Prevalence of nonconvulsive seizures and nonconvulsive status epilepticus in dogs and cats with a history of cluster seizures: A retrospective study

Abstract

Background: Nonconvulsive seizures (NCS) and nonconvulsive status epilepticus (NCSE) are frequently observed in human patients. Diagnosis of NCS and NCSE only can be achieved by the use of electroencephalography (EEG). Electroencephalographic monitoring is rare in veterinary medicine and consequently there is limited data on frequency of NCS and NCSE.

Objectives: Determine the prevalence of NCS and NCSE in dogs and cats with a history of cluster seizures.

Animals: Twenty-six dogs and 12 cats.

Methods: Retrospective study. Medical records of dogs and cats with cluster seizures were reviewed. Electroencephalography was performed in order to identify electrographic seizure activity after the apparent cessation of convulsive seizure activity.

Results: Nonconvulsive seizures were detected in 9 dogs and 2 cats out of the 38 patients (29%). Nonconvulsive status epilepticus was detected in 4 dogs and 2 cats (16%). Five patients had both NCS and NCSE. A decreased level of consciousness was evident in 6/11 patients with NCS, 3/6 also had NCSE. Mortality rate for patients with NCS (73%) and NCSE (67%) was much higher than that for patients with no seizure activity on EEG (27%).

Conclusion and clinical importance: Prevalence of NCS and NCSE is high in dogs and cats with a history of cluster seizures. Nonconvulsive seizures and NCSE are difficult to detect clinically and are associated with higher in hospital mortality rates. Results indicate that prompt EEG monitoring should be performed in dogs and cats with cluster seizures.

Keywords: burst-suppression pattern (BSP); electroencephalography (EEG); electrographic seizure; wireless video-EEG.

FIGURE 1

(A) Electrode placement dorsal view. F = frontal; C = central; O = occipital; T = temporal; G = ground; R = reference. Even numbers are located on the right side of the patient and odd numbers are located on the left side. (B) Electrode placement lateral view.

FIGURE 2

The search in the medical database of the neurology service for the term “seizure” resulted in a total of 390 patient records between June 2021 and December 2022. All inclusion criteria were ultimately met by 38 patients.

FIGURE 3

Modified double‐banana montage (impedance <10 kΩ; high‐pass filter at 0.53 Hz; low‐pass filter at 70 Hz; amplifier sensitivity at 70 μV/cm; time base 3 cm/s; 50 Hz notch filter). (A) EEG and ECG recording of a 10‐year‐old male Domestic Shorthair cat with reactive seizures showing an electrographic seizure. The cat was in a comatose state and showed no clinical seizure activity: ictal generalized rhythm at 6 Hz constantly, monomorphic; notice the amplitude increase at the end of these 10 seconds capture. The ECG revealed a temporary technical artifact. (B and C) EEG and ECG recording of a 2‐year‐old female Yorkshire Terrier with reactive seizures showing bilateral frontal rhythmic discharges at 1 Hz with evolution in frequency, morphology, and amplitude lasting >10 seconds (= electrographic seizure). The dog was in a comatose state and showed mild ear twitching time locked with recurrent electrographic seizures during the 76 minutes EEG recording (= electrographic status epilepticus).

FIGURE 4

Modified double‐banana montage (impedance <10 kΩ; high‐pass filter at 0.53 Hz; low‐pass filter at 70 Hz; amplifier sensitivity at 70 μV/cm; time base 3 cm/s; 50 Hz notch filter). (A) EEG and ECG recording of an 11‐year‐old male Chihuahua with structural epilepsy showing an electrographic status epilepticus. Notice the rhythmic spike‐wave complexes more prominent on the right side (F4‐C4, F4‐T4). This pattern was detectable during the entire EEG recording (132 minutes) and showed mild improvement after IV anticonvulsive medications. No clinical seizure activity was detectable during EEG. (B) EEG and ECG recording of a 6‐year‐old male Chihuahua with structural epilepsy showing electrographic status epilepticus with no convulsive movements. Notice the rhythmic lateralized high‐voltage epileptiform discharges on the right side (F4‐C4, C4‐O2, F4‐T4, T4‐O2). This pattern was visible during the entire EEG recording and showed no improvement after anticonvulsive medication.

FIGURE 5

Electroencephalographic and ECG recording of the 6‐year‐old male Chihuahua (same patient as in Figure 3B) with structural epilepsy during inhalant anesthesia with isoflurane (1%‐2% MAC) showing a burst‐suppression pattern (BSP). Isoelectric line (suppression) interrupted by bursts every 4‐5 seconds defining the burst‐suppression pattern. Modified double‐banana montage (impedance <10 kΩ; high‐pass filter at 0.53 Hz; low‐pass filter at 70 Hz; amplifier sensitivity at 70 μV/cm; time base 3 cm/s; 50 Hz notch filter.