Primary hypoparathyroidism in dogs and cats: Physiology, clinical signs, and initial diagnostic tests - Veterinary Medicine
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Primary hypoparathyroidism in dogs and cats: Physiology, clinical signs, and initial diagnostic tests
To understand this rare condition, you must be familiar with the physiology of calcium regulation. The net effect of a low PTH concentration is hypocalcemia, and the severity of the clinical signs and the timing of their onset are a reflection of the duration and magnitude of the hypocalcemia.


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Clinical signs

The clinical signs of hypocalcemia are identical, regardless of the cause.5 Clinical signs of hypocalcemia may be nonspecific, such as anorexia, nervousness, stilted gait, panting, ataxia, episodic weakness, vomiting, diarrhea, and weight loss.1,18 In a case series of 37 dogs with naturally occurring primary hypoparathyroidism, the most commonly noted abnormalities at presentation (in descending order of incidence) included fever, tense or splinted abdomen, stiff gait, thinness, generalized muscle fasciculations, growling, tachyarrhythmias, muffled heart sounds, weak pulses, and cataracts.1 Although these dogs were persistently hypocalcemic, signs of tetany were intermittent.1 In cats, anorexia and lethargy are more common signs, but vomiting has also been reported.2,4,5 Raised nictitating membranes and ear twitching may be observed in cats.2,4

Clinical signs may be present from one day to two years before presentation.1,2 Animals presenting for seizures may be misdiagnosed if a pretreatment biochemical profile is not performed.1,2 The owners may perceive that the clinical signs appeared acutely, but the disease probably had been present for a long time.1

Neuromuscular and neurologic signs

Neuromuscular and neurologic signs usually predominate in dogs and cats with primary hypoparathyroidism. Patients usually present for an acute onset of tetany or seizures. In retrospect, owners often identify more subtle signs of muscle stiffness.1-3,10,18 On occasion, the tetany may be latent, and excitement or activity increases neuromuscular activity, inducing tetany.1,2,10 Neuromuscular signs are intermittent and include mild muscle tremors and fasciculations, muscle cramping, and stilted gait and may progress to neurologic signs, such as generalized seizures.1,2,4,6,8,10 Seizures are often preceded by muscle tremors in one limb that gradually spread throughout the body and culminate in a seizure.1,10 Seizure activity is atypical in that dogs remain conscious and seizure activity usually ceases without intervention.2 Secondary to prolonged seizure activity, it is not unusual for the body temperature to exceed 105 F (40.6 C).2 In the same case series of 37 dogs, tetany was noted on presentation in nearly 50% of dogs, and seizure activity was directly observed in 86% of the dogs with hypoparathyroidism during their initial hospitalization.1 In cats, induction of seizure activity with excitement has not been observed.5

Other neuromuscular effects include disorientation, chewing and licking at the paws, and behavioral changes.1,4-6,10 Facial rubbing, which can be dramatic and lead to self-mutilation, is a common observation.1 In the case series of the 37 dogs, 23 of the dogs exhibited facial rubbing. People with hypoparathyroidism report a paresthesia or tingling in the lips and distal extremities, possibly from hyperexcitability of sensory neurons.6 The facial rubbing seen in dogs may be due to a similar phenomenon.6 Another possible explanation for facial rubbing is pain caused by cramping of the masseter or temporal muscles.1,2,18 Behavioral changes associated with hypocalcemia include nervousness, reluctance to be touched, decreased activity level, and aggressive behavior. Gastrointestinal signs may be secondary to smooth muscle spasms due to autonomic nerve hyperexcitability.6 The neuromuscular signs should resolve with treatment of the hypocalcemia.1,2

Cardiac manifestations

In addition to the neuromuscular effects of hypocalcemia, there may be cardiac manifestations. Calcium normally acts as a positive inotrope and as a mild positive chronotrope.1 With decreased calcium concentrations, the duration of the action potential in cardiac cells is prolonged and may result in decreased myocardial function and bradycardia.1,2,4 In people, hypocalcemia may result in 2:1 heart block.1


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Source: VETERINARY MEDICINE,
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