Alteration in mental state – an introduction

4 Alteration in mental state – an introduction



INTRODUCTION


This is the most common reason for animals to be presented to the veterinarian. Owners remark that the animal is ‘not himself’, ‘not right’. When pressed for details, the next observation is just as non-specific: lethargy, depression. This should be no surprise; many diseases result in a less reactive mental state, drowsiness or apparent indifference to stimuli.


Alertness is promoted by the ascending reticular activating system (ARAS), a network of neurons projecting from the brainstem to the cerebral cortex. Damage to the ARAS alters the level of consciousness.



CLINICAL TIP


Not all lethargic animals have brain disease.



Presenting signs


Some animals are described as having a ‘far-off’ gaze, or staring, vacant expression. This may signal a depressed state of alertness. Mydriasis, blindness, a lack of blinking, and a rigid neck posture can create the same impression. Deafness and exercise intolerance will alter an animal’s responsiveness to commands.



What is consciousness?


Consciousness is the state of awareness of the self and the environment. The former is anthropomorphized by owners, while the latter is used by veterinarians to judge the level of mental responsiveness. This state of awareness relies on the animal being alert.


Consciousness is graded by the decreasing level of function from alert to comatose. The term semi-comatose has been used in texts, but as coma is defined as a total lack of responsiveness to stimuli, using the prefix ‘semi-’ can confuse communication.



Describing the mental state



1. Alert
• Conscious; capable of responding to sensory stimuli. A more clinically useful statement would be to describe what the animal is responsive to and if that response is normal or appropriate.

2. Obtunded/depressed/lethargic
• A mild to moderate decreased interaction with or interest in the environment

• Responds slowly to verbal stimuli

• Increased hours of sleep.

3. Stupor
• Unresponsive unless roused by a noxious stimulus

• Appears to be in a deep sleep to which it returns once the stimulus has ceased.

4. Coma
• Complete unresponsiveness to any stimuli. Unrousable

• Graded according to a scale for monitoring progression of causative disease.




Delirium

In the human this is characterized by disorientation, fear, irritability, agitation and misperception of sensory stimuli which may be episodic and interspersed with lucid intervals. It results from a generalized impairment of brain function and occurs in metabolic or toxic disorders or multifocal cerebral disease. It may precede or follow stupor/coma.



Dementia

This is a persistent insidious reduction in mental awareness or understanding in an alert patient. The term is generally applied, in humans, to those with a chronic problem with little chance of improvement, e.g. degeneration, trauma, tumour.



Localizing the lesion


Stupor and coma result from one or both of the following:


• Diffuse, bilateral cerebral impairment

• Failure of the rostral brainstem ARAS within the mid-pons, extending rostrally to the thalamus.


CLINICAL TIP


Stupor and coma usually result from sub-acute or acute brain disease


A lesion in the compact brainstem damages a larger proportion of the ARAS with a more profound effect on consciousness than would the same sized lesion in the cerebrum which only picks off a small fraction of the now-dispersed fibres.



CLINICAL TIP


It is more common for coma to result from brainstem lesions than from cerebral disease


Five observations aid in specifying the area of dysfunction:


1. State of consciousness

2. Pattern of breathing

3. Size and reactivity of the pupils

4. Eye movements and oculovestibular responses

5. Skeletal muscle motor responses.


RESPIRATORY PATTERN



Normal respiration


Respiration maintains normal oxygenation and acid–base balance. Abnormalities can alter mental state, exercise ability and voice production.



Inspiratory respiratory muscles



• Diaphragm: accounts for most of thoracic volume change during quiet inspiration. Innervated by the phrenic nerve which originates from cord segments and nerve roots C5–7, found at vertebral level C4–C6

• External intercostal muscles elevate the lower ribs. Innervated by segmental spinal nerves.

Either muscle set alone can provide adequate ventilation at rest. Most of the work of breathing is moving the elastic tissues of the thoracic wall and lungs.



Expiratory respiratory muscles



• Used in forced expiration. The internal intercostals muscles and the abdominal muscles contract the rib cage caudally, decreasing intrathoracic volume. Intra-abdominal pressure is increased, pushing the diaphragm cranially.

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Alteration in mental state – an introduction
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