Central nervous system

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TOPIC 4 Central nervous system

Topic Contents

Assessment of consciousness 58

Test: Glasgow Coma Scale (GCS) 58
CSF analysis 60

Test: Lumbar puncture 60
Test: CSF appearance (spectrophotometry) 61
Test: CSF cell counts 62
Test: CSF glucose 63
Test: CSF microbiology 64
Test: CSF opening pressure 64
Test: CSF protein 64
Electroencephalogram derivatives 65

Test: Bispectral index (BIS) 66
Evoked potentials 68

Test: Somatosensory evoked potentials (SSEPs) 69
Test: Motor evoked potentials (MEPs) 70
Test: Auditory evoked potentials (AEPs) 71
Imaging 73

Test: Computerized tomography (CT) brain 73
Test: MRI brain 75
Cervical spine in trauma 77

Test: Plain cervical radiographs 77
Test: Cervical CT scan 80
Test: Cervical MRI 81
Intracranial pressure (ICP) monitoring 81
Malignant hyperthermia susceptibility 85

Assessment of consciousness

Test: Glasgow Coma Scale (GCS)

Indications

• Assessment of consciousness initially used after traumatic brain injury but now also applied to other situations.

How it is done

• Sum the scores of the three eye, verbal and motor tests (Tables 4.1 and 4.2).

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

Interpretation

Data presented as a final score.

• The highest score is 15 with a fully awake patient.
• The lowest possible score is 3 when patients are dead or in deep coma.
• Ideally the breakdown of the separate components should be recorded.

Causes of reduced GCS include:

• Acute brain injury: traumatic, vascular and infections
• Metabolic disorders: hypoglycaemia, hepatic or renal failure
• Drugs: including sedatives and alcohol.

Management principles

• Patients with a GCS of 8 or less should be intubated to ensure airway protection, oxygenation and CO2 clearance.
• NICE guidelines advise the frequency of the observations after traumatic brain injury.

Limitations and complications

• Failure to incorporate brainstem reflexes.
• It is most accurate in assessing altered levels of consciousness due to trauma, but is often used to assess medical causes of coma.
• The presence of an endotracheal tube precludes use of the verbal assessment. ‘T’ is then recorded in this section (e.g. M5 VT E3).
• In spinal cord injury the stimulation and assessment of the motor response needs to be applied above the level of injury.
• Orbital trauma may prevent assessment of eye opening.

CSF analysis

Test: Lumbar puncture

Indications

• Diagnostic Lumbar Puncture.

Analysis of CSF is required for the diagnosis of the following CNS conditions:

• Infections, including bacterial, viral and fungal meningitis, Inflammatory CNS disease; including encephalitis, myelitis, Guillain Barré syndrome and multiple sclerosis, CNS malignancy, and Intracerebral haemorrhage.
• Reduce CSF volume for therapeutic purposes.
• Administer intrathecal drugs, e.g. chemotherapy, spinal anaesthesia, etc.,

Conditions in which lumbar punctures are commonly undertaken include:

• Infection, including bacterial, viral and fungal meningitis
• Inflammatory CNS disease; including encephalitis, myelitis, Guillain Barré syndrome and multiple sclerosis.
• CNS malignancy.
• Intracerebral haemorrhage.

Contraindications

• Reduced GCS.
• Focal neurological signs.
• Papilloedema.
• Coagulation disorders.
• Local infection at site.

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%

Limitations and complications

• Headache:

– Incidence of post dural puncture headache (PDPH) is reduced with smaller needle size. The average frequency of headache is 20–40% using 20–22 G and 5–12% using 24–27 G needles
– Incidence also reduced by using a noncutting (Whittacre or Sprotte) type needle rather than cutting (Quinke) type, and by replacement of stylet before needle withdrawal. There is no evidence increased fluids or bed rest prevents headache.

• Traumatic tap – defined as CSF containing >1000 RBC per high-powered field.
• Backache – may occur in up to 40% of patients.
• Failure.
• Spinal cord or nerve root damage–replacement of stylet before needle withdrawal may avoid damage to nerve roots and dura.
• Cerebral herniation is rare and can be avoided by using CT to exclude a space-occupying lesion (SOL) prior to lumbar puncture.
• Subdural, epidural haematomas.
• Cranial nerve palsies.
• Discitis.
• Pneumocephalus.
• Infection.

Test: CSF appearance (spectrophotometry)

• Xanthrochromia is the yellowish discoloration of Cerebrospinal fluid (CSF) due to the presence of bilirubin, a haemoglobin breakdown product.
• Visual inspection alone is not a reliable method to detect xanthochromia. Spectrophotometry is necessary.

Indications

• In a suspected subarachnoid haemorrhage (SAH), an LP should be done ideally 12 hours after the suspected haemorrhage to allow for the in vivo formation of CSF bilirubin.

How it is done

• If possible collect four sequential CSF specimens and ensure the last sample is sent for bilirubin analysis.
• Check with the laboratory for the volume required and ensure availability of spectrophotometry.
• Protect sample from light and when possible avoid vacuum transport systems that may haemolyze red blood cells (RBCs), produce oxyhaemoglobin (oxyHb) and hence a false-positive result.
• A simultaneous blood specimen should be taken for serum bilirubin and total protein.
• After centrifugation the supernatant is analyzed with a spectrophotometer.
• Haem pigments produce characteristic absorbance peaks at 400–460 nm.

Interpretation

Normal CSF appearance is crystal clear and colourless

Abnormalities

• Need to be interpreted carefully as causes of xanthochromia include raised serum bilirubin and CSF protein.
• The exact ratios of RBC, oxyHb and bilirubin found in the CSF will depend upon the timing of the lumbar puncture in relationship to the bleed.
• If a CT scan and CSF spectrophotometry are normal within 2 weeks of a sudden severe headache then SAH is excluded.

Further investigations

• Patients with a CT positive for subarachnoid blood should proceed to either a cerebral angiogram or CT angiogram to try and find the cause of the SAH (an aneurysm in >85% of cases). Treatment can then be undertaken (surgical or radiological) with the aim of preventing a re-bleed and allowing the aggressive management of vasospasm.
• The small group of patients with a history suggestive of a SAH but with a negative CT, can be identified with an appropriately timed lumbar puncture followed by spectrophotometer CSF analysis. They can then also be referred for cerebral angiography.

Limitations and complications

• Sensitivity of bilirubin to diagnose a subarachnoid haemorrhage has been shown to be 96% when undertaken more than 12 hours after haemorrhage.
• A traumatic tap will produce a CSF sample with an increased RBC count, but unlike SAH the sample will not contain bilirubin. Spectrophotometry is the only reliable way to distinguish SAH from a traumatic tap.
• Spectrophotometric findings taken on a second or subsequent LP only reflect the probability that blood has been introduced into the subarachnoid space at an earlier puncture.

Test: CSF cell counts

Indications

• Diagnostic lumbar puncture.
• Monitoring of partially treated meningitis/ventriculitis.

How it is done

• Cell count must be performed manually by an experienced operator using a Neubauer chamber within 30 minutes of sampling.
• White blood cell (WBC) differential is usually performed on concentrated CSF by centrifugation.

Interpretation

Normal range

• Adults <5 WBC/mm3 (70% lymphocytes, 30% monocytes, occasional eosinophil or neutrophil).
• Neonates <20 WBC/mm3, including <60% neutrophils.

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  

WBC differential

Viral, fungal and tuberculosis (TB) infections characteristically show lymphocytosis, but early infection may show polymorphonuclear neutrophil (PMN) preponderance. Eosinophilic meningitis >10 eosinophils/mm3 is rare. Causes include parasitic infections, other infections, VP shunts, malignancy and allergic drug reactions.

Limitations and complications

• Cell counts diminish after 30 minutes due to settling, binding to surfaces and cell lysis. Neutrophil counts fall by 50% by 2 hours whereas lymphocytes and monocytes do not.
• Allow 1 WBC per 500 RBCs in the event of traumatic tap.

Test: CSF glucose (Table 4.5)

Interpretation

Normal range

• CSF glucose should be approximately two-thirds serum glucose: a simultaneous serum sample should always be taken.
• Levels rarely go above 17 mmol/L regardless of serum levels.
• Beware: glucose levels are usually normal in viral infections and can be normal in up to 50% of bacterial CNS infections.

Table 4.5 Causes of altered CSF glucose

Low CSF glucose High CSF glucose
CNS infections Hyperglycaemia
Chemical meningitis  
Subarachnoid haemorrhage  
Hypoglycaemia  

Test: CSF microbiology

Indications

• Diagnostic lumbar puncture.

How it is done

• Gram stain for suspected bacterial infections.
• Acid-fast staining for suspected TB.
• India ink stain for cryptococcus.
• CSF culture is essential to determine antimicrobial sensitivity and resistance. A minimum of 2 mL is required. However, fungal and TB cultures require 20–40 mL to provide reasonable sensitivity. This requires multiple CSF samples.
• Polymerase chain reaction (PCR) has replaced tissue culture for most viral and some bacterial CNS disease.
• CSF antigen testing. For cryptococcus this has a sensitivity of 80–95%.
• Latex agglutination and coagglutination are methods that allow detection of bacterial antigens in CSF. For Haemophilus influenza sensitivity is quoted to be 60–100%, but is lower for other bacteria. This can be useful in partially treated meningitis where cultures may not yield an organism.

Test: CSF opening pressure

Indications

• All lumbar punctures.

How it is done

• Connect manometer to lumbar puncture needle hub once CSF is draining. Allow CSF pressure to equilibrate with atmospheric pressure in the manometer tubing.
• Read the pressure in cmH2O/mmH2O calibrated onto manometer tubing.

Interpretation

Normal data (Table 4.6)

Pressures can become elevated if the patient holds their breath or strains. It will reduce with hyperventilation. Fluctuations may be seen with respirations.

Table 4.6 Normal range of CSF pressure

Age (years) Pressure (cmH2O)
<8 1–10
>8 6–20
Obese adult <25

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

Test: CSF protein

Indications

• CSF protein concentration is one of the most sensitive indicators of pathology within the CNS.

Interpretation

Normal range

• Adult range 0.15–0.45 g/L is reached between 6 and 12 months of age.
• Newborns have up to 1.5 g/L.

Abnormalities (Table 4.8)

• CSF: serum ratio of albumin is 1:200. Immunoglobulins are normally excluded from CSF; their CSF:serum ratio is >1:500 and essentially consists only of IgG.
• Oligoclonal bands present in the CSF but not serum suggest an immune response within CNS.
• CSF protein levels >2 g/L suggest bacterial infections.

Table 4.8 Causes of altered CSF protein

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