Pinniped electroencephalography: Methodology and findings in California sea lions (Zalophus californianus)

Abstract

This study was designed to identify abnormalities in the electroencephalograms (EEGs) recorded from stranded California sea lions (Zalophus californianus) with suspected domoic acid (DA) toxicosis. Recordings from animals presenting for non-neurological issues were also obtained to better understand the normal EEG (background activity and transient events) in this species, as, to date, studies have focused on examining natural sleep in pinnipeds. Most animals were sedated for electrode placement and EEG acquisition with some receiving antiepileptic medications or isoflurane during the procedure. A total of 103 recordings were read and scored from 0 (normal) to 3 (severely abnormal). Epileptiform discharges, consisting of spikes, sharp waves, slow waves, and/or spike waves, were present in all EEGs with scores of 1, 2, or 3. The distribution of these events over the scalp varied. While often generalized, others were lateralized over one hemisphere, bifrontal, bioccipital, and/or bitemporal, while some discharges were multifocal. Findings were different between sea lions and occasionally changed within the EEG on a given sea lion. No clinical seizures were observed during the recording but a few sea lions had findings consistent with electroencephalographic seizures. When available, supporting diagnostic results obtained from magnetic resonance imaging (MRI) and/or necropsy/histopathology were described, as well as the status of those sea lions that recovered and were released with satellite tags.

Keywords: EEG; domoic acid (DA); epilepsy; hippocampus; memory; seizures; spatial ability.

Figure 1

A sea lion with subcutaneous electrodes in place for EEG/EOG (A). Only the hubs are visible (the 12 mm needles project rostrally). A diagram of electrodes with designated labels (B). The modified double banana montage utilized (C). F, frontal; C, central; P, parietal; O, occipital; T, temporal; A, aural; odd numbers, left side; even numbers, right side; z, midline; Z, ground; OS, left eye; OD, right eye (ECG not shown).

Figure 2

Neurologically Normal sea lion (7347) wakefulness EEG pattern with alternating beta activity (>13 Hz, low amplitude at middle and end) and alpha activity (8–13 Hz, medium amplitude at the beginning and between middle and end) (A). Slow wave sleep in a Neurologically Normal sea lion (6326) with ongoing delta activity (<4 Hz high amplitude) and a well-developed sleep spindle before the 4 s mark (vertical line) with smaller ones at the beginning and end of this epoch (B). Both had EEG scores of 0. The same sea lion (6326) during isoflurane anesthesia (C). A 1-s suppression period occurred between the 3 and 4 s marks. Multiple sharp (duration 70–200 ms) and slow waves (>200 ms) can be seen. Superimposed muscle artifact (fast, darker, jagged-looking activity) is present bifrontally (F3 and F4).

Figure 3

Recordings from Neurologically Normal sea lions with mild abnormalities (a score of 1). Sea lion 7155 recording with a large slow wave in the middle of the epoch (A). An example of a typical generalized slow wave preceded by a deep positive (downward) deflection at the same time point in sea lion 8488 (B).

Figure 4

Segment of EEG from a sea lion initially thought to be Neurologically Normal (7689). Continuous generalized abnormal slow wave activity was present throughout this recording (EEG score = 3). The frequency varied between 4 and 5 Hz and may represent an electroencephalographic seizure (no clinical signs were apparent).

Figure 5

A sample of EEG from a Suspected DA toxicosis sea lion (8659, a score of 3) during slow-wave sleep. Several generalized large slow waves are apparent in addition to right hemisphere spikes (duration <70 ms, after the 1- and 2-s marks). A spike-wave discharge is shown between the 5- and 6-s marks. Muscle artifact is present in the EOG channels.

Figure 6

Five s burst of generalized abnormal activity consisting of slow waves, spikes, and sharp waves between normal slow-wave sleep background segments in Suspected DA toxicosis sea lion 6741 (a score of 3).

Figure 7

Quasiperiodic (irregular intervals) generalized and multifocal discharges in Suspected DA toxicosis sea lion 6887 (a score of 3).

Figure 8

Primarily left-sided EEG abnormalities (spikes, sharp waves, and slow waves) with a few spike waves extending into the right hemisphere in Suspected DA toxicosis sea lion 6731 (A). A segment of EEG in Suspected DA toxicosis sea lion 9935 demonstrating various abnormalities with a bitemporal distribution (B). Both had a score of 3.

Figure 9

An unusual recording in Suspected DA toxicosis sea lion 7160 (a score of 3) consisting of continuous spikes (A), spikes devolving into sharp waves (B), both voltage maximum caudally (occipital region). This appears to be an electroencephalographic seizure. Isoflurane administration had profound effects on the EEG pattern (C).

Figure 10

Change in abnormal activity associated with state. Suspected DA toxicosis sea lion 8659 (same as in Figure 5) near the end of the recording with a right-sided discharge during slow-wave sleep (A) and normal wakefulness (B).

Figure 11

Change in EEG associated with isoflurane anesthesia in Suspected DA toxicosis sea lion 6731 (compare with Figure 8A).

Figure 12

The effect of a benzodiazepine on the EEG in sea lion 6667 (a score of 2). Before administration, there are numerous spikes (only the largest one is likely blink artifact) (A). Loss of abnormal fast activity after lorazepam administration (B) and disappearance of all abnormalities associated with wakefulness (C).

Figure 13

Individual generalized events and bursts of sharp and slow waves in Suspected DA toxicosis sea lion 9724 (a score of 3). This animal was diagnosed with white matter lesions on MRI but hippocampi were considered normal. Mild encephalitis and cardiomyopathy were found at necropsy consistent with a protozoal etiology.

Figure 14

Large slow waves and a positive (pointing away) right temporal spike between the 3 and 4 s marks in Suspected DA toxicosis sea lion 6079 (a score of 1). This animal was diagnosed with porencephaly at necropsy.

Figure 15

Histological section of a normal left hippocampus from sea lion 9724 (A). Hippocampal section from one (9821) with severe bilateral atrophy [(B), left shown]. Bar is 1 mm, the same scale for both.