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Basics ⬇

Outline

BASICS

Overview!!navigator!!

  • SDF (“thumps”) is a rhythmic contraction of the diaphragm that is synchronous with the heartbeat
  • Results from hyperexcitability of the phrenic nerve secondary to acid–base (alkalosis) and/or electrolyte (hypocalcemia, hypomagnesemia) imbalances; the electric impulse from atrial depolarization stimulates the phrenic nerve, causing diaphragmatic contractions synchronic with the heartbeat
  • Pathophysiology
    • Electrolyte and acid–base imbalances may increase phrenic nerve excitability
    • The phrenic nerve is stimulated by atrial depolarizations where it passes over the right atrium
    • Decreased calcium, potassium, and magnesium concentrations increase nerve excitability
    • Alkalosis (respiratory or metabolic) increases calcium and magnesium binding to albumin, leading to ionized hypocalcemia and hypomagnesemia
  • Systems affected—nervous, cardiac, and muscular

Signalment!!navigator!!

Any breed, age, or sex. Rare in young animals.

Signs!!navigator!!

Historical Findings

  • Prolonged exercise accompanied by intense sweating (endurance horses, Thoroughbred or Standardbred racehorses)
  • Also associated with GI disease, blister beetle toxicosis, lactation tetany, transportation, and trauma

Physical Examination Findings

  • Pathognomonic sign—a spasmodic contraction of the flank that is synchronous with the first heartbeat (atrial depolarization) and independent of the respiratory cycle
  • Flank twitching may not occur with every heartbeat
  • Strong contractions may produce a thumping noise, which has led to the name “thumps”

Causes and Risk Factors!!navigator!!

  • Electrolyte-deficient diets, diuretic use, excessive sweating (metabolic alkalosis), hyperventilation (respiratory alkalosis), GI disease, transportation, lactation tetany, and hypoparathyroidism
  • Hypocalcemia, hypomagnesemia, hypokalemia, hypochloremia, and alkalosis, either individually or in combination

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Hiccups and nonsynchronous diaphragmatic flutter have been observed. In these cases, diaphragmatic twitching is not synchronous with cardiac contraction
  • Forceful contractions of the abdominal muscles may accompany severe respiratory disease. Affected horses present with signs of respiratory disease, and abdominal contractions are associated with expiration, not a heartbeat

CBC/Biochemistry/Urinalysis!!navigator!!

  • Hypocalcemia, hypokalemia, hypochloremia, hypomagnesemia, and alkalosis (metabolic or respiratory) are common laboratory abnormalities
  • Serum ionized calcium is more accurate than total calcium to diagnose hypocalcemia
  • Serum ionized magnesium is more accurate than total magnesium to diagnose hypomagnesemia

Other Laboratory Tests!!navigator!!

Blood gas analysis for acid–base status.

Diagnostic Procedures!!navigator!!

Simultaneously record the ECG and the diaphragmatic contractions, either manually or with electromyography or phonocardiography.

Treatment ⬆ ⬇

TREATMENT

  • Correction of underlying condition and restoring electrolyte and acid–base abnormalities
  • Oral or parenteral administration of balanced electrolyte solutions
  • Hypocalcemia may be corrected by IV administration of 23% calcium gluconate solution (21.4 mg/mL calcium; add 50–100 ml/L of isotonic fluids). If SDF does not resolve, increase calcium dose or supplement fluids with MgSO4 (1–2 g/L) or KCl (10–20 mEq/L), depending on laboratory abnormalities
  • Rapid administration of calcium gluconate (e.g. 100 mL over 15–30 min) may be indicated if there are other signs of hypocalcemia (e.g. muscle fasciculations, excitability)
  • Administration of 0.9% NaCl helps correct metabolic alkalosis and hypokalemia

Medications ⬆ ⬇

MEDICATIONS

Contraindications/Possible Interactions

Administration of alkalinizing solutions (e.g. bicarbonate) may worsen clinical signs by decreasing available free calcium and free magnesium.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Monitor electrolytes, acid–base status, and clinical signs.

Prevention/Avoidance!!navigator!!

  • When large electrolyte losses are anticipated (e.g. endurance ride), administer electrolytes before, during, and after losses occur
  • Avoid chronic calcium supplementation because it reduces parathyroid hormone secretion and may impair mobilization of calcium from bone when needed

Possible Complications!!navigator!!

Electrolyte abnormalities may lead to ileus, muscle weakness, and cardiac arrhythmias.

Expected Course and Prognosis!!navigator!!

SDF is not life-threatening; in most cases; it is a transient condition that resolves either spontaneously or in response to treatment of the underlying problem. In horses with primary hypoparathyroidism SDF could last for days, is often recurrent after calcium therapy, and the prognosis for recovery is guarded.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

  • Prolonged exercise in hot, humid conditions
  • Lactation
  • GI disease

Abbreviations!!navigator!!

  • GI = gastrointestinal
  • SDF = synchronous diaphragmatic flutter

Suggested Reading ⬆ ⬇

Toribio RE. Disorders of calcium and phosphorus. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine, 4e. St. Louis, MO: Elsevier, 2018:1029–1052.

Toribio RE. Magnesium and Disease. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine, 4e. St. Louis, MO: Elsevier, 2018:1052–1058.

Toribio RE. Disorders of calcium and phosphate metabolism in horses. Vet Clin North Am Equine Pract 2011;27:129–147.

Author(s) ⬆

Author: Ramiro Toribio

Consulting Editors: Michel Levy and Heidi Banse

Acknowledgment: The author and editors acknowledge the prior contribution of Laurent Couëtil.