6 Metabolic encephalopathy
Altered behaviour (agitation or lethargy), decreased mental alertness, seizures, exercise intolerance, weakness or collapse commonly occur. Stupor and coma may result.
Cerebral dysfunction appears acutely, episodically, or as an insidiously worsening process depending on the underlying disease, its speed of onset and the duration and severity of the metabolic derangement. Signs may be triggered by additional stresses such as eating, exercise, excitement, drug therapy, surgery and concurrent disease.
Cases of metabolic coma typically have intact and symmetrical PLR and VOR reflexes. Respiration may have altered depth or rate but is regular, in comparison to structural diseases causing coma.
Distinct areas of the CNS may be preferentially susceptible, producing focal signs in a diffuse disease.
Mixed metabolic derangements occur, with no part seemingly severe enough to produce the clinical signs. Mild changes can have an additive effect on cerebral function. Each must be addressed.
Cardiac dysfunction secondary to severe electrolyte disturbances and acute metabolic acidosis may be rapidly lethal.
A description of the animal’s activity and responsiveness is vital. When and how often an abnormal event occurs and how long it lasts is useful for diagnosis and also for monitoring progression of disease and the success of future treatment.
The dog was presented for assessment of a 6-month history of episodic events which commenced with digging activity indoors and hiding in corners. Twitching of the face and body, vocalizing, defecating, and running about the house then followed. The dog was mentally unresponsive during the event and if restrained, its limbs were felt to flex and extend. The events lasted up to 2 hours and occurred at any time of day, with weeks or months between the events. The dog was described as being otherwise normal.
Neurological examination of this case was normal. No video of the event was available.
The lesion was localized to the cerebrum due to the behaviour change. Episodic changes can occur with a persistent structural lesion of the brain or an intermittent metabolic defect. Episodic behaviour change also occurs with psychological conditions in small animals. The normal approach is to rule out organic CNS disease before diagnosing psychological causes.
In all three causes of episodic behaviour change, the animal may appear normal between the episodes. Structural or metabolic lesions can cause subtle abnormalities (e.g. change in house training, change in personality or sleep patterns) between the flamboyantly abnormal events, so it is vital to thoroughly quiz the owner.
An unresponsive mental state can also occur when an animal is agitated or distracted by severe pain, or an acute balance loss, or intense pruritus: the owner tries to distract the animal but the animal pays no attention to the owner. If physical examination of the animal fails to find any other abnormality, then the unresponsive mental state probably results from cerebral dysfunction of some kind.
Some aspects of this patient’s episodes were suggestive of seizure activity; the localization would be the same.