Tests Used in Neurology

Lumbar Puncture

Overview

Lumbar punctures (LPs) are used in many scenarios when there is suspected inflammatory or infectious disease of the nervous system. It is usually performed at the intervertebral space at L4/L5 at the level of the iliac crest. 

Contraindications

• Features of raised intracranial pressure
  • Reduced or fluctuating GCS
  • Bradycardia and hypertension
  • Abnormal posturing
  • Pupillary abnormalities – unresponsive/unequal/dilated
  • Papilloedema
  • Bulging fontanelle in infants
• Shock
• Convulsions until stable
• Coagulopathy or anticoagulation use
• Infection at the lumbar puncture site
• Respiratory insufficiency – may cause respiratory failure

Complications

• Post-lumbar puncture headache – most common
  • Usually within 24-48 hours after an LP
  • Usually managed supportively but IV caffeine and epidural saline can be considered
• Infection
• Bleeding
• Cerebral herniation – rare but can be fatal

Cerebrospinal Fluid Interpretation

Overview

Cerebrospinal fluid (CSF) is found in the subarachnoid space of the brain, within the ventricles, and in the spinal canal. The choroid plexus of the ventricles produces CSF at around 500ml/day. Production and reabsorption are kept in a dynamic equilibrium, and generally, the average adult has about 150ml of CSF at a given time.

Assessment

When analysing CSF, we look at the following factors:

• Appearance of the CSF
• Opening pressure (normal range is 10-20cm H2O)
• Protein
• Glucose – CSF glucose is generally half of the plasma glucose
• Cell count – white cell count with differentials and red cells
• Gram stain – for bacterial organisms
• Cultures – if infection suspected

Other investigations may be considered in specific scenarios:

• Xanthochromia – usually seen in subarachnoid haemorrhages
• Oligoclonal bands – usually seen in multiple sclerosis and neurosyphilis
• Viral cultures
• Polymerase chain reaction (PCR) – for tuberculosis/viral or bacterial meningitis
• India ink staining – for cryptococcus 

Findings

These are some CSF findings commonly seen in exams. It is important to remember that CSF findings should be used alongside the history, examination findings, and other investigations and should not be used alone for diagnosis.

Normal:

• Appearance: clear and colourless
• Protein: 0.2-0.4g
• Glucose: around 50% of plasma glucose
• White cell count: <5 per mm3
• Red cell count: 0-10 per mm3
• Organisms: none
• Opening pressure: 10-20cm H2O
• Other findings: none

Bacterial meningitis:

• Appearance: cloudy/turbid
• Protein: elevated, >1.5g/L
• Glucose: <50% of plasma glucose
• White cell count: raised >1000 per mm3, mostly neutrophils (polymorphs)
• Organisms: may be present e.g. gram-negative diplococci in Neisseria meningitidis
• Opening pressure: usually high, >25cm H2O

Viral/aseptic meningitis or encephalitis:

• Appearance: clear
• Protein: protein raised/higher end of normal
• Glucose: usually within normal limits
• White cell count: raised, mostly lymphocytes
• Organisms: no organisms seen; PCR usually needed to identify cause
• Opening pressure: may be normal/raised

Tuberculous meningitis:

• Appearance: slightly cloudy, a fibrin web may be seen
• Protein: elevated, >1.5g/L – usually much higher than bacterial meningitis
• Glucose: <50% of plasma glucose
• White cell count: raised, mostly lymphocytes
• Organisms: PCR can be used to rule tuberculosis in or out quickly
• Opening pressure: raised, but can be upper limit of normal

Fungal meningitis:

• Appearance: cloudy
• Protein: elevated
• Glucose: <50%
• White cell count: elevated, mainly lymphocytes
• Opening pressure: elevated

Neurosyphilis:

• Protein: raised
• White cell count: raised
• Other findings: VDRL positive, TPPA positive

Subarachnoid haemorrhage:

• Appearance: may be bloodstained or xanthochromia seen ≥12 hours later
• Protein: elevated or upper limit of normal
• Glucose: normal/low
• White cell count: elevated 
• Red cell count: elevated, can be significantly higher than white cells
• Opening pressure: elevated

Multiple sclerosis:

• Appearance: clear
• Protein: may be elevated
• Glucose: normal
• White cell count: slightly raised, primarily lymphocytes
• Opening pressure: normal
• Other findings: oligoclonal bands seen

Guillain-Barré syndrome:

• Appearance: usually clear
• Protein: elevated
• Glucose: normal
• White cell count: normal/slightly elevated, mainly lymphocytes
• Opening pressure: normal/slightly elevated

Electroencephalography (EEG)

Overview

This involves placing sensors on the scalp to assess the electrical activity of the brain. Signals are recorded on an encephalogram and interpreted. Ambulatory EEGs may also be performed, which involves using a portable EEG over one or more days. They are often used in epilepsy. 

Computed Tomography & Magnetic Resonance Angiography (CTA/MRA)

Overview

Angiography is used to highlight and assess blood vessels and look for problems including stenosis, occlusions, and aneurysms. This is generally done by injecting a contrast agent and using CT or MRI scans to produce images (angiograms) of the blood vessels.

Intracranial Pressure Monitoring

Overview

This involves inserting a pressure monitor through the skull by drilling a hole in the skull. Intracranial pressure monitoring can be affected by movements such as leaning forward and breathing.

Nerve Conduction Studies

Overview

Small electrodes are placed on the skin which release electrical impulses to stimulate nerves. The speed and amplitudes of nerve impulses are recorded. It may be used in carpal tunnel syndrome, Guillain-Barré syndrome, Charcot-Marie-Tooth disease, and other peripheral neuropathies such as diabetic neuropathy.

Repetitive nerve stimulation

A variant of nerve conduction studies includes repetitive nerve stimulation which can be used to assess for neuromuscular junction disorders such as myasthenia gravis.

Electromyography (EMG)

Overview

A small needle is inserted through the skin into the muscles and electrical activity is recorded on an electromyograph. EMGs may be considered in motor neurone disease.

Evoked Potentials

Overview

Evoked potentials record electrical impulses in the brain following a stimulus that evokes them. Electrodes are attached to the scalp and record these impulses. Some types of evoked potential include:

• Visual evoked potentials (VEP):
  • A stimulus such as flashing lights or alternating patterns is presented and brain impulses are recorded
  • These can confirm damage to the visual pathway (retina, optic nerve, optic chiasm, optic radiations, occipital cortex)
• Brainstem auditory evoked potentials (BAEP):
  • A stimulus such as sound is presented
  • These can confirm damage to the auditory pathway (cochlea, cochlear nerve, auditory cortex)
• Somatosensory evoked potentials (SEEP):
  • A peripheral nerve is continually stimulated (usually via electrical simulation)