Testing for Weakness
What may be causing my pet to be weak?
Weakness is a non-specific symptom that can be caused by many different diseases or conditions. Problems with the cardiovascular system are an important cause of weakness or fainting spells (called syncope) because of poor circulation or irregular heart rate or rhythm. Severe anemia from any cause will also cause lethargy or weakness. Severe hypoglycemia (low blood sugar), hypothyroidism, marked increases or decreases in serum potassium concentrations (termed hyperkalemia and hypokalemia, respectively), and neoplasia (cancer) are common causes of weakness.
"Weakness is a non-specific symptom that can be caused by many different diseases or conditions."
Spinal cord disease such as disc disease may also present as weakness, usually of the hind limbs. Musculoskeletal diseases such as arthritis may cause an apparent weakness.
More uncommon causes of weakness include pheochromocytoma (a tumor of the adrenal gland), and muscle related diseases such as myasthenia gravis.
How do we determine the cause of weakness in my pet?
A thorough history and physical examination will give us 'clues' to the underlying cause of weakness in your pet. The presence of abnormal heart or lung sounds may signal the presence of underlying cardiovascular disease. The presence of other clinical signs such as hair loss or weight gain may point to hypothyroidism as the most likely cause of weakness.
A series of screening tests will be recommended if the cause of weakness is not obvious. These screening tests include a complete blood count (CBC), a serum biochemistry profile, a urinalysis and a serum thyroxine (total T4) concentration.
What might these screening tests tell us?
The complete blood count (CBC) provides us with an evaluation of the red blood cells, the white blood cells, and the platelet components of a blood sample. A hematology (blood) analyzer will provide us with the total numbers of these cells, and evaluation of a blood smear will allow us to look at the physical characteristics of these cells.
Anemia is indicated by a decrease in red blood cell numbers, hemoglobin and packed cell volume (PCV). Since anemia is a common cause of weakness, it is important to determine whether it is present. In some cases, the underlying cause of the anemia) may be suggested by the CBC results, while in others, further testing may be required.
Marked increases in white blood cells is suggestive of underlying inflammation or infection. Because hypoglycemia (low blood sugar), one cause of weakness, may be due to an underlying infection, an evaluation of your pet's white blood cells in important.
The serum biochemistry profile provides us with an evaluation of many organ systems including the kidneys and liver. In addition to organ related enzymes, the biochemistry profile evaluates blood glucose (sugar), proteins, and electrolytes, including potassium.
"Changes in either glucose or potassium are seldom related to diet in adult animals..."
Hypoglycemia (low blood sugar) and alterations in the serum potassium concentrations are common causes of weakness. Changes in either glucose or potassium are seldom related to diet in adult animals; usually another underlying condition is to blame. Such conditions include Addison's disease or tumors with respect to hypoglycemia, and kidney disease or Addison's disease with respect to potassium. The biochemistry profile provides an evaluation of kidney function and may support Addison's disease if other electrolytes changes are noted.
A urinalysis provides us with an evaluation of the chemical and physical properties of a urine sample. A urinalysis is important in any sick animal, but is crucial for the appropriate interpretation of any changes found on the biochemistry profile, especially in the kidney related metabolites.
Serum thyroxine (total T4) concentrations should be evaluated in both cats and dogs showing clinical signs of weakness. Cats typically suffer from a condition termed hyperthyroidism in which the thyroid glands make excessive amounts of thyroid hormone. Dogs suffer from reduced thyroid hormone concentrations, a condition called hypothyroidism. Both conditions may cause weakness; in cats, this is due to the direct effect of excess thyroid hormone on the breakdown of skeletal muscle (seen as weight loss) as well as a decrease in serum potassium concentration. In dogs, the lack of hormone can affect nerves, which may be seen as weakness in the muscles that the nerves control.
What other tests might be indicated for the evaluation of weakness in my pet?
Any additional testing will depend entirely upon the combined results of the history, physical examination, and screening tests.
Suspected heart disease, disc disease, or tumors of the abdominal cavity may be investigated by imaging studies such as x-rays or ultrasound. An electrocardiogram may be performed if heart disease is of concern.
The causes of any documented anemia may require further investigation such as a Coombs' test, serum iron testing, or fecal occult blood testing.
Suspected Addison's disease will need to be confirmed by an ACTH stimulation test.
If the weakness is related to a rare condition called myasthenia gravis, then all of the screening tests may be essentially within normal range. Confirmation of this condition requires a specialized blood test that demonstrates circulating antibodies to specific muscle receptors.