TOPIC 4 Central nervous system

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

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.

CSF analysis

Test: Lumbar puncture

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.

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.

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