Abstract
A yearling California sea lion (Zalophus californianus) was admitted into rehabilitation with signs of cerebellar pathology. Diagnostic imaging that included radiography and magnetic resonance imaging (MRI) demonstrated space-occupying lesions predominantly in the cerebellum that were filled partially by CSF-like fluid and partially by gas, and cerebral lesions that were fluid filled. Over a maximum period of 4 months, the brain lesions reduced in size and the gas resorbed and was replaced by CSF-like fluid. In humans, the cerebellum is known to be essential for automating practiced movement patterns (e.g., learning to touch-type), also known as procedural learning or the consolidation of “motor memory.” To test the animal in this study for motor memory deficits, an alternation task in a two-choice maze was utilized. The sea lion performed poorly similar to another case of pneumocerebellum previously reported, and contrary to data acquired from a group of sea lions with specific hippocampal injury. The learning deficits were attributed to the cerebellar injury. These data provide important insight both to the clinical presentation and behavioral observations of cerebellar injury in sea lions, as well as providing an initial model for long-term outcome following cerebellar injury. The specific etiology of the gas could not be determined. The live status of the patient with recovery suggests that the most likely etiologies for the gas are either de novo formation or air emboli secondary to trauma. A small air gun pellet was present within and was removed from soft tissues adjacent to the tympanic bulla. While no evidence to support the pellet striking bone was found, altered dive pattern associated with this human interaction may have provided the opportunity for gas bubble formation to occur. The similarity in distribution of the gas bubble related lesions in this case compared with another previously published case of pneumocerebellum suggests that preferential perfusion of the brain, and more specifically the cerebellum, may occur during diving events.
Highlights
Diving marine mammals have elaborate anatomical, physiological, and behavioral adaptations to reduce N2 gas loading the detailed mechanics of gas kinetics is a contentious subject area of renewed and growing interest among scientists (Hooker et al, 2012)
Microfilaremia due to the filarid nematode Acanthocheilonema odenhali is a common finding in California sea lions admitted to The Marine Mammal Center (TMMC) (Dailey, 2001)
The lesions in the cerebellum were strikingly similar to those documented in our initial case (Van Bonn et al, 2011)
Summary
Diving marine mammals have elaborate anatomical, physiological, and behavioral adaptations to reduce N2 gas loading the detailed mechanics of gas kinetics is a contentious subject area of renewed and growing interest among scientists (Hooker et al, 2012). A single case of pneumocerebellum in a California sea lion was recently described (Van Bonn et al, 2011) While this case has striking similarities to the previously published case, the clinical and behavioral assessments for motor learning that were possible combined with the survival of the animal provides valuable information for veterinarians, veterinary technicians, animal care specialists, rehabilitators, behavioralists, trainers, and physiologists on both gas bubble related pathology and space-occupying encephalopathies in sea lions in general. These data provided here will assist care-givers in the prognosis and expectations following marine mammal cerebellar injury. The data raises questions about perfusion during dives, with apparent preferential perfusion to the cerebellum, as well as demonstrating the potential for gas bubble related pathologies when normal protective mechanisms are disrupted
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