Central nervous system

Published on 06/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

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TOPIC 4 Central nervous system

Assessment of consciousness

Test: Glasgow Coma Scale (GCS)

How it is done

Table 4.1 Glasgow coma scale scores for the three tests, plus variables in children

Best eye response
Score Description
4 Eyes open spontaneously
3 Eyes open to speech. Do not confuse with arousal of sleeping patient
2 Eyes open to pain. Try fingernail bed pressure. Supraorbital pressure will cause grimace and eye closure
1 No eye opening, ensure painful stimulus is adequate
Best verbal response
Score Description
5 Orientated in time, person and place
4 Responds to questions but is disorientated and confused
3 Inappropriate, random words
2 Incomprehensible sounds and moans but no words
1 None
Verbal response is adjusted in children
Score Verbal response Preverbal/grimace response
5 Appropriate babbles, words or phrases to usual ability Normal facial oromotor activity
4 Inappropriate words, or spontaneous irritable cry Less than usual ability, response only to touch
3 Cries inappropriately Vigorous grimace to pain
2 Grunts to pain, occasional whimpers Mild grimace to pain
1 No vocal response No response to pain
Best motor response, test and record in each limb*
Score Description
6 Obeys commands
5 Localizes pain. Hand should cross midline or get above clavicle in attempt to remove the stimulus
4 Withdraws from pain. Pulls limb away from fingernail bed pressure. Normal flexion observed
3 Abnormal flexion, decorticate response (spastic wrist flexion)
2 Extension to pain, decerebrate response (extensor posturing)
1 No motor response. Ensure adequate painful stimulus and no spinal injury

* Upper limb responses are more reliable as lower limb responses could be spinal reflexes.

Table 4.2 Severity of acute head injury

GCS score Coma
≤8 Severe
9–12 Moderate
≤13 Minor

CSF analysis

Test: Lumbar puncture

Normal values

See Table 4.3

Table 4.3 Lumbar puncture results

Measure Normal values
Opening pressure 7–20 cmH2O
Cell count 0–5/mm3, all lymphocytes
Protein concentration 0.15–0.45 g/L
Glucose concentration 2.8–4.2 mmol/L
CSF: blood glucose ratio 65%

Test: CSF appearance (spectrophotometry)

Test: CSF cell counts

Interpretation

Abnormalities

See Table 4.4.

Table 4.4 Causes of elevated CSF white blood cell count

  Characteristics
Bacterial meningitis Often >1000/mm3, usually PMN
Viral meningitis <100/mm3, usually lymphocytes
Seizures  
Intracerebral haemorrhage  
Malignancy  
Guillain-Barré syndrome <50 monocytes/mm3
Multiple sclerosis <50 monocytes/mm3
Other inflammatory conditions  

Test: CSF opening pressure

Interpretation

Abnormalities

See Table 4.7.

Table 4.7 Causes of altered CSF pressure

High pressure (>25 cmH2O) Low pressure (<6 cmH2O)
Intracranial haemorrhage CSF leak
Space-occupying lesions Previous lumbar puncture
Meningitis Severe dehydration
Cerebral oedema Inadequate production
Congestive cardiac failure Shunt
High venous pressure Obstructive hydrocephalus Excess absorption
Idiopathic/benign intracranial hypertension Drugs: acetazolamide, diuretics

Electroencephalogram derivatives

The electroencephalogram (EEG) is produced from the electrical activity in the superficial cerebral cortex. Scalp electrodes are used, the exact location of which has been determined by an international system. Each electrode can detect activity within 2.5 cm. The resulting signal is amplified, filtered and then displayed visually on a plot of amplitude versus time. Computerized Fourier analysis can be undertaken to show amplitude at each frequency, power versus frequency or a compressed spectral array.

Monitors that use the EEG include the bispectral index, cerebral state index, patient state index, narcotrend index and spectral entropy monitors.

Test: Bispectral index (BIS)

Limitations and complications

Evoked potentials

Evoked potentials (EP) measure action potentials that occur in response to a specific stimulus. The recorded potentials test a specific neural tract, sensory or motor, peripheral or central. Evoked potentials are smaller than EEG and require computer averaging to resolve them from background signals (EEG and ECG). The EP waveform (Fig. 4.2) is a plot of voltage versus time. It is described in terms of amplitude, latency and morphology.

Guidelines and standards are available from The International Federation of Clinical Neurophysiology at http://www.ifcn.info/ and The American Clinical Neurophysiology Society at http://www.acns.org/.

Intraoperative monitoring should be considered whenever the function of the brain, brainstem, spinal cord or selected peripheral nerve is at risk. This includes surgery for scoliosis, spinal trauma, spinal cord pathologies, tethered cords, brainstem tumours, cranial nerve involvement and thoracoabdominal aortic aneurysms.

Test: Somatosensory evoked potentials (SSEPs)

Test: Motor evoked potentials (MEPs)

MEPs assess the motor pathway from the motor cortex to the NMJ. In combination with SSEP monitoring, addition of MEP improves sensitivity and specificity for detecting neuronal injury.

Test: Auditory evoked potentials (AEPs)

Imaging

Test: Computerized tomography (CT) brain

Cervical spine in trauma

Injuries to the cervical spine occur in approximately 2–6% of blunt trauma patients, and in 10% of patients with severe head injuries with a GCS <8.

Test: Plain cervical radiographs (Fig. 4.10)

The three standard cervical trauma views are the investigation of choice according to NICE guidelines (see http://www.nice.org.uk). They consist of lateral, AP and open mouth views. The sensitivity of this series ranges from 70% to 90%. In practice this is reduced as films are inadequate in up to 50% of patients. Expert interpretation is essential.

Test: Cervical CT scan

Primary CT scanning of the entire cervical spine with sagittal and coronal reconstruction is more sensitive than plain films, particularly with the advent of the newer generation multislice/multidetector CT (MDCT) machines. Thus it has become the primary imaging of choice according to some protocols.

Alternatively CT can be used as a supplement to plain films for:

Test: Cervical MRI

Cervical MRI can detect soft tissue, ligament, disc and cord injury that may not be evident even on helical or multislice CT. To date no prospective studies comparing modern multislice CT and MRI have been conducted for the evaluation of occult cervical injuries in unconscious patients.

Intracranial pressure (ICP) monitoring

Indications

An ICP monitor is used in traumatic brain injury when the following apply:

Further indications for ICP monitoring are listed in Table 4.9.

Table 4.9 Reasons for ICP monitoring

Condition Comments
Subarachnoid haemorrhage With associated hydrocephalus to allow CSF drainage
Brain tumours Selected patients at high risk of post op cerebral oedema, e.g. posterior fossa craniotomy
Reye’s syndrome Active treatment decreases mortality
Hydrocephalus Diagnostic tool in complex cases
Benign intracranial hypertension Monitoring via lumbar drain as diagnostic test and treatment response
Hypoxic brain swelling Post drowning, CO poisoning
Others Meningitis, venous sinus thrombosis, hepatic encephalopathy, stroke and craniostenosis

Interpretation

Waveform analysis over 30 minutes should be the minimum as instant CSF measurements may be misleading. Normal range varies with age, posture and clinical conditions. Values in children are not well established (Table 4.10).

Table 4.10 Waveform analysis normal ranges

Age group Normal range (mmHg)
Adults <10–15
Children 3–7
Term infants 1.5–6

Abnormalities

Lundberg A (plateau) waves (Fig. 4.12). Steep increases in ICP from near normal values to 50–100 mmHg, persisting for 5–20 minutes and then fall sharply. Always pathological and indicate greatly reduced compliance. Often accompanied by neurological deterioration.
Lundberg B waves (Fig. 4.13). Rhythmic oscillations occurring every 1–2 minutes. Sharp rhythmic oscillations, occurring every 1–2 minutes, occur in ventilated patients and with Cheyne–Stokes respiration. Indicative of failing intracranial compensation.

Malignant hyperthermia susceptibility

Malignant hyperthermia (MH) is a hypermetabolic condition that results from exposure to inhalational anaesthetics or suxamethonium. Abnormalities in excitation–contraction coupling cause loss of control of calcium movements, which may in turn cause a hypermetabolic response. The prevalence is estimated at 1 in 8500.

Further investigations

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