Page 935 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

910 CHAPTER 6

VetBooks.ir HYPOCALCAEMIA Aetiology/pathophysiology
Ionised serum calcium concentration is regu-
lated in a narrow range by PTH; however, cer-
(See also p. 1115.)
tain events can influence ionised serum calcium
Definition/overview so rapidly that this hormonal control system can-
Hypocalcaemia is not a specific disease, but not react quickly enough. Such events can include:
rather an electrolyte abnormality that can be a sudden decrease in calcium intake; increased
caused by, or associated with a variety of condi- calcium demand (e.g. lactation); faecal, urinary
tions, including those listed in Table 6.6. Primary and/or sweat loss; or decreased calcium solubility.
hypopara thyroidism is an uncommon cause of Cantharadin toxicity (caused by ingestion of blis-
hypocalcaemia. ter beetles) can also cause hypocalcaemia. Sepsis
and endotoxaemia may cause hypocalcaemia but
the mechanism by which this occurs is unknown.
It is likely that inflammatory cytokines associated
Table 6.6 Conditions associated with
hypocalcaemia with endotoxaemia suppress PTH secretion, inter-
fere with calcium mobilisation or result in tissue
Pregnancy/lactation or GI sequestration of calcium. In humans, sepsis
• Mid gestation is associated with increased serum concentrations
• Within 2 weeks of the end of gestation
• 10–86 days after parturition of calcitonin precursors (e.g. procalcitonin) and it
• 1–2 days after weaning is thought that increased procalcitonin precipitates
Sweating (loss of fluid and electrolytes) the hypocalcaemia.
• Endurance events
• Prolonged transport, especially in heat and humidity Clinical presentation
• Hot, humid environments Clinical signs vary, depending on the severity of the
Alkalaemia problem. It is important to remember that clinical
• Found in association with K and Cl loss in sweat, signs are dependent on the level of ionised calcium,
hypokalaemia associated with anorexia, not necessarily total calcium. Since calcium is pro-
hypochloraemia with severe gastric reflux or
respiratory alkalosis caused by hyperventilation tein bound in plasma, low plasma protein concentra-
Sepsis tions will result in lower measured values for total
• Gastrointestinal upsets calcium, but ionised calcium may be within normal
• Metritis limits and true hypocalcaemia is not present. The
• Pleuropneumonia percentage of calcium in the blood that is protein
• Retained placenta bound varies with acid–base status. Acidaemia
• Increased procalcitonin, perhaps an inflammatory tends to have a protective effect by increasing the
cytokine percentage of blood calcium that is ionised while
Primary hypoparathyroidism alkalaemia makes an animal more prone to clini-
Secondary hypoparathyroidism
Hypomagnesaemia cal signs of hypocalcaemia. With a normal serum
Acute kidney injury albumin concentration, total serum calcium in the
Acute rhabdomyolysis range of 2.0–2.5 mmol/l (8–10 mg/dl) causes mild
Urea poisoning signs, including colic, synchronous diaphragmatic
Hepatitis
Blister beetle poisoning (cantharadin toxicosis) flutter or signs of hyperexcitability. Tachypnoea
Pancreatitis and tachycardia, with or without arrhythmias, may
Rapid intravenous tetracycline administration be present. Total serum calcium of 1.25–2.0 mmol/l
Corticosteroids (5–8 mg/dl) may result in tetany, incoordination,
Idiopathic (miniature horses seem to be susceptible to this stiffness of gait or goose-stepping, abnormal facial
condition)
expressions (‘sardonic grin’), elevation of the tail