Channel

Montage

1

Cc to Ci

2

FZ to CZ

3

Iliac crest to T12

4

Crease of popliteal fossa to 5cm proximal

Waveform

Localisation

LP(Lumbar Potential), N22

T12, cauda equina

N37

Sensory parietal cortex

Wave

Latency(ms)

N22

18-28

N37

32-46

N22-N37

12.2-20.0

Abnormality

Interpretation

Absent LP but normal N37

Test can be interpreted as normal & extra time need not be spent in registering LP

Normal LP, abnormal LP-N37 IPL

This abnormality suggest a conduction defect in the large fibre sensory system above the cauda equina & below the sensory cortex following right/left sided stimulation. Since the tracts cross the medulla and there is no way of knowing  whether the lesion is caudal or rostral to that point, one cannot be more specific as to the vertical location of the lesion.

LP present but delayed, LP-N37 IPL is normal

The conduction defect must be peripheral to the cauda equina.

LP present but delayed AND LP-N37 is abnormal

This usually indicates both periphe5ral and central conduction delays although a single lesion at the cauda equina / lower cord is a less likely possibility.

LP absent AND N37 delayed

The conduction defect could be in the peripheral nerve; the interpretation must reflect this possibility.