Cerebral assessment 1: aEEG/EEG

What is the cerebral function monitor?

  • A CFM is an amplitude integrated EEG (aEEG) device that allows continuous neurological monitoring in infants in the NICU. It assesses global cerebral activity as compared to a formal EEG.
  • CFM allows for continuous bedside monitoring that can be commenced easily at any time of the day or night; unlike a standard 12 lead EEG that requires input from the Neurophysiology department.
  • It is not a substitute for a formal EEG as it does not localize lesions, may miss subtle brief seizures and miss focal seizures – as such an EEG will also usually be performed on infants who have CFM monitoring.
  • An example of a CFM is the Natus Olympic 6000, which is a 3-electrode (single EEG lead) set up. 2 active electrodes and 1 noise suppression electrode. The display on the top of the screen is a filtered time compressed output. Fig. 1. The display on the bottom should be set to show the raw or live EEG. Fig. 2.

 

 

Which infants should be monitored?

1. Term/ near term Infants
Cerebral Function Monitoring (CFM) should routinely be used for all infants of gestational age ≥ 35 weeks who have one or more of the following:
a) Evidence of encephalopathy .
b) Evidence of perinatal distress suggestive of possible hypoxic-ischaemic encephalopathy (HIE) and who required admission to NICU. Monitor infants who have any of the following features of perinatal compromise:
         1. foetal/neonatal acidaemia with cord pH or arterial pH within 1 hour of birth showing
             pH<7.0 or Base Deficit of > 15, and/or
         2. APGAR score of <5 at 5 mins postnatal.
c) Suspected seizures.
e) Muscle relaxed infants.

2. CFM may also provide useful information in babies with:
a) Meningitis (requiring intensive care).
b) Evidence of extensive structural brain injury or serious congenital brain
anomalies (e.g. cerebral infarction, congenital brain haemorrhage/ tumour, hydrocephalus).

3. Preterm Infants
The CFM may be less easy to interpret in preterm infants. Nevertheless it can provide very useful information and so may be considered in some infants of < 35 weeks’ gestation e.g. those with:
a) Clinical or suspected seizures.
b) Encephalopathy.
c) Grade 3 or 4 intraventricular haemorrhage.

CFM of preterm infants should be at the discretion of the attending consultant

Practical Issues

Electrodes

  • cfm-3These can be needle or disc/cup electrodes.
    Electrodes are placed according to the international 10-20 system. For a single channel CFM the two active electrodes should be placed in the biparietal C3/C4 or P3/P4 position. Electrical activity is measured between the electrodes, so it is important that they are adequately spaced apart to interrogate the cortex adequately. The third, noise suppression electrode should be placed frontosuperior to the others.
  • Do not insert a needle electrode over a fontanelle.
  • For disc/cup electrodes the skin needs to be prepared with a dedicated skin prep gel, parting hair to have as free an area as possible and a conductive gel needs to be applied to the concave side of the electrode prior to firmly pressing to the skin.
  • Steristrips are used to provide extra support at the insertion/attachment site.

Impedence

  • Measure of quality of electrode contact – important to check when reviewing data.
  • Want it as low as possible – easier to achieve with needle electrode
  • Alarm if > 20 kOhm
  • Can be used to detect lead motion artefact/noise or loose leads.

 

 

Analysing CFM

Comment on:

  • Impedence and artefacts
  • Lower margin (should be > 5 µvolts in normal CFM)
  • Upper margin (should be > 10 µvolts in normal CFM)
  • Background – Continuous, Discontinuous, Burst Suppression.
  • Sleep wake cycling status
  • Seizure activity

 

 

Normal CFM

 

 

Moderately Abnormal
• No sleep wake cycling
• Upper Margin is > 10 µVolts
• Lower Margin is < 5 µVolts
• Greatly increased variability

 

 

Severely Abnormal
• No sleep wake cycling
• Upper margin is < 10 µVolts
• Greatly reduced variability
• Burst suppression noted

 

Seizures

•Onset of seizure → continuous, high activity
•Causes CFM trace to narrow and rise up
•Injured brain has increased variability