Pancytopenia secondary to suspected idiosyncratic phenobarbital reaction in a dog

Abstract

A 4-year-old neutered male St. Bernard-mastiff crossbred dog showed clinical signs of lethargy and anorexia after being administered phenobarbital for the treatment of idiosyncratic seizures. A complete blood (cell) count revealed pancytopenia. Auto-agglutination and Coombs tests were negative suggesting that an immunemediated cause was unlikely; therefore, an idiosyncratic reaction to phenobarbital was suspected. Supportive care and control of seizures with zonisamide was initiated and clinical signs improved. Blood values were monitored closely and returned to normal after 3 wk.

On April 27, 2021, a 4-year-old neutered male St. Bernardmastiff crossbred dog was brought to Tanglefoot Veterinary Services in Cranbrook, British Columbia, with clinical signs of progressive lethargy and anorexia for 1 wk. The dog had been diagnosed with idiosyncratic epilepsy in January 2021, which was being controlled successfully with phenobarbital (Epiphen; Vétoquinol, Lavaltrie, Quebec), 2.1 mg/kg body weight (BW), PO, q12h. On examination, the dog was lethargic and mildly pyretic (39.8°C), but the remainder of the physical examination did not reveal any other abnormal findings. The dog was up-to-date on canine distemper virus, canine adenovirus type-2, canine parvovirus, canine influenza virus (DA2PP), rabies, and Bordetella vaccinations and there was no history of travel. A complete blood (cell) count (CBC) revealed pancytopenia, characterized by a mild normocytic, normochromic, non-regenerative anemia [hematocrit 34.6%; reference interval (RI): 37.3 to 61.7%)], severe leukopenia (2.06 × 10⁹/L; RI: 5.05 to 16.76 × 10⁹/L) characterized by severe neutropenia (0.48 × 10⁹/L; RI: 2.95 to 11.64 × 10⁹/L), mild lymphopenia (0.96 × 10⁹/L; RI: 1.05 to 5.10 × 10⁹/L), mild eosinopenia (0.05 × 10⁹/L; RI: 0.06 to 1.23 × 10⁹/L), and moderate thrombocytopenia (58 × 10⁹/L; RI: 148 to 484 × 10⁹/L). A manual platelet count was performed on a blood smear and confirmed the presence of moderate thrombocytopenia [3.5 platelets per high power field (hpf ), equating to 52 × 10⁹/L; RI: 148 to 484 × 10⁹/L]. Biochemistry revealed a moderate decrease in urea (1.3 mmol/L; RI: 2.5 to 9.6 mmol/L), a moderate increase in globulins (GLOB) (53 g/L; RI: 25 to 45 g/L), and a mild increase in alkaline phosphatase activity (ALKP) (254 U/L; RI: 23 to 212 U/L). An IDEXX SNAP canine pancreas-specific lipase (cPL) test (IDEXX Laboratories, Westbrook, Maine, USA) was completed and the result was below the reference value. A slide agglutination test was performed by mixing 1 drop of EDTA blood and 2 drops of saline and no microagglutination was observed under a microscope. A focused assessment with sonography for trauma (FAST) scan of the thorax and abdomen revealed no abnormalities. Urinalysis of a free-catch sample received in-clinic revealed a urine specific gravity (USG) of 1.008, pH of 7.0, 1+ protein, and 1+ blood; all other values were normal. Urine sediment revealed infrequent red blood cells (3/HPF) and white blood cells (< 1/HPF). Treatment with lactated Ringer’s solution (LRS) (1 L, IV, over 1.5 h), ampicillin (Ampicillin Sodium; Novopharm, Toronto, Ontario) 22 mg/kg BW, IV, q8h, and metronidazole (Metronidazole; Hospira, Saint-Laurent, Quebec), 10 mg/kg BW, IV, q12h was initiated. Phenobarbital was discontinued immediately, and treatment with zonisamide (Zonegran; Eisai, Mississauga, Ontario), 600 mg, PO, q12h was initiated for seizure control. The dog went home overnight.

The next day, the dog was returned to the clinic where he continued to be lethargic but ate wet food. The dog’s vital parameters were within normal limits. No seizures were noted overnight by the owner. Treatment with ampicillin, metronidazole, and LRS (1 L, IV, over 3 h) continued. A CBC was repeated and revealed a further decrease in hematocrit (33.9%; RI: 37.3 to 61.7%), neutrophils (0.16 × 10⁹/L; RI: 2.95 to 11.64 × 10⁹/L), and platelets (54 × 10⁹/L; RI: 148 to 484 × 10⁹/L). Biochemistry was not performed. Treatment with amoxicillin/clavulanic acid (Clavaseptin; Vétoquinol), 1000 mg, PO, q12h for 7 d, oral metronidazole, 625 mg, PO, q12h for 7 d), and omeprazole (Omeprazole; Sandoz, Boucherville, Quebec), 30 mg, PO, q24h, for 7 d was initiated. Treatment with zonisamide continued. The dog went home overnight.

The following day, April 29, the dog continued to be lethargic, and spontaneous epistaxis developed. The remainder of the physical examination was normal, and no seizures were noted by the owner. A CBC was repeated and revealed a further decrease in neutrophils (0.11 × 10⁹/L; RI: 2.95 to 11.64 × 10⁹/L) and increase in hematocrit (35.9%; RI: 37.3 to 61.7%) and platelets (83 × 10⁹/L; RI: 148 to 484 × 10⁹/L). A FAST scan of abdomen and thorax revealed no abnormalities. Treatment with amoxicillin/clavulanic acid, metronidazole, omeprazole, and zonisamide continued. The dog was sent home.

The dog was brought in for a recheck CBC on May 5, 2021. The owner reported that the dog was less lethargic, had increased appetite, and experienced several episodes of epistaxis that were successfully controlled at home. The dog was less lethargic, and no abnormalities were noted during the physical examination. A CBC was performed and revealed a normal hematocrit (42.8%; RI: 37.3 to 61.7%) and platelets (206 × 10⁹/L; RI: 148 to 484 × 10⁹/L), and increased neutrophils (0.44 × 10⁹/L; RI: 2.95 to 11.64 × 10⁹/L). Treatment with amoxicillin/clavulanic acid, metronidazole, and omeprazole was discontinued, and treatment with zonisamide was continued for control of seizures.

The dog was seen for a recheck on May 19, 2021. The owner reported that the dog’s lethargy had improved, and no other abnormalities were noted at home. No abnormalities were noted on physical examination in the clinic. A CBC was performed and revealed no abnormalities aside from a mild eosinophilia (1.56 × 10⁹/L; RI: 0.06 to 1.23 × 10⁹/L). These findings were consistent with adequate bone marrow recovery and the dog was no longer required to return for further laboratory analysis. Treatment with zonisamide was continued for control of seizures.

Discussion

Pancytopenia in dogs can be due to either reversible or irreversible bone marrow stem cell injury (1). Irreversible injury consists of an intrinsic defect in hematopoiesis, whereas reversible injury is transient (1). Both can be induced by drugs, chemicals, viruses, radiation, neoplasia, and immune-mediated injury; however, the cause is often undetermined (1). Although an idiosyncratic reaction to phenobarbital is rare and the mechanism is unknown, phenobarbital-induced cytopenia has been documented several times in dogs (2). Recorded cellular reactions vary and include neutropenia, thrombocytopenia, and anemia; with neutropenia usually being the most severe (3–6). In humans, it is thought that phenobarbital induces a hemophagocytic syndrome in which cytokine activation of cytotoxic T-cells and macrophages results in a non-selective phagocytosis of cells in the bone marrow (7). Due to the frequency and multilineage cytopenia that often occurs, this is a plausible theory for the mechanism of phenobarbital-induced pancytopenia in dogs (8).

Diagnosis of phenobarbital-induced pancytopenia is difficult as the clinical signs are vague and often do not reflect the severity of the underlying disease. Diagnosis is based on history, clinical findings, and, most importantly, a positive response to discontinuing phenobarbital. The most common complaints are lethargy, inappetence, and vomiting (8), of which lethargy and inappetence were evident in this case. Physical examination findings are usually unremarkable with some dogs showing pyrexia, pale mucous membranes, and lymphadenomegaly (8). When a CBC is completed and reveals cytopenia in multiple cell lineages, the extent of the dog’s disease is determined. Neoplasia can only be ruled out by bone marrow examination but was unlikely in this case based on this dog’s signalment and through imaging of the thorax and abdomen. Infectious causes of pancytopenia are ruled out based on vaccination history, travel history, 4Dx SNAP test, blood and urine cultures, and tick PCR tests. A thorough history of drug usage can be obtained to determine if a particular drug may be the cause of pancytopenia. In this case, the dog had recently started treatment with phenobarbital to control idiopathic epilepsy, thus a phenobarbital-induced pancytopenia was the top differential diagnosis. Neoplasia was less likely given the dog’s young age, normal FAST scan results and lack of abnormal hematopoietic cells on the blood smears. Infectious causes of pancytopenia were ruled as the dog was up-to-date on vaccinations, had no history of travel, and results from the 4Dx SNAP test were normal. A tick PCR test and blood and urine cultures were not completed as a phenobarbital reaction was strongly suspected; however, these tests could have been completed if the dog had not responded positively to discontinuing phenobarbital.

Additional testing of the bone marrow, including aspirate and biopsy, could have been performed to further evaluate for abnormalities, such as neoplasia or infection. Common bone marrow findings with phenobarbital-induced pancytopenia include marrow hypercellularity with “lower numbers of mature cells than expected” despite active hematopoiesis (5). Evidence of phagocytosis has been noted in some cases, suggesting an immune-mediated mechanism (5). Rarely, a bone marrow biopsy will reveal necrosis and myelofibrosis and the relation of these findings to an idiosyncratic phenobarbital reaction is undetermined (9). The diagnosis of a phenobarbital-induced pancytopenia is strongly suspected when the drug is discontinued, and the dog’s bone marrow positively responds. It usually takes 1 to 3 wk for the bone marrow to fully recover, with neutropenia resolving in 2 to 3 wk (3–6,10). The most important aspect of treatment for idiosyncratic phenobarbital reaction is discontinuing treatment with phenobarbital and using an alternative method to control seizures (2), in this case zonisamide. In addition to this, dogs require supportive care to prevent the occurrence of serious complications that can arise in a pancytopenic state. Severe neutropenia is a common occurrence and dogs should be administered prophylactic broad-spectrum antibiotics to prevent secondary infections for which they are predisposed (11). Severe thrombocytopenia (< 30 × 10⁹/L) can predispose dogs to spontaneous bleeding and requires that dogs remain quiet with reduced activity until the thrombocytopenia resolves or a platelet transfusion is received (12). If severe anemia occurs, a blood transfusion may be necessary to correct the anemia and maintain the hematocrit above 20% (13). Additional supportive care, such as fluids, gastro-protectants, and anti-emetics can be used based on need until adequate recovery is achieved.

Idiosyncratic phenobarbital reactions are rare and unpredictable in occurrence. Upon initiating treatment with phenobarbital, it is important to be diligent and follow-up with clients regularly to monitor for signs of an adverse drug reaction in their dogs. Diagnosis of this condition and discontinuing phenobarbital promptly is paramount for survival and recovery. This case highlights both the difficulty in diagnosing the cause of pancytopenia and the importance of excellent supportive care that is tailored to the individual animal once a diagnosis is made.