OPEN a ACCESS Freely available online
`
`(0; PLOS ONE
`
`Clinical Risk Factors Associated with Anti-Epileptic Drug
`Responsiveness in Canine Epilepsy
`
`CrossMark
`
`up.dates
`
`Rowena M. A. Packer', Nadia K. Shihabill'2, Bruno B. J. Torres", Holger A. Volk'*
`
`1 Department of Clinical Science and Services, Royal Veterinary College, Hatfield, Hertfordshire, United Kingdom, 2 Department of Neurology/Neurosurgery, Southern
`Counties Veterinary Specialists, Ringwood, Hampshire, United Kingdom, 3 Department of Veterinary Medicine and Surgery, Federal University of Minas Gerais, Belo
`Horizonte, Minas Gerais, Brazil
`
`Abstract
`
`The nature and occurrence of remission, and conversely, pharmacoresistance following epilepsy treatment is still not fully
`understood in human or veterinary medicine. As such, predicting which patients will have good or poor treatment
`outcomes is imprecise, impeding patient management. In the present study, we use a naturally occurring animal model of
`pharmacoresistant epilepsy to investigate clinical risk factors associated with treatment outcome. Dogs with idiopathic
`epilepsy, for which no underlying cause was identified, were treated at a canine epilepsy clinic and monitored following
`discharge from a small animal referral hospital. Clinical data was gained via standardised owner questionnaires and
`longitudinal follow up data was gained via telephone interview with the dogs' owners. At follow up, 14% of treated dogs
`were in seizure-free remission. Dogs that did not achieve remission were more likely to be male, and to have previously
`experienced cluster seizures. Seizure frequency or the total number of seizures prior to treatment were not significant
`predictors of pharmacoresistance, demonstrating that seizure density, that is, the temporal pattern of seizure activity, is a
`more influential predictor of pharmacoresistance. These results are in line with clinical studies of human epilepsy, and
`experimental rodent models of epilepsy, that patients experiencing episodes of high seizure density (cluster seizures), not
`just a high seizure frequency pre-treatment, are at an increased risk of drug-refractoriness. These data provide further
`evidence that the dog could be a useful naturally occurring epilepsy model in the study of pharmacoresistant epilepsy.
`
`Citation: Packer RMA, Shihab NK, Torres BBJ, Volk HA (2014) Clinical Risk Factors Associated with Anti-Epileptic Drug Responsiveness in Canine Epilepsy. PLoS
`ONE 9(8): e106026. doi:10.1371/journal.pone.0106026
`
`Editor: Giuseppe Biagini, University of Modena and Reggio Emilia, Italy
`
`Received April 7, 2014; Accepted July 29, 2014; Published August 25, 2014
`
`Copyright: © 2014 Packer et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
`unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
`
`Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its
`Supporting Information files.
`
`Funding: The authors have no funding or support to report.
`
`Competing Interests: Nadia K. Shihab is employed by Southern Counties Veterinary Specialists. There are no patents, products in development or marketed
`products to declare. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for
`authors.
`
`* Email: hvolk@rvc.ac.uk
`
`NKS and MT are joint second authors on this work.
`
`Introduction
`
`Epilepsy is the most common chronic neurological condition in
`humans and dogs, with estimated prevalences of 0.4-1% [1] and
`0.6%, respectively [2]. In human medicine, the best improvement
`in Quality of Life (QoL) for epilepsy patients is achieved when
`treatment leads to remission (seizure freedom) [3-5]. Indeed, in
`one study, no significant change in QoL was found after treatment
`for subjects that did not achieve seizure freedom [4]. In addition to
`anti-epileptic drug (AED) therapy, surgical interventions are
`utilised to achieve seizure freedom in medically intractable cases
`[6]. The dog has been considered as a naturally occurring model
`of human epilepsy [7,8]. There are considerable parallels in the
`diagnosis of human and canine epilepsy, with similarly high levels
`of workup, for example and the use of advanced diagnostic
`imaging and in limited cases, the use of electroencephalography
`(EEG) [9]. However, in veterinary medicine, most epilepsy trials
`have primarily focused on reducing seizure frequency, rather than
`achieving seizure freedom. Indeed, an 513°/0 reduction in seizure
`frequency has been the defmition of AED efficacy in the majority
`of canine epilepsy studies (e.g. [10-17]). This may not be a
`
`satisfactory outcome for the carers (the owners), with nearly one
`third considering only complete seizure freedom as an acceptable
`outcome [18]. More than two thirds of dogs with epilepsy will
`continue to have seizures long-term [19-22] and around 20-30%
`will remain poorly controlled (<50% reduction of seizure
`frequency) despite adequate treatment with phenobarbitone (PB)
`and/or potassium bromide (KBr) [23-25]. Consequently, there is
`a need to identify those dogs that are likely to have poor outcomes
`so that owners have realistic, evidence-based expectations of their
`dog's treatment. This has been an area of focus in human epilepsy,
`with analyses identifying risk factors for pharmacoresistance and
`poor outcome (e.g. [26-28]). In contrast, it has been recognised
`that more epidemiologic studies are needed to further document
`the nature and occurrence of remission of epilepsy in dogs [29],
`and identify risk factors associated with positive and negative
`outcomes. For those dogs that are unresponsive to AEDs,
``alternative' non-pharmacological treatment options need to be
`developed to improve their quantity and quality of life, for
`example, dietary and surgical interventions [30].
`Remission with or without medication has been observed in
`canine epilepsy cases, demonstrating that epilepsy in dogs is not
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`necessarily a lifelong condition. Remission rates vary between
`studies, for example in a study of Danish Labrador Retrievers,
`24% of dogs were classed as being in remission; with only 1 (6%) of
`these receiving antiepileptic treatment (drug-induced remission)
`[21]. In a further Danish study of 63 dogs with epilepsy, the
`remission rate (both spontaneous remission and remission with
`treatment) was 15% [22]. In these studies, remission was classified
`as being seizure free for two years or three years seizure free,
`respectively. In a Swiss study of Labrador Retrievers, 30% of dogs
`treated with phenobarbitone became seizure-free, with an average
`follow-up period of 4.8 years [19]. In a study of the efficacy of
`phenobarbital compared with KBr as a first line treatment,
`complete seizure freedom was achieved in 85% and 52%,
`respectively, of treated dogs [31]. This study only lasted for six
`months however, and it is possible that the percentage of dogs
`experiencing seizure freedom would be lower given a longer
`follow-up period. In addition, higher % treatment success rates
`may reflect studying animals in first opinion practice environment,
`where seizure phenotypes are likely to be less severe than animals
`seen at referral practices.
`Several factors related to the natural history of the disease and
`clinical factors have been implicated in both the experimental and
`clinical literature as influencing the likelihood of successful
`treatment with AEDs (either remission or <50% reduction in
`seizure frequency). For example, recent rodent studies found that
`early treatment [32] had a positive influence on the likelihood of
`remission being achieved in certain types of epilepsy. Indeed, in
`human epilepsy it was thought that patients should be treated with
`AEDs immediately after a seizure to increase the likelihood of
`achieving remission. However, evidence that remission rates in
`countries with and without ready access to AEDs are similar [33]
`implies that AEDs may act to suppress seizures, but have no
`influence on achieving remission. In addition, there is increasing
`evidence from both canine, rodent and human studies, that other
`aspects of disease e.g. different markers of severity can influence
`drug responsiveness and treatment outcome [19,29,34-36]. This
`includes a high seizure frequency before treatment, and the
`presence of cluster seizures and/or status epilepticus. Much of the
`canine epilepsy literature in this area is derived from single breed
`studies, thus the aim of this retrospective study was to investigate
`factors associated with remission in a large population of dogs with
`epilepsy treated at a multi-breed canine specific epilepsy clinic.
`
`Materials and Methods
`
`Data from dogs treated at a multi-breed canine specific epilepsy
`clinic at the Royal Veterinary College Small Animal Referral
`Hospital (RVC SARH) between 2005-2011 was retrospectively
`collected from RVC's electronic patient records. Clinical data was
`originally gained via standardised owner questionnaires for
`epilepsy patients at their first appointment, and longitudinal
`follow up data was gained via telephone interview with the dogs'
`owners. All dogs received a uniform diagnostic protocol (including
`complete blood cell count; serum biochemical profile and dynamic
`bile acid testing; MRI of the brain, 1.5-Tesla Gyroscan NT,
`Philips Medical Systems) and a neurological examination to rule
`out an underlying cause of the seizure activity. Only dogs which
`were reported in the records to be diagnosed with idiopathic
`epilepsy, for which a cause was not identified (no remarkable
`findings on interictal neurological examination, haematology,
`biochemistry, brain magnetic resonance imaging and cerebrospi-
`nal fluid examination), were included in the study. A genetic or
`hereditary basis cannot be confirmed for every case included in the
`study, and it is possible that the cause could have been identified
`
`Drug Canine Epilepsy
`
`with continuous EEG recording. Only dogs receiving AEDs were
`included in the study.
`Seizures were classified according to the former guidelines of the
`International League Against Epilepsy, modified for veterinary
`patients (Berendt and Gram, 1999; Licht et al., 2002). Epilepsy
`was defined of at least two unprovoked seizures >24 h apart.
`Cluster seizures were defined as an episode where more than one
`seizure occurred within a 24 h period, with full recovery of
`consciousness between seizures. Status epilepticus was defined as
`seizure activity lasting longer than 10 min without gaining
`consciousness. Seizure activity lasting less than 10 min without
`gaining consciousness was classed as a single seizure episode. A
`consistent history was collected with the help of a questionnaire
`developed for a previous study [10]. The data collected included:
`signalment, age presented to the hospital (days), age of dog at the
`time of the first seizure (days), time until diagnosis (days), duration
`of the disorder before treatment (days), number of seizures prior to
`any treatment with an AED, seizure frequency per month before
`medication, type of seizures experienced, and experience of cluster
`seizures (yes/no) and status epilepticus (yes/no). Medication
`administered was recorded, specifically whether phenobarbitone
`(PB), potassium bromide (KBr) or other 3rd line drugs were
`prescribed, and response to these drugs recorded as responsive or
`unresponsive. Follow up time was recorded in days. Treatment
`success was recorded as:
`
`(i) Seizure-free remission (with or without medication) (1/0)
`(ii)
`.50% reduction in seizure frequency (1/0)
`
`Non-responsiveness to an AED was classified as a less than 50%
`reduction in seizure frequency, despite being within the reference
`range for the prescribed AED(s) and titrated to the maximum
`tolerated effective dose. As these data were derived from a clinical
`population, decision-making leading to the maximum dose of any
`AED was made by both the clinician and the owner, taking into
`account adverse effects of the drug and its efficacy. Serum levels of
`phenobarbitone and/or potassium bromide were checked by the
`attending clinician, and recorded from the clinical records where
`available, to ensure the dog was within the reference range for
`these AEDs and receiving adequate therapy, and to test the effect
`of this variable.
`
`Ethics statement
`This study was approved by the Royal Veterinary College's
`Ethics and Welfare Committee. The owners of the dogs gave
`permission for their animals to be used in this study.
`
`Statistical analysis
`Differences between outcome variables were tested with a
`Fisher's exact test for categorical variables with expected values <
`10, and the Pearson's chi squared test for expected values >10.
`The Mann-Whitney U-test was used for continuous variables.
`Generalised linear mixed models for binary outcomes were then
`used to identify risk factors in a multivariate analysis for successful
`treatment outcomes, using the lmer function in R from the lme4
`package. Treatment outcomes (i) seizure free remission with or
`without medication (1/0) and (ii) 50°/o reduction in seizure
`frequency (1/0) were used as the response variables in models.
`Follow-up time and serum AED values were tested in the models
`to verify that they did not have an effect on treatment success.
`Breed was included as a random effect, with all cross breeds coded
`plainly as 'cross breed' due to the unknown parentage of many of
`these dogs. This random effect took into account the genetic non-
`independence of multiple members of the same breed in the study
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`Drug Canine Epilepsy
`
`Table 1. Association between clinical variables and being in seizure-free remission in canine epilepsy patients.
`
`Remission
`
`No (%)
`
`75.1
`
`24.9
`
`53.2
`
`46.8
`
`20.0
`
`80.0
`
`62.8
`
`37.2
`
`Yes (%)
`
`Fishers exact (2 sided)
`
`P
`
`Statistics
`
`53.6
`
`46.4
`
`75.0
`
`25.0
`
`0.0
`
`100.0
`
`17.9
`
`82.1
`
`5.56
`
`4.53
`
`0.25
`
`0.024
`
`0.038
`
`0.802
`
`19.63
`
`<0.001
`
`Median (25th-75th percentile)
`
`Median (25th-75th percentile)
`
`Mann Whitney U
`
`1080 (720-1800)
`
`1440 (1080-2085)
`
`180 (62.3-378.8)
`
`720 (441-1286)
`
`90 (30-180)
`
`5 (3-85)
`
`3 (1-6)
`
`90 (15-225)
`
`1170 (720-1725)
`
`60 (26-120)
`
`4 (3-53)
`
`2 (1.25-3.75)
`
`1933
`
`1204
`
`2971
`
`578
`
`1286
`
`1582
`
`0.61
`
`0.79
`
`0.026
`
`0.31
`
`0.09
`
`0.39
`
`Sex (cid:9)
`
`Male
`
`Female
`
`Neuter status (cid:9)
`
`Neutered
`
`Seizure severity (cid:9)
`
`Entire
`
`Status
`epilepticus
`
`No Status
`epilepticus
`
`Cluster
`seizures
`
`No Cluster
`seizures
`
`Age presented to hospital
`(days)
`
`Time until diagnosis (days)
`
`Age at onset seizures (days)
`
`Duration of disorder before
`treatment (days)
`
`Number of seizures before
`start of treatment
`
`Seizure frequency per month
`before medication
`
`doi:10.1371/journal.pone.0106026t001
`
`population, and possible demographic and environmental factors.
`Predictors including age, sex and neuter status were tested in all
`models. Multicollinearity was checked for in all models, identified
`from inflated standard errors in the models, and thus avoided.
`Model fit was assessed using the deviance and Akaike's informa-
`tion criterion. Data is presented as median with 25th and 75th
`percentiles and all tests were used two-sided with P<0.05 being
`considered statistically significant.
`
`Results
`
`Population demographics
`122 dogs were lost to follow and 344 dogs were included in the
`analysis, of which 89.5% were pure bred and 10.5% were cross-
`breeds. The five most common breeds were the Labrador
`Retriever (14.8%), cross breed (10.5%), Border Collie (9.9%),
`German Shepherd Dog (8.7%) and the Staffordshire Bull Terrier
`(5.5%). The majority of dogs were male (70.3%), with 57% of all
`dogs neutered. The median age (in days) at presentation to the
`small animal referral hospital was 1260 days (720-2008) (approx-
`imately 3.5 years).
`
`Clinical data
`The median age at onset of seizures was 780 days (360-1447.5).
`The median time until diagnosis was 150 days (38-360), with the
`median duration of the disorder before treatment 67.5 days (30-
`180). The median number of seizures before the start of treatment
`was 4.5 (3-7.25) with a median seizure frequency (per month)
`before medication of 3 (1-5). The median follow up time was 656
`days (330-960).
`
`A minority of dogs had experienced status epilepticus (13.1%),
`whereas nearly half of dogs had experienced cluster seizures (48%).
`There was a significant association between the presence of status
`epilepticus and cluster seizures (X2 = 8.05, P= 0.004), with 9.8% of
`dogs experiencing both status epilepticus and cluster seizures.
`There was no difference between male and female dogs
`experiencing cluster seizures (48.9% vs. 45.8%; X2 = 0.26,
`P = 0.61); however, more male dogs experienced status epilepticus
`than female dogs (15.5% vs. 5.2%; X2 =4.12, P = 0.041). At the
`univariate level (Table 1) dogs without cluster seizures were
`significantly more likely to go into remission, but there was no
`difference in dogs with or without status epilepticus.
`The most common seizure type was complex-focal seizures with
`secondary tonic-clonic generalisation (35.7%), followed by gener-
`alised tonic-clonic (32.6%), complex-focal (14.1%), and simple-
`focal seizures with secondary tonic-clonic generalisation (13.7%).
`The rarest seizure type was simple-focal seizures with only 11 cases
`(3.8%).
`Of the 113 dogs for which PB concentrations were available,
`they were well within the reference range (29.1±1.60 j.tg/rnl,
`reference range from our laboratory of 15-45 ig/m1). KBr
`concentrations were available for 53 dogs and were
`1.61 -±0.11 mg/nil, again well within the reference range from
`our laboratory of 0.5-1.9 mg/ml.
`The majority of dogs were receiving PB at follow up (67.2%),
`with a further 38.4% of cases receiving KBr, and 27% of all cases
`receiving PB and KBr in combination. A minority of cases (10.2%)
`were prescribed a third line AED (e.g. gabapentin, pregabalin,
`levetiracetam and zonisamide). In addition, 5.4% of cases received
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`Drug Canine Epilepsy
`
`Table 2. Association between clinical variables and (cid:9)
`
`50% reduction in seizure frequency in canine epilepsy patients.
`
`50% reduction
`
`No (%)
`
`Yes (%)
`
`Fishers exact (2 sided)
`
`P
`
`Statistics
`
`Sex (cid:9)
`
`Neuter status (cid:9)
`
`Seizure severity (cid:9)
`
`Male
`
`Female
`
`Neutered
`
`Entire
`
`Status
`epilepticus
`
`No Status
`epilepticus
`
`Cluster
`seizures
`
`No Cluster
`seizures
`
`78.5
`
`215
`
`50.0
`
`50.0
`
`21.1
`
`78.9
`
`71.7
`
`28.3
`
`64.5
`
`355
`
`63.2
`
`36.8
`
`10.2
`
`89.8
`
`33.5
`
`665
`
`5.54
`
`3.62
`
`4.35
`
`0.025
`
`0.040
`
`0.052
`
`34.01
`
`<0.001
`
`Median (25th-75th percentile)
`
`Median (25th-75th percentile)
`
`Mann Whitney U
`
`990 (720-1514.8)
`
`14245 (840-2094.5)
`
`5795
`
`4225.5
`
`0.011
`
`0.216
`
`Age presented to hospital
`(days)
`Time until diagnosis (days)
`
`Age at onset seizures (days)
`
`Duration of disorder before
`treatment (days)
`
`Number of seizures before
`start of treatment
`
`Seizure frequency per
`month before medication
`
`doi:10.1371
`
`
`
`183 (72.5-360)
`
`720 (360-1125)
`
`37.5 (22.5-142.5)
`
`5 (3.3-8.8)
`
`3 (1-5)
`
`150(34-360)
`
`968 (447.8-1699)
`
`90(30-180)
`
`45 (3-7.8)
`
`2 (1-5)
`
`9893
`
`833.5
`
`2762
`
`5022.5
`
`0.007
`
`0.064
`
`0.276
`
`0.569
`
`emergency rectal diazepam treatment and 8.1% received pulsed
`intermittent treatment with levetiracetam.
`
`Risk factors for remission
`Fourteen per cent of dogs were in remission on PB treatment.
`When _50% reduction in seizure frequency is used as the
`outcome measure, success rates are markedly higher with 64.5% of
`dogs achieving this level of seizure reduction. At the univariate
`level, several factors were associated with an increased likelihood
`of achieving remission (Table 1), namely: being female, neutered,
`no previous experience of cluster seizures and an older age at onset
`of seizures. The same four factors were also associated with an
`.50`)/0 reduction in seizure
`increased likelihood of achieving an
`frequency, with the addition of an older age at presentation to
`hospital (Table 2).
`
`When tested in a multivariate mixed model (Table 3), two
`categorical variables were significantly associated with the
`likelihood of remission being achieved; sex and cluster seizures,
`with female dogs over two times more likely to achieve remission,
`and dogs with no previous experience of cluster seizures over six
`times more likely to achieve remission. No effects of neuter status
`or previous episodes of status epilepticus were found in any model,
`and were not found to improve model fit (determined by Akaike
`Information Criterion [MC] and % correct classification), and as
`such they were not included in the final model. There were no
`significant effects of time until diagnosis, duration of time before
`treatment, the number of seizures before treatment or the seizure
`frequency per month before medication. No effects of follow up
`time or serum AED values were found. There were no significant
`effects of seizure type on the likelihood of remission (p = 0.208);
`
`Table 3. Risk factors for remission in canine epilepsy cases.
`
`Predictor (cid:9)
`
`Sex
`
`Female
`
`Male
`
`Cluster Seizures
`
`No
`
`Yes
`
`doi:10.1371/journal.pone.0106026.t003
`
`Odds Ratio (95% CI OR)
`
`SE (coef)
`
`2.39 (1.01-5.64)
`
`Ref
`
`6.08 (2.35-15.70)
`
`ref
`
`0.44
`
`0.49
`
`2.00 (cid:9)
`
`0.047
`
`S
`
`3.75 (cid:9)
`
`<0.001
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`Drug Canine Epilepsy
`
`Table 4. Risk factors for an L750% reduction in seizure frequency in canine epilepsy cases.
`
`Predictor
`
`Sex
`
`Female
`
`Male
`
`Cluster Seizures
`
`No
`
`Yes
`
`Age at onset of seizures (days)
`
`doi:10.1371/journal.pone.0106026.t004
`
`Odds Ratio (95% CI OR)
`
`SE (coef)
`
`Z
`
`P
`
`2.15 (1.12-4.15)
`
`ref
`
`4.66 (2.58-8.39)
`
`ref
`
`1.00 (1.00-1.01)
`
`0.33
`
`2.32
`
`0.021
`
`0.30
`
`0.00
`
`5.14
`
`<0.001
`
`2.51
`
`0.013
`
`however the seizure types with the lowest remission rates were
`simple-focal (0% remission) and complex-focal seizure with
`secondary tonic-clonic generalisation (14.1% remission).
`When an'50°/0 reduction in seizure frequency is used as the
`outcome measure (Table 2 and 4), the same two factors were
`found to significantly predict the likelihood of achieving remission
`in a multivariate model (Table 4), with the addition of age at onset
`of seizures. As age at onset of seizures increases, the likelihood of
`achieving an -?-.50% reduction in seizure frequency increases.
`
`Breeds
`Dogs of fifteen different breeds achieved seizure freedom, and
`dogs of fifty-two breeds achieved an
`.50°/0 reduction in seizure
`frequency. There was no statistically significant effect of breed on
`the likelihood of dogs going into remission or having an 50%
`reduction in seizure frequency when tested at the univariate level.
`Of the breeds with over 10 dogs for which data was available (the
`Labrador Retriever, Cross Breed, German Shepherd, Border
`Collie and Staffordshire Bull Terrier), the breed least likely to go
`into remission or have an
`.50°/0 reduction in seizure frequency
`was the Border Collie (0% and 40% respectively), followed by the
`German Shepherd (11% and 35%) and Staffordshire Bull Terrier
`(0% and 57%). Fishers exact tests revealed only significant effects
`of being a Border Collie or German Shepherd on the likelihood of
`entering remission or experiencing an -?-50% reduction in seizure
`frequency (Table 5). When these breeds were included in
`multivariate analyses as binary variables, no significant effects
`were found.
`
`Discussion
`
`The results of this retrospective study provide evidence that the
`presence of cluster seizures and thus seizure density (the temporal
`pattern of seizure activity) is a more influential risk factor on the
`likelihood of achieving remission in canine epilepsy than seizure
`
`Table 5. Top five breeds most likely to lack drug response.
`
`frequency or the total number of seizures prior to treatment.
`Nearly half (48%) of dogs in the study population had experienced
`cluster seizures, of which only 17.9% achieved remission and
`33.5% achieved an
`.50°/0 reduction in seizure frequency. This
`result has previously been found in human epilepsy [37]. The
`number of epileptic dogs that experience cluster seizures varies
`between studies, with recent reports between 38% and 64%
`[20,38]. The breed least likely to achieve remission in this study
`was the Border Collie, a breed previously demonstrated to have a
`higher level of cluster seizures than other breeds (84.6% affected)
`[20], with similar levels reported in other studies (e.g. 94%; [29]).
`A remission rate of 14.2% was observed in this study, similar to a
`previous Danish study of canine epilepsy (15%) [22]. These were
`both mixed study populations; however, in studies of Labrador
`Retrievers in isolation, higher levels of remission have been
`observed (24-40%) [19,21]. When >50% reduction in seizure
`frequency is used as the outcome measure, success rates are
`markedly higher at 64.5%.
`Seizure density as well as frequency has been demonstrated to
`influence the likelihood of remission in humans, with individuals
`who experience an episode of status epilepticus [39-41], or cluster
`seizures [37] less likely to go into remission. These results were also
`seen in a recent study of predictors of pharmacoresistance in rats,
`where the average seizure frequency per day of 13 rats
`nonresponsive to medication was 4.31/day, indicating some rats
`having cluster seizures [36]. This frequency was significantly
`higher than 20 drug-responsive rats (mean 0.54/day). It is further
`notable, that of the 13 rats that were unresponsive to medication, a
`subgroup of six rats (18%) experienced high levels of cluster
`seizures, with an average of 8.94 seizures per day [36]. Intact male
`and female dogs have a higher likelihood of having cluster seizures
`[42] which may have a negative impact on their prognosis.
`Evidence from canine epilepsy is not clear however, with 89% (8/
`9) of Border Collies in remission having a history of cluster
`seizures, status epilepticus, or both [29]. A severe epilepsy
`
`Breed
`
`Border Collie
`
`German Shepherd
`
`Staffordshire Bull Terrier
`
`Cross Breed
`
`Labrador Retriever
`
`doi:10.1371/journal.pone.0106026.t005
`
`% remission
`
`0
`
`11
`
`0
`
`19
`
`23
`
`P
`
`0.02
`
`0.51
`
`0.18
`
`0.30
`
`0.14
`
`% (cid:9)
`
`50°/r, reduction
`
`40
`
`35
`
`57
`
`61
`
`76
`
`0.01
`
`0.01
`
`0.37
`
`0.38
`
`0.07
`
`PLOS ONE I www.plosone.org (cid:9)
`
`5 (cid:9)
`
`August 2014 I Volume 9 I Issue 8 I e106026
`
`Page 00005
`
`

`
`phenotype is often seen in this breed, thus data from a larger
`population with a diversity of breeds represented would be
`valuable to gain an insight into this relationship in a wider
`population with a variety of disease phenotypes.
`No evidence was found to support the results of a recent rodent
`study that found early treatment [32] influenced the likelihood of
`remission being achieved. There are divergent opinions within the
`veterinary profession regarding time to treatment after diagnosis of
`epilepsy, a topic also debated in human medicine [43]. One school
`of thought advises treatment of seizures as soon as a dog is
`diagnosed as having recurrent seizures (i.e. after the second seizure
`episode). However, the impact of AED side effects on QoL may be
`considerable, with this being the top reason cited by owners for a
`decreased QoL in their dogs (28% of 25 owners questioned) [44].
`As such, the second school of thought considers that there should
`be a balance between the benefits gained from using AEDs with
`the potential adverse effects they cause. The results of this study
`indicated no effect of time to treatment; however, there is mixed
`evidence regarding its effects on treatment outcome. In clinical
`studies of epilepsy in dogs, decreased time to treatment has not
`been observed as a positive influence upon treatment outcome,
`indeed, one study demonstrated that Labrador Retrievers that
`were in remission received medication a longer period of time after
`their first seizure than those dogs which continued to seizure [19].
`It should be acknowledged that this result may be biased by
`animals with a more severe seizure phenotype receiving treatment
`earlier, due to owner and/or veterinarian concerns. It is currently
`not veterinary practice to initiate treatment after the first seizure.
`Early initation of treatment has also proven unsuccessful in several
`human studies [45-47]. Time to treatment is additionally likely to
`be influenced by disease severity, for example it was shorter in
`dogs with episodes of status epilepticus [48], thus being
`confounding factors in statistical analyses.
`A large number of seizures before treatment has been identified
`as a poor prognostic factor in several previous human studies of
`epilepsy [34,41,49], with patients experiencing a greater number
`of seizures prior to initiation of treatment more likely to have
`refractory epilepsy. In rats, it was recently demonstrated that
`seizure frequency in the early phase of epilepsy is a strong
`predictor of refractoriness [36]. This has also been seen in dogs,
`with refractory dogs having a significantly higher number of
`seizures prior to presentation and beginning of treatment in
`Labradors [19] and an initially higher seizure frequency in Border
`Collies [29]. It has been discussed whether this initial high seizure
`frequency and subsequent refractoriness may be an effect of
`kindling (Reynolds, 1995). However, as time to treatment has not
`been found to be a strong predictor of refractoriness in dogs and
`humans, initial high seizure frequency has been considered more
`likely to be the result, rather than the cause of the pathophysi-
`ological changes that are later manifested as refractory epilepsy
`[34,50]. Indeed, in this study and another previous study of canine
`epilepsy, the number of seizures before treatment was not
`significantly different between dogs positive vs. negative treatment
`outcomes [48]. In addition, no effect was found of seizure type
`upon the likelihood of remission; however, the most common
`seizure type in dogs that did not achieve remission (39.6%) was
`complex-focal seizures, also seen in human epilepsy [51,52],
`adding evidence to the belief that focal seizures are more
`challenging to treat.
`Males were found to be less likely to achieve remission than
`female dogs. Historically, male dogs are thought to seizure more
`than female dogs [53], and recent epidemiological studies of
`idiopathic epilepsy have confirmed a male overrepresentation for
`this disorder [2,38]. With regard to the impact of sex upon
`
`Drug Canine Epilepsy
`
`treatment outcome, little existing data is available. One study
`noted that female dogs with epilepsy lived longer with the disorder
`than male dogs, with a median age at death two years greater (8 vs.
`6 years, respectively) [22]; however, this outcome measure may be
`influenced by owner euthanasia decisions, so can only be a proxy
`of treatment success. In previous studies, male dogs were found to
`be more highly affected by cluster seizures than female dogs [42].
`This result was not found in the current study, and indeed sex and
`the presence of cluster seizures were found to be independently
`significant risk factors, thus further investigation is warranted into
`the effect of sex on treatment outcome.
`Age at onset of disease was found to significantly influence the
`likelihood of achieving an
`-50°/0 reduction in seizure frequency,
`with dogs experiencing their first seizure at an older age more
`likely to achieve this level of reduction. This has previously been
`demonstrated in Border Collies, with the mean age at onset
`significantly higher in dogs with remission compared to those with
`active epilepsy [29], and in Labradors, with dogs classed as having
`excellent or good results (defined as those that were seizure-free, or
`had an improvement in their seizure frequency, strength and/or
`duration