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Neurophysiological assessment of coma

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Definitions

• Consciousness is the state of awareness of self and the environment and coma is its opposite. That is the total absence of awareness of self and the environment even with external stimulation.

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Between these poles there is :

– Confusion…………bewildered but attentive– delerium…………..irritable,out of contact but

alert– obtundation……….reduced alertness, slowed

responses– stupor……………..responsive only with

vigorous and repeated stimuli

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Other states of importance• Vegetative state……The return of alertness but

no evidence of cognitive function• Apallic syndrome…..Essentially the same as

vegetative state• Akinetic mutism……Silent alert immobility,

minimal motor response to noxious stimulation.• Locked in syndrome..Evidence of cognitive

function as distinct from akinetic mutism.

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The state of consciousness reflects

• 1. The level of arousal – Arousal depends on the integrity of brain stem

function, in particular the ARAS

• 2. Cognitive function– Cognitive function predominantly depends on

cortical and thalamocortical integrity.

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Clinical approach to the comatose patient

• Pathology must involve either– Bilateral hemispheres……supratentorial– The ARAS……………….subtentorial– Diffuse…………………...toxic/metabolic

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Patients can usually be categorised into these groups – History– Assessment of arousal– pupillary responses– eye movements– corneal responses– breathing pattern– motor patterns– deep tendon reflexes

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Supratentorial

• Initiating signs usually of focal cerebral dysfunction

• Signs of dysfunction progress rostral to caudal

• Motor signs often asymmetrical

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Fig 19

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Fig 20

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Fig 21

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Fig 22

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Fig 23

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Subtentorial

• History of brain stem dysfunction or sudden onset coma

• Oculovestibular features often precede or accompany coma

• Cranial nerve palsies present

• Bizarre respiratory patterns common and at onset

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Toxic/Metabolic

• Confusion,delirium,stupor,obtundation precede motor signs

• Pupillary responses usually preserved

• motor signs usually symmetrical

• Asterixis,myoclonus,seizures common

• Acid-base imbalance with hypo or hyperventillation

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Psychogenic unresponsiveness

• eyelids actively close

• icewater calorics induce nystagmus

• EEG normal

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The role of EEG in diagnosis

• Differentiates coma from psychogenic unresponsiveness• Identifies non convulsive status epilepticus• Bilateral delta indicates the patient is either deeply

asleep or unconscious• A normal EEG rules out metabolic brain disease as a

cause of coma.• A “normal” EEG in delerium strongly suggests an

alcohol or drug withdrawal state• The degree of slowing usually reflects the severity of the

metabolic encephalopathy

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• In diffuse metabolic encephalopathy the EEG is usually more sensitive than the clinical assessment with slowing still present when the patient has returned to clinical normality.

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The role of EEG in prognosis of anoxic cerebral injury.

• Five Grades

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Grade 1. “Near normal”

– Excellent prognosis unless “locked in”or alpha pattern coma

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Grade 2. Theta dominant

– If reactive the prognosis is very good– If nonreactive survival is usually accompanied

by neurological sequelae

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Grade 3. Delta dominant

– If reactive the prognosis can be good– If non-reactive the prognosis is grave provided

drugs and hypothermia excluded.

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Grade 4. Burst suppression & continuous bilateral periodic

sharp waves– Prognosis grave if drugs and hypothermia

excluded– Often associated with clinical myoclonus.

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Grade 5. Isoelectric

– Prognosis grave if drugs and hypothermia excluded.

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Rare Variants

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Alpha pattern coma

– Anterior predominance– Unreactive alpha frequency activity.– Rare survivors but only if brain stem reflexes

intact.

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Theta pattern coma

– Usually elderly– 5 Hz theta with low amplitude burst

suppression morphology– Grave prognosis

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Spindle coma

– Usually head injury, rarely anoxic injury– resembles stage II sleep – prognostically benign.

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The role of EEG in coma prognosis in anoxic injury

• The difficult group are grade II nonreactives and grade III. These are also the most common groups.

• SEPs are useful to further define the prognosis in these groups.

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The role of EEG in prognosis in severe head injury

• EEG is considerably more limited in prognostication in severe head injury.

• Reactivity may be the most useful parameter for classifying outcome into “good” vs “bad” – Good being moderately disabled or better– Bad being worse than moderately disabled.

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Reactivity can be • Attenuation• Paradoxical (high amplitude slow waves)• Doubtful/Uncertain• absent.

• 90 + % of patients with preserved reactivity of either type have “good”outcomes

• 90 +% of patients with absent reactivity have bad outcomes.

• 20 +% have “uncertain” reactivity & 70+ % of these have good outcomes.

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Fig 1

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The role of SEPs in anoxic cerebral injury and severe head

injury– The bilateral absent of the “thalamo-cortical” wave forms (N19, N20, N1)signifies that the patient will not recover to better than PVS………….100% specificity

– However sensitivity is low (20-30 %).– Hence the interest in the N70

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N70

• Madl et al– “Of 113 patients with a bilateral N70 peak latency

>130 msec or absent all but one had a poor outcome”

– Sensitivity of 94% and specificity of 97%

• Sherman et al– Using a bilateral N70 peak latency > 176 msec all

had a poor outcome– Sensitivity 78% and specificity of 100%