Second edition

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Figure 1.2 A T1-weighted magnetic resonance imaging scan
demonstrates a laminar band heterotopia, as indicated by the
arrow, in exquisite detail. (Reproduced from Hopkins et al. 1995.)
PITUITARY ADENOMA
Pituitary macroadenomas may be seen on both CT and MR
scanning; microadenomas, however, are generally seen only
with MR scanning (Levy and Lightman 1994), which, in the
case of prolactinomas, may be used to monitor the results
of treatment with bromocriptine (Pojunas et al. 1986).
1.5 ELECTROENCEPHALOGRAPHY
The existence of cerebral electrical activity was demonstrated
in animals in 1875 by an English physician, Richard
Caton (1875), and the first human electroencephalogram
(EEG) was reported by Hans Berger in 1929 (Berger 1929).
By the middle of the twentieth century, the EEG had become
very important in the diagnosis of such intracranial lesions
as tumors but, with the advent of CT and MRI the indications
for electroencephalography have changed, and most
EEGs are currently obtained in the course of the diagnosis
or management of seizures or epilepsy and in the evaluation
of delirium. This chapter discusses EEG instrumentation,
the normal EEG, various EEG abnormalities, activation
procedures (e.g., hyperventilation), normal variants, and the
various artifacts that may mimic pathologic abnormalities.
As with any other diagnostic test, electroencephalography
must be properly performed to yield the most useful
data (Epstein et al. 2006a). In particular, the awake EEG
1.5 Electroencephalography 21
should include at least 20 minutes of artifact-free recording,
followed, when appropriate, by the activating procedures
of hyperventilation, photic stimulation, and sleep,
which should itself last an additional 20 minutes.
In contrast with CT and MR scanning, there is nothing
‘intuitively’ obvious about an EEG tracing: anyone familiar
with neuroanatomy can almost immediately grasp an MR
scan. Looking at an EEG tracing is, however, like looking at
an electrocardiogram (ECG); without a considerable amount
of preparation on the part of the physician, the EEG tracing
is no more informative about the state of the brain than
the ECG is about the heart. Consequently, this section on
EEG is relatively longer than that on neuroimaging, as well
as more detailed.
Instrumentation
Electrodes are attached to the scalp and are connected via
wires to the EEG machine. Pairing of these wires, and the
electrodes from which they stem, allows one to construct
numerous different channels. In older, analog machines,
this pairing is performed utilizing ‘selector switches’: however,
in the now standard digital machines, an analog-todigital-converter
allows for the creation of channels at the
touch of a keyboard. Within the EEG machine itself, one
finds amplifiers and filters that respectively amplify the
very weak electrical signals arising from the cortex and filter
out as much as possible electrical activity that arises
from either extracerebral sources or from the brain, and
which is of little clinical interest.
The amplified and filtered electrical impulse of each
channel is then used, in analog machines, to cause a deflection
of the appropriate pen over a continuously moving
sheet of paper, thus creating the actual tracing (EEG). With
digital machines, there are, of course, no pens or paper
tracings; however, this terminology has stayed with us. In a
standard recording, the sheet moves at a constant rate of
30 mm/s, and the sensitivity of the pen is set such that an
impulse of 50 μV causes a deflection of 7 mm.
The specific arrangement of electrodes on the scalp is
known as an array, and the international 10–20 system
described by Jasper (1958) remains a world-wide standard
(Epstein et al. 2006b). In this system, imaginary lines are
drawn on the head between specific landmarks (e.g., the
nasion and inion) and the electrodes are placed along them
at certain fractional intervals, i.e., either 10 percent or 20
percent of the total length of the imaginary line. These electrodes
are designated with letters that refer to their location,
and with numbers that indicate whether they are on
the left side of the head, the right side or in the sagittal midline;
thus, F p � frontopolar, F � frontal, T � temporal,
O � occipital, C � central, P � parietal, and A � auricular;
odd numbers indicate the left side of the head, even
numbers the right side, and zero (‘z’) the sagittal midline.
Figure 1.3 demonstrates these placements, and Table 1.1
provides the full name for each electrode. Note, however,