The relevant investigations to be undertaken in the individual patient will be determined by the differential
Box 1.2 Classification of differential diagnosis of coma
Coma without focal or lateralising signs and without meningism Anoxic-ischaemic conditions Metabolic disturbances Intoxications Systemic infections Hyperthermia/hypothermia Epilepsy
Coma without focal or lateralising signs but with meningeal irritation Subarachnoid haemorrhage Meningitis Encephalitis
Coma with focal brain stem or lateralising cerebral signs Cerebral tumour Cerebral haemorrhage Cerebral infarction Cerebral abscess.
diagnosis. In general the role of the investigation in the patient in coma is to help establish the aetiology of the coma and will vary from simple blood tests through more complex blood tests, examination of the cerebrospinal fluid, electrophysiological tests, and imaging. Although the EEG has shown hierarchical value in the assessment of depth of coma and has been used, to an extent, to identify the prognosis in coma,21,22 its major role is in identifying patients who are in a subclinical status epilepticus or who have complex partial seizures, because this will significantly alter their management.23 It may also be useful in distinguishing between feigned or psychiatric coma, in which it will be normal, and genuine cerebral disease, when it shows diffuse abnormalities, or to help identify a focal lesion. The prognostic value of the EEG is probably not as great as that obtained from careful observation of clinical signs,21 though there are suggestions that a combination of clinical assessment and EEG may improve prediction.24
Evoked potentials, predominantly brain stem evoked potentials, and somatosensory evoked potentials may give information relating to the intactness of brain stem pathways and to the existence of a cortical component. Theoretically the use of brain stem evoked responses could provide evidence for the presence and site of the brain stem disease and, as they are relatively unaffected by drug coma, they may provide evidence on the aetiology.24-26 Regrettably there is as yet little correlation between evoked response studies and coma and prognosis, but it seems that the use of somatosensory evoked potentials and brain stem auditory evoked potentials will become of value in identifying the prognosis of patients in coma. One technical problem is the need to undertake these recordings in the busy premises of an intensive care unit where considerable other electrical interference may occur.
Brain imaging techniques, including computerised tomography (CT) and magnetic resonance imaging (MRI), are important in coma in providing evidence of diagnosis.27 CT has a very significant role to play in identifying those patients who have a structural cause for coma. MRI can be even more informative, though there are inevitable problems in inserting the patient in coma, together with the necessary life support systems, into the field of the magnet. Other, more complex techniques, such as intracranial pressure monitoring and cerebral blood flow studies are rarely of help in the diagnosis of medical coma; their role in prognosis is not fully evaluated, and their usefulness is likely to be limited by their invasiveness.28 Measures of biochemical parameters in coma are predominantly diagnostic but some measures, such as brain type creatine kinase and neuron specific enolase in the cerebrospinal fluid (CSF), may help in determining prognosis.29,30
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