Evaluation of therapeutic phenobarbital concentrations and application of a classification system for seizures in cats: 30 cases (2004–2013)

Katherine E. Finnerty Department of Medical Sciences, College of Veterinary Medicine, University of Wisconsin, Madison, WI 53711.

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Heidi L. Barnes Heller Department of Medical Sciences, College of Veterinary Medicine, University of Wisconsin, Madison, WI 53711.

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Miyu N. Mercier VCA Aurora Animal Hospital, 2600 W Galena Blvd, Aurora, IL 60506.

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Carley J. Giovanella Gulf Coast Veterinary Neurology and Neurosurgery, 3800 SW Freeway, Ste 136, Houston, TX 77027.

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Vivian W. Lau Gulf Coast Veterinary Neurology and Neurosurgery, 3800 SW Freeway, Ste 136, Houston, TX 77027.

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Helena Rylander Department of Medical Sciences, College of Veterinary Medicine, University of Wisconsin, Madison, WI 53711.

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Abstract

Objective—To determine the percentage of cats with a phenobarbital (PB) concentration between 15 and 45 μg/mL that had a ≥ 50% reduction in the number of seizures and to investigate applicability of the 2011 International League Against Epilepsy (ILAE) classification system in cats.

Design—Retrospective case series.

Animals—30 cats with suspected or confirmed epilepsy.

Procedures—Medical records for 2004 to 2013 at 3 veterinary hospitals were searched. Information collected included signalment, duration of observation before treatment, frequency of seizures before PB administration, seizure phenotype, dose of PB, serum PB concentration, number of seizures after PB administration, duration of follow-up monitoring, and survival time. A modified 2011 ILAE classification system was applied to all cats.

Results—Seizure control was achieved in 28 of 30 (93%) cats with a serum PB concentration of 15 to 45 μg/mL. This comprised 10 of 11 cats with structural epilepsy, 14 of 15 cats with unknown epilepsy, and 4 of 4 cats with presumptive unknown epilepsy. Thirteen cats had no additional seizures after initiation of PB treatment.

Conclusions and Clinical Relevance—Seizure control was achieved in most cats with a serum PB concentration between 15 and 45 μg/mL, regardless of the cause of the seizures. A modified 2011 ILAE classification was applied to cats with seizures and enabled classification of cats without specific genetic testing and without identified structural or inflammatory disease. This classification system should be incorporated into veterinary neurology nomenclature to standardize communication between veterinarians and improve comparisons among species.

Abstract

Objective—To determine the percentage of cats with a phenobarbital (PB) concentration between 15 and 45 μg/mL that had a ≥ 50% reduction in the number of seizures and to investigate applicability of the 2011 International League Against Epilepsy (ILAE) classification system in cats.

Design—Retrospective case series.

Animals—30 cats with suspected or confirmed epilepsy.

Procedures—Medical records for 2004 to 2013 at 3 veterinary hospitals were searched. Information collected included signalment, duration of observation before treatment, frequency of seizures before PB administration, seizure phenotype, dose of PB, serum PB concentration, number of seizures after PB administration, duration of follow-up monitoring, and survival time. A modified 2011 ILAE classification system was applied to all cats.

Results—Seizure control was achieved in 28 of 30 (93%) cats with a serum PB concentration of 15 to 45 μg/mL. This comprised 10 of 11 cats with structural epilepsy, 14 of 15 cats with unknown epilepsy, and 4 of 4 cats with presumptive unknown epilepsy. Thirteen cats had no additional seizures after initiation of PB treatment.

Conclusions and Clinical Relevance—Seizure control was achieved in most cats with a serum PB concentration between 15 and 45 μg/mL, regardless of the cause of the seizures. A modified 2011 ILAE classification was applied to cats with seizures and enabled classification of cats without specific genetic testing and without identified structural or inflammatory disease. This classification system should be incorporated into veterinary neurology nomenclature to standardize communication between veterinarians and improve comparisons among species.

Seizure disorders are the most common neurologic conditions encountered in dogs and cats.1–5 However, a consensus has not been reached on the most appropriate classification system for seizures in veterinary patients. Several classification systems for seizures in humans have been used in veterinary medicine, but none have been universally accepted.3,4,6,7 In 2011, the Commission of Classification and Terminology of the ILAE developed a new classification system to account for the wealth of genetic understanding of human seizure disorders. For the previous ILAE classification system, idiopathic epilepsy was defined as recurrent seizures with no known underlying cause and of suspected genetic origin.8,9 The new ILAE classification system provides the ability to classify a seizure disorder as genetic if a known genetic defect has been identified. If a genetic cause has not been identified, patients are classified as unknown epileptics. These 2 categories replace the previous term idiopathic epilepsy.10 Finally, the new ILAE classification system substitutes the term structural epilepsy for previously described symptomatic epilepsy.

Genetic epilepsy is considered rare in cats because a genetic basis for seizures has not been determined in most cats.7,10,11 Furthermore, the new ILAE classification system would allow veterinarians to use the term unknown epilepsy to label seizure disorders of cats that did not fit the previous traditional definition of idiopathic epilepsy. In addition to providing new definitions for the cause of seizures, the 2011 ILAE classification system could be used to address the concept of generalized and focal seizures.

Phenobarbital has long been considered the AED of choice for cats without metabolic causes of seizures. This fact is largely attributable to the low cost, twice-daily dosing frequency, and low incidence of adverse effects.1–3,12–17 The therapeutic range for PB in dogs is 15 to 45 μg/mL.16,18 This range of PB concentrations has also been used for cats; however, the therapeutic range in cats has not been established. Administration of an AED is considered effective when a patient has a ≥ 50% reduction in the number of seizures during a given treatment period1,19,20 The main purpose of the study reported here was to determine the efficacy for treating seizures in cats with a serum PB concentration between 15 and 45 μg/mL. The second purpose of the study was to modify and apply the ILAE 2011 seizure classification system to cats.

Materials and Methods

Case selection—Medical databases were searched at 3 veterinary hospitals. At the University of Wisconsin Veterinary Medical Teaching Hospital, Madison, Wis, medical records from 2004 to 2013 were searched. At VCA Aurora Animal Hospital, Aurora, Ill, medical records from 2004 to 2012 were searched. At Gulf Coast Veterinary Neurology and Neurosurgery Hospital, Houston, medical records from 2005 to 2012 were searched. Search terms were feline seizures, epilepsy, brain tumor, encephalitis, cat seizures, PB, PB serum concentration, and brain MRI. Criteria for inclusion were ≥ 2 seizures prior to starting PB treatment, PB as the only AED during the evaluated time frame, and a serum PB concentration measured ≥ 14 days after PB treatment was started. Cats were excluded if they had evidence of abnormal hepatic enzymatic activity prior to starting PB treatment.

Medical records review—Age at seizure onset, duration of observation before treatment, sex, breed, body weight, results of diagnostic testing, frequency of seizures before initiation of PB treatment, seizure phenotype, dosage of PB, serum PB concentration ≥ 14 days after the start of PB treatment, number of seizures ≥ 14 days after the start of PB treatment, duration of follow-up monitoring, and survival time were recorded for each cat. If the PB dosage was adjusted, the observation period was calculated on the basis of the longest period the patient received a specific dosage with an available therapeutic serum PB concentration that reflected that dose. The client or primary veterinarian was contacted when information was unavailable in the record. Seizure control was defined as a ≥ 50% reduction in the number of seizures for each cat.

Procedures—Cats were classified in accordance with a modified 2011 ILAE classification system as having structural epilepsy, unknown epilepsy, or presumptive unknown epilepsy on the basis of results of diagnostic testing (Appendix). Genetic screening was not available; therefore, no cats were classified as having genetic epilepsy. A cat was classified as having structural epilepsy when CSF analysis and advanced brain imaging (CT or MRI) identified a structural, inflammatory, or infectious disease. If these tests were performed and there were no abnormal results, cats were classified as having unknown epilepsy. Cats were classified as having presumptive unknown epilepsy if they had a history of seizures for ≥ 2 years with no abnormalities detected on neurologic examinations conducted during the interictal periods and if CSF analysis or advanced brain imaging was not performed.

Statistical analysis—Descriptive statistics including median and ranges were calculated for continuous variables such as age and body weight. The Mann-Whitney U test was used to compare the dose of PB among groups (cats with structural epilepsy, cats with unknown epilepsy, and cats with presumptive unknown epilepsy). The Spearman correlation was used to evaluate the association between dose and serum PB concentration. For all analyses, values of P < 0.05 were considered significant.

Results

Thirty cats met the inclusion criteria. The study included 23 domestic shorthair cats, 1 Siamese, 1 domestic longhair cat, 1 domestic medium-hair cat, 1 British Shorthair, 1 Himalayan, 1 Maine Coon, and 1 Persian-crossbred cat. Sixteen were neutered males, and 14 were spayed females. The cats were assigned to 3 groups: structural epilepsy (11/30 [37%]), unknown epilepsy (15/30 [50%]), and presumptive unknown epilepsy (4/30 [13%]). Diagnostic imaging was performed on 26 cats (MRI for 25 cats and CT for 1 cat). As described by the owners, 26 cats had generalized seizures, 2 cats had both partial and generalized seizures, and 2 cats had partial seizures. Median age of onset of seizure activity was 8.0 years (range, 0.14 to 16 years) for cats with structural epilepsy, 6.0 years (range, 0.75 to 18 years) for cats with unknown epilepsy, and 2.5 years (range, 2 to 7 years) for cats with presumptive unknown epilepsy. Median body weight was 5.2 kg (11.4 lb; range, 2.3 to 6.5 kg [5.1 to 14.3 lb]) for cats with structural epilepsy, 5.1 kg (11.2 lb; range, 2.5 to 8.1 kg [5.5 to 17.8 lb]) for cats with unknown epilepsy, and 4.6 kg (10.1 lb; range, 2.6 to 5.4 kg [5.7 to 11.9 lb]) for cats with presumptive unknown epilepsy. Median duration of the observation period prior to initiation of PB was 14 days (range, 1 to 670 days) for all cats, 21 days (range, 1 to 122 days) for cats with structural epilepsy, 6 days (range, 1 to 72 days) for cats with unknown epilepsy, and 14 days (range, 1 to 670 days) for cats with presumptive unknown epilepsy.

Median number of seizures per cat for all cats before and after initiation of PB treatment was 2.5/wk (range, 0.04 to 108 seizures/wk) and 0.03/wk (0 to 2.5 seizures/wk), respectively. Thirteen (43%) cats did not have additional seizures after starting PB treatment. Median duration of the follow-up monitoring period after starting PB treatment was 14 months (range, 0.63 to 91 months) for all cats, 11.5 months (range, 1.3 to 29.3 months) for cats with structural epilepsy, 8 months (range, 0.63 to 72.3 months) for cats with unknown epilepsy, and 75.8 months (range, 27.3 to 91 months) for cats with presumptive unknown epilepsy. All cats continued to receive PB during the follow-up monitoring period. Conditions diagnosed in the 11 cats with structural epilepsy included meningoencephalitis (n = 3 cats), extra-axial mass (2), sinusitis with secondary meningitis (1), multifocal infarcts (1), porencephaly (1), occipital lobe cysts (1), meningioma (1), and hydrocephalus (1). Concurrent medical problems for all cats included infection attributable to FeLV (n = 1), congestive heart failure (1), and feline lower urinary tract disease (1).

The median PB dosage for all cats was 4.0 mg/kg/d (1.8 mg/lb/d; range, 1.5 to 8.6 mg/kg/d [0.7 to 3.9 mg/lb/d]). The median PB dosage was 4.0 mg/kg/d (range, 1.5 to 5.8 mg/kg/d [0.7 to 2.6 mg/lb/d]) for cats with structural epilepsy, 4.0 mg/kg/d (range, 2.0 to 7.6 mg/kg/d [0.9 to 3.5 mg/lb/d]) for cats with unknown epilepsy, and 4.3 mg/kg/d (2.0 mg/lb/d; range, 2.8 to 6.5 mg/kg/d [1.3 to 3.0 mg/lb/d]) for cats with presumptive unknown epilepsy. The median serum PB concentration was 23.3 μg/mL (range, 12.1 to 37.3 μg/mL) for cats with structural epilepsy, 29.0 μg/mL (range, 8.0 to 44.0 μg/mL) for cats with unknown epilepsy, and 28.0 μg/mL (range, 26.0 to 31.0 μg/mL) for cats with presumptive unknown epilepsy. Seizure control was achieved in 28 of 30 (93%) cats with a serum PB concentration between 15 and 45 μg/mL, which comprised 10 of 11 cats with structural epilepsy, 14 of 15 cats with unknown epilepsy, and 4 of 4 cats with presumptive unknown epilepsy. In the remaining 2 cats, seizure control was achieved but at serum PB concentrations outside the therapeutic reference range for dogs. Seizures were controlled in 1 cat with unknown epilepsy at a serum PB concentration of 8.0 μg/mL and in 1 cat with structural epilepsy at a serum PB concentration of 12.1 μg/mL.

Routine serum biochemical analysis was performed for 21 of 30 cats during the follow-up monitoring period, and results for liver enzymes were within the reference range in 20 of the 21 cats. In 1 cat, an increase was detected in the activity of alanine transferase, but the activity of this enzyme had returned to within the reference range at the time of a subsequent analysis in that cat without the addition of other medications and without an adjustment to the dosage of PB. Two additional cats developed azotemia during the follow-up monitoring period. These 2 cats were 17 and 18 years old, respectively; therefore, azotemia was not attributed to PB administration. Of the 11 cats with structural epilepsy, 1 was treated surgically, 6 received prednisone, and 1 received clindamycin.

For statistical analysis of the PB dosage, cats with unknown epilepsy and presumptive unknown epilepsy were grouped, and their results were compared with the results for cats with structural epilepsy. The median and range of the PB dosage were not significantly (P = 0.33) different between these 2 groups. A significant correlation (r = 0.66; P < 0.001) was detected between PB dosage and serum PB concentration.

Twenty-four of 30 (80%) cats were alive at the completion of the study. This consisted of 14 of 15 cats with unknown epilepsy, 6 of 11 cats with structural epilepsy, and 4 of 4 cats with presumptive unknown epilepsy.

Discussion

The classification of epilepsy in dogs and cats can be confusing because of a lack of consistent terms used among studies.4,5 We used a modified 2011 ILAE classification system to determine the applicability of this human classification system for use in cats. Previous human seizure classification schemes categorized epilepsy as symptomatic (or secondary) when an underlying intracranial cause was identified, presumptive symptomatic (or cryptogenic or primary) when no underlying cause was identified, or idiopathic when a genetic origin was suspected.1,4,7,11 Seizures from metabolic causes are not classified under epilepsy, but instead are called reactive seizures. All seizure classification systems are dependent on the sensitivity of the diagnostic testing used to include or exclude a patient from a category. Similarly, the proposed modified 2011 ILAE classification system was dependent on advanced diagnostic imaging, genetic testing, and inflammatory and infectious disease testing for accurate classification of the cats. Magnetic resonance imaging of the brain was performed on most cats in the present study, although CT was performed on 1 cat. Because of the inherent limits of CT, it is possible that this cat was misclassified.

Historically, idiopathic epilepsy is considered to be rare in cats. Because of a lack of available genetic testing in cats, the term unknown epilepsy may be more appropriate than the term idiopathic. Investigators have identified 2 generations of cats with spontaneous seizure activity that may have had a genetic origin.21 Although the mode of inheritance was not identified, the investigators proposed a genetic cause.21 In those cats, the term idiopathic epilepsy should be replaced with the term genetic epilepsy. Genetic epilepsy should be used only when genetic or familial transmission has been confirmed.

We added a fourth classification to the 2011 ILAE classification system, presumptive unknown epilepsy, for cats on which diagnostic imaging and CSF analysis were not performed. A 2-year history of seizures was chosen to encompass an expected survival interval for cats with untreated vascular, neoplastic, and inflammatory disease. Without treatment, the median survival time after diagnosis of meningioma, lymphoma, and gliomas in 1 retrospective study22 was 18, 21, and 35 days, respectively. To the author's knowledge, data on survival time are unavailable for cats with untreated vascular or inflammatory disease. It is unlikely that a cat with structural disease and not receiving a specific treatment would survive for 2 years; therefore, the most likely etiology in these cats was unknown epilepsy. A limited ability to pursue diagnostic testing is a problem that is frequently encountered in veterinary medicine. Use of the term presumptive allows veterinarians to classify a patient, but it clearly demarcates that testing was not performed in that patient.

Phenobarbital is the most common AED used for epileptic cats. Despite its frequent use in cats, to the authors’ knowledge, there are few studies on the efficacy of PB for treating seizures in cats. Administration of AEDs is considered effective when a patient has a ≥ 50% reduction in the number of seizures during a given treatment period.1,19,20 In a recent study,23 investigators evaluated cats with suspected primary epilepsy and found that PB was effective in 50% to 80% of these cats. This range of percentages is lower than that in the present study and may reflect differences in categorization of seizure control between studies. Additionally, investigators in the present study evaluated cats before and after PB treatment, whereas investigators in that other study23 evaluated cats on the basis of the absolute number of seizures annually. In the present study, we found that seizure control was achieved in a majority (93%) of cats with a serum PB concentration between 15 and 45 μg/mL, regardless of the cause of the seizures (diagnosis). Seizures were controlled in all cats receiving PB; therefore, a comparison between cats with controlled and uncontrolled seizures could not be performed in the present study.

Phenobarbital is relatively affordable and historically has a low incidence of adverse effects; thus, PB is an excellent AED choice for cats.1,12–14 The authors are not aware of any published reports of fatal hepatotoxicosis in cats, but mild elevations in both alkaline phosphatase and alanine transferase activities have been described.2,12,13,15 One of the cats in the present study had evidence of elevated alanine transferase activity after long-term administration of PB. The PB dosage was not reduced in this cat, and the alanine transferase activity in this cat returned to within the reference range at a subsequent biochemical analysis. These findings supported the low incidence of hepatic enzyme induction by PB in cats.

Typical dosing recommendations for PB are 3 to 5 mg/kg/d (1.4 to 2.3 mg/lb/d) for cats.13,14 In the present study, cats that received PB at 2.5 to 8.6 mg/kg/d (1.1 to 3.9 mg/lb/d) had a therapeutic serum PB concentration between 15 and 45 μg/mL. The therapeutic range is the drug concentrations at which there is a high chance of therapeutic success.24 It is determined from the minimally effective drug concentration in most patients and the maximal drug concentration without adverse effects, as observed in most patients. It is understood that an animal may still have toxicosis or a lack of efficacy for drug concentrations within this range because the range represents a population average. The PB dosage was adjusted for 12 cats in the present study so that the therapeutic concentration was between 15 and 45 μg/mL. In addition, seizures were controlled in 1 cat at a therapeutic serum PB concentration of 8.0 μg/mL and in another cat at a serum PB concentration of 12.1 μg/mL. Therefore, it is possible that seizures may have been controlled at a lower serum PB concentration in the cats for which we proactively adjusted the PB dosage. Information regarding the timing of blood sample collection in relation to PB administration was not available because of the retrospective nature of the present study. In a previous study25 in dogs, timing of blood sample collection in relation to PB administration resulted in a < 30% change in PB concentration. This would also be expected in cats because the dosing interval is shorter than the half-life; however, fluctuations in therapeutic serum PB concentrations in cats receiving long-term PB treatment have not been evaluated. Until additional studies can be performed to evaluate seizure control at different serum PB concentrations, we recommend a reference range of 15 to 45 μg/mL be used as the therapeutic serum PB concentration in cats.

Analysis of the Spearman coefficient indicated that a relationship existed between the dose and serum concentration of PB. This relationship was not found for PB administration to dogs,18 and it indicates that linear drug adjustment is more valid in cats. However, it remains to be determined whether PB dosage or serum concentration is the most reliable variable when monitoring seizures in cats. Although we achieved excellent seizure control in cats with a serum PB concentration within the therapeutic range for dogs, a prospective study conducted to evaluate the seizure control obtained with various serum PB concentrations may reveal a different therapeutic range for cats.

The present study had a few limitations. These included the retrospective nature of the data, small sample size, and nonnormal population distribution.

In the present study, it was easy to apply a modified 2011 ILAE classification to cats with seizures, which allowed us greater freedom to classify cats for which we did not have results of specific genetic testing and without identified intracranial disease. We recommend incorporating this classification into the veterinary neurology nomenclature to standardize communication among veterinarians and improve comparisons among publications. Furthermore, seizure control was achieved in most cats with a serum PB concentration within the published therapeutic range for dogs, regardless of the cause of the epilepsy.

ABBREVIATIONS

AED

Antiepileptic drug

ILAE

International League Against Epilepsy

PB

Phenobarbital

References

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Appendix

An ILAE classification system for epilepsy in cats modified from a classification system used in humans.10

Traditional classificationILAE 2011 classificationDefinitionInclusion criteria for cats
IdiopathicGeneticSeizures are a direct result of a known or presumed genetic defectUndetermined in cats
SymptomaticStructural or metabolicStructural or metabolic condition or disease that is confirmed to be associated with a substantially increased risk of developing epilepsyAdvanced diagnostic imaging and CSF analysis
CryptogenicUnknownUnderlying cause is unknown; it may be a fundamental genetic basis or the consequence of an unrecognized structural or an unidentified metabolic disorderNo abnormalities detected during diagnostic testing
Probably symptomaticPresumptive unknownPresumptive diagnoses of unknown epilepsy on the basis of no abnormalities during the interictal interval and no metabolic abnormalitiesSurvival time of > 2 years after starting PB treatment
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