INTRODUCTION

Although coma can be caused by an array of direct and indirect neurologic disorders affecting the intracranial structures in small animals. The development of coma scales has focused on dogs and cats with head trauma. These tools can be used objectively to decide when to initiate aggressive treatment, to monitor the success of therapy and, in some cases, provide a prognosis. Such scoring systems are in their infancy in veterinary medicine and are meant to be only a guide to management. They can be used to monitor patients with any cause of coma based on this premise. This chapter is focused on coma scales as they specifically relate to head trauma.

Appropriate therapy for patients with head trauma remains controversial in veterinary medicine (see Chapter 152, Traumatic Brain Injury). Treatment must be immediate if the animal is to recover to a level that is both functional and acceptable to the owner. It is important therefore to be aware of the optimal way to assess these patients; many dogs and cats can recover from severe brain injuries if the clinician is able to identify treatable systemic and neurologic abnormalities in a timely manner.

Intracranial Pressure After Head Trauma

Increases in intracranial pressure (ICP) are often responsible for the clinical decline seen in many animals after head trauma.¹ Following traumatic brain injury, the volume of the brain tissue compartment increases, usually a result of edema or hemorrhage. As this tissue compartment increases, the cerebrospinal fluid (CSF) and blood compartments must decrease to prevent an increase in ICP. Compensation for increased brain tissue volume initially involves shifting of cerebrospinal fluid out of the skull, decreased production of CSF and, eventually, decreased cerebral blood flow.² These compensatory mechanisms prevent increases in ICP for an undetermined period. In general, the compensation is more effective when the increases in ICP are slow. Once the capacity for compensation is exhausted, a further small increase in intracranial volume will result in dramatic elevations of ICP, with the immediate onset of clinical signs.² Unfortunately, the clinical signs of significantly increased ICP often become evident too late for any therapy to be effective. A clear understanding of what clinical signs necessitate immediate intervention, those that indicate a grave prognosis, and those that need to be closely monitored will help with the successful treatment of patients following head trauma.

NEUROLOGIC ASSESSMENT

Initial neurologic assessment should include an evaluation of the patient’s state of consciousness, breathing pattern, size and responsiveness of pupils, ocular position and movements, and skeletal motor responses. This should be repeated at least every 30 to 60 minutes in patients with severe head injuries to assess for deterioration or to monitor the efficacy of therapy. This would require an objective mechanism to “score” the patient’s condition so that treatment decisions could be made logically.

MODIFIED GLASGOW COMA SCORING SYSTEM

In humans, traumatic brain injury is graded as mild, moderate, or severe on the basis of the level of consciousness or the Glasgow Coma Scale.³ Mild traumatic brain injury in humans is usually due to a concussion, and full neurologic recovery routinely occurs. A patient with moderate traumatic brain injury is lethargic or stuporous, and a patient with severe injury is comatose. Patients with severe traumatic brain injury have a high risk of hypotension, hypoxemia, and brain swelling.³ If these sequelae are not prevented or managed properly, they can exacerbate the existing brain damage and increase the risk of death.³ The clinical point at which to initiate therapy for a veterinary patient with head trauma, the extent of appropriate therapy, and the length of time that such treatment is necessary are poorly documented. The effectiveness of specific treatment and the prognosis for any given animal will always be difficult to assess because of the multifactorial nature of the injury.

A modification of the Glasgow Coma Scale used in humans has been proposed for use in veterinary medicine.⁴ This scoring system enables grading of the initial neurologic status and serial monitoring of the patient. Such a system can facilitate the assessment of prognosis, which is crucial information for both the veterinarian and owner.⁴ An almost linear correlation between this scoring system and survival of dogs with head trauma has now been evaluated (Figure 97-1).⁵ It should be noted that long-term survival and functional outcome have not been evaluated using these scales in dogs or cats.

Figure 97-1 Probability of survival for patients with head trauma based on their individual modified Glasgow Coma Scale score at admission.

Reprinted with permission from Platt SR, Radaelli ST, McDonnell JJ: The prognostic value of the modified Glasgow Coma Scale in head trauma in dogs, J Vet Intern Med 15:581, 2001.

Each of the three categories of the examination (i.e., level of consciousness, motor activity, brain stem reflexes) is assigned a score from 1 to 6 (Table 97-1). The level of consciousness provides information about the functional capabilities of the cerebral cortex and the ascending reticular activating system in the brain stem.⁶

Table 97-1 Small Animal Modified Glasgow Coma Scale

Assessment Parameter	Score
Motor Activity
Normal gait, normal spinal reflexes	6
Hemiparesis, tetraparesis, or decerebrate activity	5
Recumbent, intermittent extensor rigidity	4
Recumbent, constant extensor rigidity	3
Recumbent, constant extensor rigidity with opisthotonus	2
Recumbent, hypotonia of muscles, depressed or absent spinal reflexes	1
Brain Stem Reflexes
Normal pupillary light reflexes and oculocephalic reflexes	6
Slow pupillary light reflexes and normal to reduced oculocephalic reflexes	5
Bilateral, unresponsive miosis with normal toreduced oculocephalic reflexes	4
Pinpoint pupils with reduced to absent oculocephalic reflexes	3
Unilateral, unresponsive mydriasis with reduced to absent oculocephalic reflexes	2
Bilateral, unresponsive mydriasis with reduced to absent oculocephalic reflexes	1
Level of Consciousness
Occasional periods of alertness and responsiveness to environment	6
Depression or delirium, capable of responding but response may be inappropriate	5
Semicomatose, responsive to visual stimuli	4
Semicomatose, responsive to auditory stimuli	3
Semicomatose, responsive only to repeated noxious stimuli	2
Comatose, unresponsive to repeated noxious stimuli	1

A score is given to each of three categories of the neurologic examination: motor activity, brain stem reflexes, and level of consciousness. Within each category, a score of 1 to 6 exists, representing the most severe to the mildest of clinical pictures. A total score can then be helpful (1) in estimating the severity of the initial condition, which determines the most appropriate level of therapy, (2) assessing the prognosis for survival within the first 72 hours, and (3) assessing the effect of therapy.

Reprinted with permission from Platt SR, Radaelli ST, McDonnell JJ: The prognostic value of the modified Glasgow Coma Scale in head trauma in dogs, J Vet Intern Med 15:581, 2001.