A practical definition of status epilepticus (SE) is that of a continuous series of two or more discrete seizures lasting at least 5 minutes between which there is incomplete recovery of consciousness. This definition helps to guide the clinician in treatment specifically intended to reduce neurologic injury and is distinct from our definition of cluster seizures. Cluster seizures are two or more seizures occurring over a relatively brief period (i.e., minutes to 24 hours) but with the patient regaining consciousness between the seizures. The aim of this article is to address what we do know and what we still need to evaluate. There is little veterinary literature that evaluates SE and cluster seizures but what is available is compiled below to aid the reader get an evidence-based overview of these conditions.

A sagittal T2-weighted magnetic resonance image of an 11-year-old cross-breed with an olfactory/frontal lobe tumour (arrow). It must be remembered that status epilepticus may be the result of such lesions.

Cerebrospinal fluid analysis can reveal inflammatory CNS disease in cases of status epilepticus

Epidemiology
Twelve to 30% of adult human patients with a new diagnosis of epilepsy first present in status epilepticus. The prevalence of dogs with either SE and / or cluster seizures has been estimated to be 0.44% of the total hospital admissions. The prevalence of SE in dogs with idiopathic epilepsy has just been evaluated by the authors; 407 dogs with idiopathic epilepsy confirmed with normal CSF and MRI testing were evaluated for the historical presence of SE or cluster seizure activity. There were a total of 10 cases with SE (2.5%) and 166 cases with cluster seizures (41%) and there was no association between dogs with either condition. In this study there were no breed influences on either SE or cluster seizures. One study found the English Foxhound, Pugs, Teacup Poodles, Boston Terriers and Lakeland Terriers were significantly over-represented, although dogs evaluated had many different causes of the seizure activity. Another study has shown that an increased body weight was the only variable significantly different between dogs with idiopathic epilepsy that did and did not have episodes of SE.

The mean age of dogs with SE based on three studies is 4.5 years (range = 0.1-15.0 years). When idiopathic epileptic dogs were evaluated, there was no influence of gender (even when the effect of neutering was evaluated) on the presence of SE which agrees with other studies. However, entire dogs with idiopathic epilepsy were found to be 1.9 times more likely than neutered dogs to experience cluster seizures!

A study of 50 dogs with SE found that up-to 44% of dogs may experience this event without any prior evidence of a seizure abnormality. The underlying cause of the SE in this study was variable. It was nearly 5 times more likely for SE to occur when there had been no previous evidence of seizure activity than when dogs had experienced previous generalised tonic clonic seizure activity.

Diagnostic Testing
Of 50 dogs with SE of variable causes, 36% of CSF samples were abnormal based upon elevated cell counts, and / or protein levels, and / or abnormal cytological interpretation of a cytocentrifuged sample. When non-SE dogs had cisternal CSF samples analysed only 12% of these samples were abnormal based upon the above criteria. Although these results were clinically different and would suggest that SE dogs would be more likely to have an abnormal CSF tap in comparison to non-SE dogs, this was not found to be statistically significant. Cerebrospinal fluid (CSF) abnormalities were documented in 73.5% (75/102) of dogs with either SE or cluster seizures in another study but these results were not compared to another population of dogs. The results of CT scans evaluated for SE dogs found 46% to be abnormal. The abnormalities demonstrated included mass lesions, multifocal parenchymal hyperdensities and contrast-enhancing lesions. When these results were compared to non-SE seizure dogs 32% were interpreted as abnormal and no statistical difference between the two groups was found. Neuroimaging examinations (magnetic resonance imaging or computed tomography) performed on 25 dogs with either SE or cluster seizures yielded a positive finding in 19 (76%) of the cases There have been no veterinary studies looking solely at the frequency of MRI abnormalities found in dogs with SE.

Underlying Causes
A case-controlled cohort study evaluating 50 dogs with generalised convulsive SE found 28% of the dogs were diagnosed with primary epilepsy, 32% were diagnosed with secondary epilepsy and 12% were diagnosed with reactive epilepsy. A specific cause could not be determined in 28% of the cases. In another study evaluating 156 dogs with either SE or cluster seizures, approximately 27% of the cases were diagnosed with primary epilepsy, with 35% of the cases having secondary epilepsy whilst reactive epileptic seizures were seen in 7% of the cases. Chronic processes that cause SE include pre-existing epilepsy in which SE is caused by breakthrough seizures or the discontinuation of anti-epileptic drugs. A low anti-epileptic drug concentration was determined to be the cause of the seizures in almost 6% of the cases in the above study.

Outcome
Approximately 25% of 156 dogs with SE or cluster seizures, died or were euthanased in one study. No significant associations were observed between the outcome of dogs with SE or cluster seizures and the breed of the dog, the dog’s age at onset of seizure activity, the type of seizure activity at admission or the findings on CSF analysis. However a significant negative association was identified between the outcome and the diagnosis of granulomatous meningoencephalitis (GME) and the outcome and loss of control of the seizure activity at 6 hours after admission. Six of 19 (32%) dogs that had episodes of SE in another investigation, died of causes directly attributed to the seizure disorder. Mean life spans of dogs that did and did not have episodes of SE were 8.3 and 11.3 years, respectively in this study and survival time was significantly different between groups. As more evidence accumulates about SE and cluster seizures, we can become more comfortable advising the owners about their concerns. Until all the information is at hand, aggressive early treatment and diagnosis with realistic expectations remain pivotal.

References

Platt SR, Haag M. Canine status epilepticus: a retrospective study of 50 cases. J Small Anim Pract. 2002 Apr;43(4):151-3.

Saito M, Munana KR, Sharp NJ, Olby NJ. Risk factors for development of status epilepticus in dogs with idiopathic epilepsy and effects of status epilepticus on outcome and survival time: 32 cases (1990-1996). J Am Vet Med Assoc. 2001 Sep 1;219(5):618-23

Bateman SW, Parent JM. Clinical findings, treatment, and outcome of dogs with status epilepticus or cluster seizures: 156 cases (1990-1995). J Am Vet Med Assoc. 1999 Nov 15;215(10):1463-8.

Platt SR, McDonnell JJ. Status epilepticus: clinical features and pathophysiology. Compend Cont Ed Pract Vet 2000; 22(7): 660-669.