Complimentary examinations

Although the bedside examination is the standard method of measuring neurological function, complimentary examinations such as electroencephalography, evoked potentioals and brain scanning are of help to objectively measure brain damage and residual neuronal function in coma and related conditions.

 

Electroencephalogram (EEG)

Electroencephalography (EEG) is an objective tool that permits continuous and online monitoring of brain function.   It detects spontaneous brain electrical activity from the scalp. To measure the EEG, electrodes are attached to the scalp with a conducting paste or with needles.

In comatose states, traditional EEG measures provide valuable insight into brain function by demonstrating focal or diffuse background abnormalities and epileptiform abnormalities [1, 2]. The EEG also has prognostic value in coma.

 

Evoked potentials (ERPs)

An evoked potential (EP) or event-related potential (ERP) is the time-locked average of the EEG in response to a specific sensory, motor or cognitive event.

Evoked potentials have been used for a long time to assess comatose patients.

Most important are the prognostic value of somatosensory evoked potentials in coma.

“Exogenous” ERPs are tightly time-locked to the presentation of an external stimulus and depend on the physical properties of the sensory stimuli used to elicit them (e.g., brainstem auditory, somatosensory, and visual evoked potentials). Somatosensory evoked potentials (SEPs) are elicited by electrical stimulation of a peripheral nerve (e.g., the median nerve at the wrist). They are an important and routinely used means of monitoring the functional integrity of sensory pathways. Early components of these potentials arising within 100 milliseconds are known to persistent even in unconscious states.

The later components of exogenous potentials and other so-called “endogenous” ERP components (e.g., auditory mismatch negativity [3] and P300) are more reliably related to the (conscious or unconscious) cognitive processing of the information, and less frequently observed in disorders of consciousness [for review see for example 4]. The presence of a mismatch negativity correlates with recovery of consciousness or minimal consciousness [3].

 

Brain X-ray CT

Despite its wide availability, CT (computed tomography), has been replaced by the more sensitive MRI as the procedure of choice for brain imaging. CT is mainly useful when rapid information about the state of the brain is desired. In particular, it helps making choice between surgical and medical management of patients with coma. Such conditions include trauma, and stroke (where a differentiation between hemorrhage and infarction is important). CT is also widely used for the evaluation of lesions that involve bone (e.g., fractures) of the brain.

 

Brain Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) stands for a vast and varied array of techniques that provide an enormous range of information. From an established ability to provide high-quality structural information, MR techniques are rapidly advancing and provide other clinically relevant physiologic information as spectroscopic studies illuminating the details of biochemical status (MR spectroscopy), blood oxygenation level allowing functional activation studies (functional MRI or fMRI), cerebral blood compartment (MR angiography); perfusion (perfusion-weighted MRI), water molecular diffusion (diffusion-weighted imaging), cerebral microstructure and fiber tracking (using diffusion anisotropy effects measured by diffusion tensor imaging or DTI), magnetization transfer (MT) imaging, et cetera.
At present, MRI is the procedure of choice for the structural imaging of the brain. However, it is susceptible to movement artifacts and patients who are on life support systems, have gunshot wounds, or who have implanted MRI incompatible material (pace-makers, prostheses…), still represent problems.

fMRI is the procedure of choice for hemodynamic functional activation measurements (measuring the brain’s response to external stimuli). It can detect an increase in oxygen that occurs in an area of heightened neuronal activity.

 

Positron Emission Tomography

Although PET is primarily a research tool for brain imaging, its increasing availability in medical centers for oncology and cardiac imaging makes likely more widespread application to neurological diseases. The most frequently performed PET studies measure resting regional cerebral metabolic rates for glucose. PET measuring changes in regional cerebral as indirect index of neural synaptic activity is being replaced by functional MRI.

PET scanning involves the administration of positron-emitting radionuclides with short half-lives in which particle disintegration is captured by multiple sensors positioned around the head. The radiotracer is administered into a vein in the arm and is taken up by the brain through the bloodstream. PET studies involve the use of a cyclotron to produce the radioactive tracers. The type of information of the PET image is determined by the administered radiolabeled compound.

To study regional cerebral glucose utilization, a positron-labeled deoxyglucose tracer is used (i.e., [18F]fluorodeoxyglucose - FDG). This tracer is taken up by active brain regions as if it was glucose. An FDG-PET scan summates approximately 30 minutes of cerebral glucose metabolism and allows assessment of regional variations.

 

Cerebral angiography and transcranial Doppler sonography

Cerebral angiography and transcranial Doppler sonography [5] can be used with very high sensitivity and 100% specificity to document the absence of cerebral blood flow in brain death [6,7].

 

References

Brain imaging
Laureys S, Peigneux P, Goldman S In: Biological Psychiatry (Vol 1) D’haenen H, den Boer JA, Willner P (Eds), John Wiley & Sons Ltd, New York (2002) 155-166

 

1.         Firosh Khan, S., Ashalatha, R., Thomas, S.V., and Sarma, P.S. (2005). Emergent EEG is helpful in neurology critical care practice. Clin Neurophysiol  116, 2454-2459.
2.         Brenner, R.P. (2005). The interpretation of the EEG in stupor and coma. Neurologist  11, 271-284.
3.         Fischer, C., et al. (2006). Improved prediction of awakening or nonawakening from severe anoxic coma using tree-based classification analysis. Critical Care Medicine  34, 1520-1524.
4.         Kotchoubey, B., Event-related potential measures of consciousness: two equations with three unknown, in The boundaries of consciousness: neurobiology and neuropathology, S. Laureys, Editor. 2005, Elsevier: Amsterdam. p. 427-444.
5.         Ducrocq, X., et al. (1998). Consensus opinion on diagnosis of cerebral circulatory arrest using Doppler-sonography: Task Force Group on cerebral death of the Neurosonology Research Group of the World Federation of Neurology. J Neurol Sci  159, 145-150.
6.         Wijdicks, E.F.M., Confirmatory testing of brain death in adults, in Brain death, E.F.M. Wijdicks, Editor. 2001, Lippincott Williams & Wilkins: Philadelphia. p. 61-90.

7.         Laureys, S. (2005). Death, unconsciousness and the brain. Nat Rev Neurosci  6, 899-909.