Canine and Feline Epileptic Seizures and the Lunar Cycle: 2,507 Seizures (2000–2008)
Epileptic seizures in 211 canine and feline patients diagnosed with idiopathic epilepsy were evaluated for temporal significance in relation to the lunar cycle. Seizure counts were compared among each of the eight individual lunar phases, among each of eight exact lunar phase dates, and by percent of lunar illumination using generalized estimating equations. No statistical significance was found in any of these comparisons excluding a relationship between the onset of epileptic seizures and the phases of the moon. Alteration in anticonvulsant treatment or monitoring of canine and feline patients with idiopathic epilepsy at large was not warranted based on the lunar cycle.
Introduction
The lunar cycle, also referred to as the synodic month, has a mean length of 29 days, 12 hr, 44 min, and 2.9 sec, or 29.53 days1. Due to the differences in the elliptical orbit of the moon in relation to the sun's gravity, this time can range from 29.27 to 29.83 days1. There are eight phases of the synodic month classified by the amount of light reflected from the moon's surface that is visible from earth. The eight lunar phases include the new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and the waning crescent.
Both the general population and the scientific community have long held a fascination with the notion that the lunar cycle may influence many medical as well as sociological events. This interest is reflected in medical literature, with studies showing both positive and negative correlations to the full moon, with events ranging from epileptic and psychogenic seizures to crime, dog bites, birth, poisoning, myocardial infarction, and gout, as well as emergency room and psychiatric ward admissions.2-20 Recently, a 2006 paper published in Neurology positively correlated the incidence of primary epileptic seizures in human patients with the full moon.2 In that study, the investigators divided the lunar cycle into quarters and were able to show that epileptic seizures significantly increased in frequency during the full moon quarter as opposed to the new moon or the first and last quarters.2 No published reports in the veterinary literature have explored, in depth, the relationship between epileptic seizures and the lunar cycle, although a study published in 2007 did show a statistically significant increase in the overall rate of emergency room visits during fuller moon phases and included those patients admitted for seizures.20 No specific etiologic incidence rates were specified in that study.20
Occasionally, an owner will inquire if the lunar cycle plays a role in the onset of their pet's epileptic seizures. The target of this question was to determine if increased observation of their pet or transient increases in anticonvulsant therapy were needed during these times. The aim of this study was to definitively determine if a temporal relationship exists between the onset of epileptic seizures and the phases of the lunar cycle, with a particular interest in the full moon phase. Based on the results of the two studies described previously, the authors hypothesized that there would be a statistically significant increase in seizure frequency during the phase of the full moon.
Materials and Methods
Medical records of patients diagnosed with idiopathic epilepsy at the Veterinary Neurological Center in Las Vegas, NV, were retrospectively reviewed from January 2001 to December 2008. All patients were required to have a normal complete blood count and biochemical profile on file as well as a documented normal interictal neurologic examination. Patients that underwent advanced brain imaging (MRI or computed tomography [CT]) and cerebrospinal fluid analysis with normal results were categorized as having idiopathic epilepsy. Patients that did not undergo advanced imaging were categorized as having a diagnosis of presumptive idiopathic epilepsy based on history and compatible clinical signs. Information pertaining to patients’ signalment, age of seizure onset, diagnostic testing, and specific seizures dates was collected and recorded for analysis. Ages at seizure onset were categorized as: <12 mo of age, between 12 and 60 mo of age, or >60 mo of age to illustrate possible differences in seizure patterns in patients with a classic onset of idiopathic epilepsy between 1 and 5 yr of age.21
Seizure dates were obtained by reviewing medical records provided by referring veterinarians as well as from each patient's individual seizure calendar used at the authors’ hospital. Seizure dates between January 4, 2000 and December 24, 2008 were recorded to encompass 111 complete lunar cycles and a total of 3,278 days. Only seizure events for which a specific date was recorded in the medical record were used for data analysis. The corresponding lunar phase was calculated based on the fraction of the moon illuminated for Pacific Standard Time.22 Each seizure date was recorded and grouped by lunar phase: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, or waning crescent.
Statistical Analysis
Generalized estimating equations, assuming a Poisson distribution and log link function, were used to assess the effect of lunar phase on seizure occurrence.23 For all comparisons, P<0.05 was considered significant.
Three statistical models were constructed for data analysis, with the first evaluating the total number of seizures in each of the eight lunar phases as the outcome variable. The second used the number of seizures occurring on the exact day of each lunar phase as the outcome. The difference between the first and second models was that, in the first model, each day of the lunar cycle was classified as falling into a particular phase, whereas in the second model, only seizures occurring on the eight exact phase dates per cycle were counted. For example, a seizure occurring on the day before the full moon would be coded as having occurred during the full moon phase in the first model but would not be considered for analysis in the second model. Follow-up contrasts consisted of an omnibus test of differential seizure rates across the eight lunar phases followed by a comparison of seizure rates in each individual phase against the seven remaining phases, for a total of nine contrasts performed. The last model coded each lunar phase according to the percentage of the lunar surface that was illuminated. For example, the full moon was coded as having 100% lunar illumination, whereas the first and last quarters were coded as having 50% lunar illumination. The linear effect of the percentage of illumination variable was then examined. All three models controlled for animal species (dog or cat), gender (male or female), age of onset (young, middle aged, or old), and diagnosis type (confirmed versus presumptive). Analysis was performed in SAS software version 9.1.3a.
Results
Of the 211 cases that met the inclusion criteria for this study, 199 (94.3%) were canine patients and 12 (5.7%) were feline patients. Mean age of these patients at the onset of their seizures was 43 mo (range 5–156 mo). There were a total of 80 (38%) females and 131 (62%) males. Of the 211 total patients, 91 (43%) were diagnosed with advanced brain imaging (41 of 91 CT, 50 of 91 MRI). The remaining 120 (57%) patients were presumptively diagnosed on the basis of history, clinical signs, normal interictal neurologic examination, and routine laboratory work showing no significant abnormalities.
A total of 211 patients had a complete blood count and biochemical profile performed. There were 107 urinalysis, 50 MRI, 41 CT scans, and 88 cerebrospinal fluid analyses completed. Of the serological tests performed, 58 patients were tested for coccidioidomycosis; 35 patients had a complete tick panel performed, including Ehrlichia, Lyme, and Rocky Mountain Spotted Fever serology; 14 patients had Ehrlichia only serology; 19 were tested for toxoplasmosis; 5 for neospora; 4 had a fungal profile, including blastomyces, histoplasma, Aspergillus, and Cryptococcus; 3 were tested for Cryptococcus only; 4 for distemper; 7 for feline immunodeficiency virus/feline leukemia virus; 5 for feline corona virus; and 105 patients had a total thyroxine performed. All serological tests were negative for exposure or active infection.
There were a total of 58 breeds of canine patients represented in this study. The most prevalent breeds included 13 golden retrievers, 11 Labrador retrievers, 10 Labrador retriever mixed breeds, 8 rottweiler, 8 Siberian huskies, 8 German shepherds, 6 Jack Russell terriers, 6 American Staffordshire terriers, 6 Shetland sheepdogs, and 5 Doberman pinschers. Patients of other breeds were represented in numbers of four or less.
A total of 2,507 individual seizure episodes among 205 patients were identified for statistical analysis occurring between January 4, 2000 and December 24, 2008. Six of the original 211 patients were excluded from this analysis based on absent seizure date information in the medical record. The information on those patients was used only for demographic description. The time period in this study encompassed a total of 3,278 days or 111 complete lunar cycles. A breakdown of the percentage of seizure episodes across each phase is shown in Figure 1.
Citation: Journal of the American Animal Hospital Association 47, 5; 10.5326/JAAHA-MS-5591
The first statistical model evaluated differences in seizure frequency across the eight lunar phases. The sample size for the analysis consisted of 205 patients and 1,640 observations (eight observations for each patient). The seizure counts did not vary across phases in the omnibus test (χ2 [7]=5.12; P=0.645). Follow-up contrasts were performed to test the mean seizure rate in each phase against the mean rates for the seven remaining phases. The difference in seizure frequency was not statistically significant (χ2 tests; P>0.05) in all comparisons. The expected seizure counts in each phase and their 95% confidence intervals are presented in Figure 2. The overlapping confidence intervals were consistent with the results of the hypothesis tests, indicating no population variability in seizure counts by lunar phase. When the data were examined to include the specified covariates (species, sex, age or onset, or the confidence of the diagnosis), dogs were found to have statistically more seizures than cats (incidence rate ratio [IRR]=2.753; P=0.005), and patients with an older onset of seizures (>60 mo) had statistically fewer seizure episodes (total) than patients with either a young (IRR=0.433; P=0.038) or medium age onset of seizures (IRR=0.417; P=0.002). Sex (IRR=1.039; P=0.850) or strength of diagnosis (IRR=0.884; P=0.618) had no effect on seizure frequency.
Citation: Journal of the American Animal Hospital Association 47, 5; 10.5326/JAAHA-MS-5591
The second statistical model evaluated differences in seizure frequency across the 8 days in each lunar cycle that fell on the exact phase dates. The sample size for the analysis again consisted of 205 patients and 1,640 observations. The seizure counts did not vary across phases in the omnibus test (χ2 [7]=2.70; P=0.911), nor did the subsequent contrasts detect a significant difference (χ2 tests; P>0.05) in seizure counts between each of the phases. The expected seizure counts on each specific phase date and their 95% confidence intervals are presented in Figure 3. When the data were examined to include the specified covariates (species, sex, age or onset, or the confidence of the diagnosis), dogs again had statistically more seizures than cats (IRR=2.335; P=0.007). Patients with an older age onset of epilepsy (>60 mo) again had significantly fewer seizures than patients with an onset of seizures between 1 and 5 yr of age (IRR=0.420; P=0.01), but had no difference in seizure rates against young onset patients (IRR=0.535; P=0.220). Sex (IRR =1.024; P=0.938) or strength of diagnosis (IRR=1.038; P=0.895) had no effect on seizure frequency.
Citation: Journal of the American Animal Hospital Association 47, 5; 10.5326/JAAHA-MS-5591
The last model examined the linear effect of lunar illumination on seizure frequency. All days in this model were coded as having 0%, 25%, 50%, 75%, and 100% illumination. The effect of lunar illumination on seizure occurrence was not statistically significant (IRR=0.933; P=0.615). When the data were examined to include the specified covariates, dogs had statistically more seizures than cats (IRR=2.42; P=0.014). Sex (IRR=1.131; P=0.607), strength of diagnosis (IRR=0.896; P=0.655), or age of onset (IRR=0.896, P=0.829) had no effect on seizure frequency.
Discussion
The results of this study indicated that there was no correlation between the lunar cycle and seizure occurrence in canine and feline patients with idiopathic epilepsy. Epileptic events were evaluated based on three models looking at seizure occurrence in each phase of the lunar cycle, on the specific phase dates of the lunar cycle, as well as by the percent illumination of the night sky. In each model, there was no significant difference in the overall rate of seizure occurrence.
The retrospective nature of this study produced inherent limitations, including a lack of standardized reporting for each of the seizure episodes by date, time, or duration. Many recorded seizure episodes had to be excluded from this study due to inability to determine from the medical record an exact date on which a seizure occurred. Additionally, it was likely that many seizure episodes went unreported by owners, thereby also affecting an accurate depiction of all seizures experienced by individual patients. These unclassified or unreported episodes could theoretically have had an effect on the overall seizure incidence rate by phase. However, it was more likely that the random nature by which these events were excluded from the analysis would ultimately be balanced across each of the eight phases, thereby negating the impact these missing episodes might have had on the overall seizure rate by phase. Time of onset for each seizure was also not available; therefore, no evaluation of a diurnal distribution of seizure activity could be evaluated.
A potential weakness of this study was that only 43% of the patients had advanced imaging to attain their diagnosis. As a result, a patient with an improper diagnosis of idiopathic epilepsy could have been included in this study. To address this concern, the overall seizure rates between patients definitively and presumptively diagnosed with idiopathic epilepsy were compared. No significant difference in seizure rate was identified in each of the three statistical models examined.
It was possible that the relatively small number of patients and individual seizure episodes evaluated in this study might have overstated the lack of influence of the lunar cycle on overall seizure incidence rates, as large numbers are often needed to show a small, but significant, effect. A prospective, multi-institutional, multigeographical study with greater patient numbers would be needed to collect enough data, such that a small effect of the lunar cycle might become evident. If such a relationship were determined between seizure events and the lunar cycle, it was worth considering that such a relationship might be too small to be of clinical significance for the general population of pets with idiopathic epilepsy. The conclusion was similar in the 2006 veterinary paper in which the small (but significant) increase in patient emergency room admissions during the full moon phase was too small to affect staffing concerns.20
It was also worth considering that a subset of pets within this study might have had a positive seizure correlation to a particular lunar phase and that this relationship was eclipsed by the lack of a relationship of the study population as a whole to the lunar cycle. This relationship could only be determined by evaluating an individual patient's seizure calendar over a period of time to reveal a pattern and thus provide an opportunity for possible pre-emptive therapy. This possibility highlighted the need for accurate and up-to-date seizure charts on all patients with idiopathic epilepsy.
In one human study published in 2008, it was the overall percent illumination based on the combination of lunar phase, area cloud cover, and precipitation that was determined to have a significant effect on the rate of epileptic seizures.12 In that study, it was determined that periods having lesser illumination corresponded positively with increased seizure occurrence.12 In this study, percent illumination by lunar phase was calculated without taking into consideration cloud cover or precipitation. These factors were not thought to play a significant role in altering illumination periods in this study, as patients in Las Vegas, NV, were exposed on average to only 73 cloudy days per year and only 4.1 inches of rainfall24. More significant in this study might be the overall effect city lights might have on illumination period, causing a relatively consistent degree of evening illumination irrespective of the lunar cycle. It was possible this factor might play a role in the relatively constant rate of seizure occurrence seen in patients in this area.
A recent theory proposed that it was the amount of light a patient was exposed to throughout the day and not lunar cycle that contributed to seizure frequency, with fewer seizures occurring on days and in seasons with a greater duration and strength of sunshine.25 The reason proposed was a possible protective effect of melatonin, a hormone produced by the pineal gland that is involved with regulation of circadian rhythm and light–dark cycles.25 Melatonin production is suppressed during periods of light and is increased at night.26 Increased strength and duration of light exposure during the day corresponds with a greater degree of melatonin suppression, and, similarly, a darker night will correspond with greater melatonin production.26 As previously mentioned, the degree of sunlight exposure in this region was fairly constant, with duration of exposure varying mainly by season. However, the greater evening exposure to city lights in this area could theoretically have altered the amount of melatonin produced at night and, therefore, increased seizure susceptibility. Under this theory, seizure activity would be expected to be relatively constant in relation to lunar cycle, as was found in this study, but could show a seasonal variation. Further studies to evaluate this possibility would be of interest.
Conclusion
The phases of the moon did not have a statistically significant effect on the rate of seizure occurrence in canine or feline patients with idiopathic epilepsy. Based on this finding, alteration in anticonvulsant treatment or monitoring of patients with idiopathic epilepsy at large was not warranted based on phases of the lunar cycle. Due to individual patient variability in seizure occurrence, certain patients might have had predictable seizure patterns that would have allowed for pre-emptive therapeutic intervention, which the authors suggested highlighted the importance of keeping accurate and up-to-date seizure records on all patients with idiopathic epilepsy.
Differential distribution (percentage) of seizure episodes by lunar phase for patients with idiopathic epilepsy between January 4, 2000 and December 24, 2008.
Mean seizure count by lunar phase, including 95% confidence intervals.
Mean seizure count by exact lunar phase date, including 95% confidence intervals.
Contributor Notes
