Testing for Weakness

By Malcolm Weir, DVM, MSc, MPH; Ruotsalo, DVM, DVSc, Dip ACVP & Margo S. Tant BSc, DVM, DVSc

How can I tell if my pet is feeling weak?

Pets that feel weak often have difficulty getting on their feet and move slowly or unsteadily. Other signs include shaky muscles, fainting, or collapse. You may find your pet does not want to exercise, seems dull, and does not respond when you call. Weakness may be constant or intermittent. Intermittent weakness is sometimes called episodic weakness because your pet may show only episodes of weakness and is fine otherwise.

What might be causing my pet’s weakness?

Weakness can be caused by many different problems and can involve a variety of body systems. Some common causes of weakness include:

  • Heart disease, such as heart failure, poor circulation, irregular heartbeat  
  • Anemia, where blood is ‘thin’ or low in iron due to many causes
  • Metabolic diseases, such as low blood sugar (hypoglycemia), low calcium (hypocalcemia), high potassium (hyperkalemia), and low potassium (hypokalemia)
  • Hormonal diseases, such as hypoparathyroidism (failure of the parathyroid gland), hypothyroidism (underactive thyroid gland in dogs), hyperthyroidism (overactive thyroid gland in cats), hypoadrenocorticism (adrenal gland failure called Addison’s disease)
  • Bone/muscle disorders, including arthritis and other joint problems
  • Nerve/muscle disorders, such as spinal cord disease, nerve damage, myasthenia gravis (a rare autoimmune disorder), and seizures
  • Cancer

How will my veterinarian determine the cause of my pet’s weakness?

Finding the cause of your pet’s weakness starts with a complete history and physical examination. Your pet’s history is the information you give the veterinarian about your pet’s illness. In a pet showing weakness, this would include details about how long the signs have been present and whether the weakness is continuous or intermittent. You should report any changes you have noticed in your pet’s activity, attitude, appetite, water intake, bowel habits, and urination. It is also important to tell your veterinarian if your pet is taking medications and if you have seen your pet eat anything unusual.

"You should report any changes you have noticed in your pet’s activity, attitude, appetite, water intake, bowel habits, and urination."

A physical examination involves looking at all parts of the body and typically includes listening to the heart and lungs with a stethoscope and palpating the abdomen (gently squeezing or prodding the abdomen with the fingertips). A complete physical examination may give clues about the cause of weakness. For example, abnormal heart sounds could signal underlying heart disease; very pale gums could be a sign of bleeding or severe organ failure; an abdominal mass could signal a tumor; hair loss could be a sign of hypothyroidism in a dog.

History and the physical examination are very important, but further testing will likely be necessary, and your veterinarian may recommend doing screening tests. These are a series of simple tests that provide information about your pet’s overall health and give clues about the underlying problem. The most commonly recommended screening tests for weakness include complete blood count (CBC), serum biochemistry profile, urinalysis, and in a dog, total thyroxine (T4). Additional tests may be recommended based on the results of these screening tests.

What can these screening tests tell us?

(a) Complete Blood Count (CBC): This simple blood test provides information about the different cell types in blood. These include red blood cells, which carry oxygen to the tissues; white blood cells, which fight infection and respond to inflammation; and platelets, which help the blood clot. The CBC provides details about the number, size, and shape of the various cell types and identifies abnormal cells in circulation.

In a pet with signs of weakness, changes seen on a CBC could include:

  • Anemia means the number of red blood cells and the amount of hemoglobin in the blood are lower than normal. Anemia is a common cause of weakness and is seen with various disorders, including bleeding, longstanding disease, hypothyroidism (dogs), parasitism, poor nutrition, or cancer.
  • A high number of white blood cells with an increased proportion of immature white blood cells is usually a sign of severe underlying infection and could explain weakness.
  • High numbers of lymphocytes and eosinophils (types of white blood cells) could indicate Addison’s disease (adrenal gland failure), which is associated with episodic weakness.
  • Abnormal cells are often a sign of bone marrow disease or bone marrow cancer (leukemia).

(b) Serum biochemistry refers to the chemical analysis of serum (the pale yellow liquid part of blood that remains after the cells and clotting factors are removed). The serum contains many substances, including enzymes, proteins, lipids (fats), glucose (sugar), hormones, electrolytes (salts and minerals in the blood), and metabolic waste products. Testing for these substances provides information about the health of various organs and tissues in the body, as well as the metabolic state of the animal.

In a pet with signs of weakness, a serum biochemistry profile might show changes such as:

  • Elevated liver or kidney values could indicate organ damage or failure. 
  • Low glucose is a cause of weakness that could be related to liver disease, some types of cancer, severe infections, and overmedication with insulin.
  • Low calcium can cause weakness related to dietary or hormonal problems.
  • Altered electrolytes, especially sodium and potassium, are often seen with Addison's disease and kidney disease.

(c) Urinalysis is a routine test that analyzes urine's physical and chemical composition. It measures how well the kidneys are working, identifies inflammation and infection in the urinary system, and helps detect diabetes and other metabolic disturbances. Urinalysis is essential for adequately interpreting the serum biochemistry profile and should be done at the same time as blood testing.

"Urinalysis is essential for adequately interpreting the serum biochemistry profile and should be done at the same time as blood testing."

In a weak pet, the urinalysis might show changes such as:

  • Low urine specific gravity (pale watery urine) with elevated kidney values on the serum biochemistry indicates kidney disease.
  • High numbers of red or white blood cells indicate inflammation or infection in the urinary system and can be associated with cancer.
  • Bacteria are associated with bladder infections, which can cause signs of weakness in older animals.

(d) Serum thyroxine (total T4) is a hormone produced by the thyroid gland. The level of T4 in the blood should be evaluated in cats and dogs showing signs of weakness.

In cats, weakness may develop due to hyperthyroidism, in which the thyroid gland makes too much thyroid hormone. This leads to the breakdown of skeletal muscle (seen as weight loss) and a drop in blood potassium concentration, causing weakness. A diagnosis of hyperthyroidism can often be made with a single blood sample.

In dogs, weakness may develop due to hypothyroidism, in which the thyroid gland does not produce enough thyroid hormone. Weakness develops due to anemia, low metabolic rate, and poor muscle tone. If the total T4 is low, hypothyroidism may be present and additional testing is recommended to confirm the diagnosis.

"If the total T4 is low, hypothyroidism may be present and additional testing is recommended to confirm the diagnosis."

What additional tests might be recommended for my pet showing weakness?

Depending on the results of your pet’s history, physical examination, and screening tests, many additional tests might be recommended, including:

  • Additional testing to assess kidney or liver function if liver or kidney values are abnormal
  • Imaging: radiographs (X-rays) and ultrasound to diagnose heart and lung disease, spinal cord disease, and tumors in the chest or abdomen
  • Heart disease testing, such as an electrocardiogram, echocardiogram, and blood tests to measure heart muscle damage
  • Tests for anemia, such as Coomb’s test, serum iron, fecal occult blood, parasite tests, and bone marrow aspiration
  • Parathyroid hormone (PTH) testing if your pet has low blood calcium levels (parathyroid hormone controls the level of calcium in the blood)
  • Joint fluid analysis in pets where joint disease may be contributing to weakness
  • ACTH stimulation test if Addison’s disease is suspected
  • Other specialized testing for rare disorders, such as myasthenia gravis (an immune-mediated disorder of the nerves) and pheochromocytoma (adrenal cancer)
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