (metabolic stress test): This is a test of aerobic capacity. VO₂max (maximum oxygen consumption), blood lactates, heart rate response, and distance & speed traveled before fatigue are measured. Exercise testing can tell you if your horse is in shape and ready for performance.

• Muscle enzymes:  elevations in creatine phosphokinase (CK) or aspartate transaminase (AST) point towards muscle damage, a common cause of stiffness, particularly in young fillies. CK is not specifically elevated in horses with SAID.
• Nuclear medicine scan:  For diagnosis of early orthopedic problems (bone remodeling disease, stress fractures) or muscle tears as a cause of lameness. Used in conjunction with a standard ‘in-hand’ lameness exam and X-rays. Not a specific diagnostic tool for SAID.

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