Anemia

Dogs will not usually manifest weakness or exercise intolerance due to anemia unless their hematocrit has fallen acutely to less than 20%. Acute severe anemia from trauma, ruptured splenic hemangiosarcoma, bleeding intestinal lymphoma, gastric ulceration, anticoagulant rodenticide intoxication, thrombocytopenia, or acute hemolysis typically results in sudden collapse or profound weakness rather than repeated episodes of inability to exercise. Chronic anemia is better tolerated but, when severe, can result in classic signs of exercise intolerance.²⁷ Chronic anemia is most often seen with low-grade gastrointestinal or urinary bleeding, neoplasia, chronic hemolysis, or bone marrow disease. Perform a complete blood count in all dogs with exercise intolerance.

Hypoglycemia

Table 4. Causes of Hypoglycemia in Adult Retrievers

Functional islet cell tumors (insulinomas) are the most common causes of hypoglycemia in middle-aged and older dogs of all breeds.²⁹ Signs of weakness, trembling, abnormal behavior, and disorientation are initially episodic but become more frequent with time and are often worse with exercise. Blood glucose concentrations may be normal when the dog is asymptomatic.^28,29 Profound hypoglycemia is most likely to be demonstrated after a 24- or 48-hour fast or when measured 15 to 30 minutes after feeding, since a rise in blood glucose stimulates tumor insulin hypersecretion. Documenting normal or elevated serum insulin concentrations in the face of hypoglycemia supports a diagnosis of insulinoma.²⁹ Thoracic radiographs and abdominal ultrasonography are recommended to look for metastatic disease. Treatment usually consists of partial pancreatectomy to remove the tumor followed by medical management using frequent small high-protein meals, prednisone, diazoxide, and other medical therapies designed to increase blood glucose or decrease insulin production or release.²⁹

Symptomatic hypoglycemia of hunting dogs is a well-recognized but poorly characterized disorder seen in young adult, highly excited, thin, upland hunting dogs, especially pointers and spaniels.²⁸ Episodes are characterized by apparent disorientation, weakness, and occasionally generalized seizures after one to two hours of vigorous exercise. Affected dogs are usually normal by the time they are evaluated by a veterinarian, so hypoglycemia can only be documented by collecting a blood sample in the field at the time of collapse. If hypoglycemia is confirmed, affected dogs should be vigorously evaluated for hypoadrenocorticism, liver dysfunction, and neoplasia.^28,29,32 Excitement and intense exercise induce hyperglycemia in normal dogs, so there is speculation that dogs experiencing exercise-induced hypoglycemia may have a glycogen storage disorder.^28,33,34 Feeding affected dogs in the morning before a hunt, frequent feeding of protein-rich foods during the hunt, or the administration of a small amount of prednisone (2.5 to 5 mg/dog) just before the hunt may prevent symptomatic hypoglycemia.²⁸