Beth P. Partington

Effects of Long-Term Phenobarbital Treatment on the Liver in Dogs

Long‐term administration of phenobarbital has been reported to cause hepatic injury in dogs. Phenobarbital induces hepatic enzymes, and it may be difficult to distinguish the effect of enzyme induction on serum liver enzyme activities from actual hepatic damage. The hepatotoxicity of phenobarbital and the impact of enzyme induction on serum liver enzyme activity were investigated prospectively in 12 normal dogs. Phenobarbital was administered for 29 weeks at 5 mg per kilogram of body weight (range, 4.8— 6.6 mg/kg) PO q12h, resulting in therapeutic serum phenobarbital concentrations (20–40 μg/mL). Serum alkaline phosphatase (ALP), alanine transaminase (ALT), aspartate transaminase (AST), γ‐glutamyltransferase (GGT), fasted bile acids (fBA), total bilirubin, and albumin were determined before and during treatment. Lateral abdominal radiographs, abdominal ultrasounds, and histopathologic examinations of liver tissue obtained by ultrasound‐guided biopsy were performed before and during treatment. Radiographs revealed a moderate increase in liver size in most dogs. Ultrasonographic examination revealed no change in liver echogenicity or architecture. No evidence of morphologic liver damage was observed histopathologically. ALP and ALT increased significantly (P < .05), GGT increased transiently, and albumin decreased transiently during the study. There were no significant changes in AST, bilirubin, and fBA. These results suggest that increases in serum ALP, ALT, and GGT may reflect enzyme induction rather than hepatic injury during phenobarbital treatment in dogs. Serum AST, fBA, and bilirubin, and ultrasonographic evaluation of the liver are not affected by the enzyme‐inducing effect of phenobarbital and can therefore be helpful to assess liver disease in dogs treated with the drug.

Effects of long-term phenobarbital treatment on the thyroid and adrenal axis and adrenal function tests in dogs.

Phenobarbital can interfere with the thyroid axis in human beings and rats by accelerating hepatic thyroxine metabolism because of enzyme induction. In human beings, it also can interfere with the low-dose dexamethasone suppression test (LDDST) used to assess adrenal function by accelerating dexamethasone metabolism. This effect can cause a lack of suppression of pituitary ACTH and subsequent adrenal cortisol release after dexamethasone administration. The effects of phenobarbital on the thyroid axis, the adrenal axis, and adrenal function tests were prospectively investigated in 12 normal, adult dogs. Phenobarbital was administered at 5 mg per kilogram of body weight (range, 4.8-6.6 mg/kg) PO q12h for 29 weeks, resulting in therapeutic serum concentrations (20-40 microg/mL). Serum total thyroxine (TT4), free thyroxine (FT4) by equilibrium dialysis, total triiodothyronine (TT3), thyrotropin (TSH), and cholesterol were determined before and during phenobarbital treatment. LDDST, ACTH stimulation tests, and ultrasonographic evaluation of the adrenal glands were performed before and during treatment. TT4 and FT4 decreased significantly (P < or = .05), TT3 had minimal fluctuation, TSH had only a delayed compensatory increase, and cholesterol increased during phenobarbital treatment. The delayed increase in TSH, despite persistent hypothyroxinemia, suggests that accelerated hepatic thyroxine elimination may not be the only effect of phenobarbital on the thyroid axis. There was no significant effect of phenobarbital on either of the adrenal function tests. With the methods employed, we did not find any effects of the drug on the hormonal equilibrium of the adrenal axis.

Hepatic Imaging with Radiology and Ultrasound

Radiographically, the diseased liver may change in size, shape, position, or opacity. Contrast studies such as peritoneography, cholecystography, portography, and arteriography may be performed to increase the specificity of the radiographic diagnosis. Ultrasound can be used to detect the changes in liver echogenicity associated with disease, identify focal verses diffuse disease processes, detect vascular and biliary abnormalities noninvasively, and direct needle aspirates and biopsies for culture, cytology, and histopathology.

TRAUMA-INDUCED ANEURYSMAL BONE CYSTS IN TWO PSITTACINE SPECIES (CACATUA ALBA AND NYMPHICUS HOLLANDICUS)

Abstract An umbrella cockatoo (Cacatua alba) and two cockatiels (Nymphicus hollandicus) were presented with rapidly enlarging masses of the head or wing joints. Historic trauma to these areas was confirmed in two cases. All birds were >2 yr of age, and two were female. Cytologic examination of fluid aspirated from masses in two cases was described as serosanguineous cytologically but failed to reveal inflammation, neoplasia, or microorganisms. Radiographic evaluation of these masses included proliferation and lysis of bone, suggestive of a neoplastic process. Histopathologic examination of surgically excised tissues revealed proliferative new bone and an absence of neoplastic tissue in all cases, consistent with aneurysmal bone cyst formation. Despite the guarded prognosis reported for other companion animals, these case results suggest a good prognosis for aneurysmal bone cyst in psittaciformes. Complete surgical excision and histopathologic examination is recommended for definitive diagnoses of aneurysmal bone cyst. Postoperative bandaging and rational antibiotic use are indicated to prevent excessive motion and secondary infection of affected sites, respectively.

A Block Vertebra in an Equine Patient

Summary A yearling Standardbred colt was presented to The Ohio State University Teaching Hospital with a primary complaint of ataxia. The diagnostic work-up identified spinal canal thinning at C4–C5 as well as a clinically insignificant block vertebra at C2–C3. The term block vertebra refers to a disturbance in the normal embryologic formation of the vertebral column, which results in the bony fusion of two vertebrae. It is the intent of this case study to document the unusual condition of a block vertebra in a horse. Key Words: Equine, Vertebra, Block vertebra