The Hands-Off Evaluation- Consciousness

The first step in the neurological examination should focus on evaluating the animal’s state of consciousness, awareness of its environment, and attitude to being handled.  

Disturbances of state of consciousness are classified in order of severity as depression or obtundation (drowsiness), stupor (semicoma), and coma.  Stupor and coma represent both a state of unconsciousness.  While a stuporous animal can be roused by a painful stimulus, a comatose animal will fail to respond to any environmental stimuli, including pain.  As a rule, altered states of consciousness relate to either a diffuse lesion or widespread multifocal lesions of both cerebral hemispheres or a focal lesion affecting the ascending reticular activating system (ARAS) of the brainstem.  The latter functions to arouse the cerebral cortex and maintain the state of wakefulness.  Acute coma usually results from extensive brainstem lesions, intoxications or a metabolic disorder.  

Common changes in the level of awareness and behaviour include disorientation, aggression, vocalizing, circling, compulsive walking, or head pressing.  Alterations in the patient's level of awareness and behaviour reflect disturbances in the ARAS and limbic system components of the cerebrum or rostral brainstem. 

Note in this dog that there is a marked decrease in the level of awareness and consciousness with an associated confused mental state.

Circling can be caused by lesions in the vestibular system as well as an asymmetrical or focal lesion in the forebrain.  Tight circles are usually but not exclusively associated with a brainstem disorder or a vestibular disorder, while wide circles are often associated with a forebrain lesion. 

With vestibular disease, circling is associated with other signs of vestibular involvement (head tilt, nystagmus, strabismus, and/or falling) and usually ipsilateral to the lesion (except eventually with lesion affecting the caudal cerebellar peduncle, fastigial nucleus and flocculonodular lobes of the cerebellum).  Circling is usually toward the side of a focal or asymmetrical forebrain lesion. 

Hemi-neglect syndrome, also known as hemi-inattention syndrome, refers to an abnormal behaviour in which an animal with structural forebrain disease ignores sensory input from one-half of its environment (e.g. eating out of one-half of the bowl, turning in the wrong direction in response to sound).  This hemineglect syndrome often indicates a thalamic lesion on the side contralateral to the side being ignored by the animal.  

Note that this cat only eats out of the right side of the bowl while it ignores the left side, suggestive of a right-sided forebrain disease.