Second edition

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28 Diagnostic assessment
F 3�C 3 show a downward or negative pen deflection. What
happens next, however, is most critical: the next two channels,
C 3�P 3 and P 3�O 1, both show an upward or positive
deflection. It is apparent here that there has been a phase
reversal as one goes from channel F 3�C 3 to channel
C 3�P 3. This indicates that, in going from channel F 3�C 3
to channel C 3�P 3, one has ‘crossed’ over the depth of the
electrical chasm; furthermore, as the electrode that both
these channels have in common is C 3, it is now clear that
the depth of the chasm lies under that electrode; that is to
say phase reversal is seen at electrode C 3.
In some cases, focal epileptiform activity will not exhibit
phase reversal with a bipolar montage. Specifically, when
the focus is either proximal to the start of the chain or distal
to its end, phase reversal is not possible. For example,
consider a longitudinal chain linking F p1, F 3, C 3, P 3, and
O 1, and then imagine that the focus is located anterior to
F p1. In this case, all the pen deflections will be positive.
Conversely, if the focus were distal to O 1, all the pen deflections
would be negative.
Ictal activity
Ictal discharges consist of rhythmic activity that, unlike
interictal epileptiform discharges, is sustained, at the very
least, for a matter of seconds, with most ictal discharges
lasting minutes. These ictal discharges may be either generalized
at the onset or display a focal onset.
Ictal discharges that are generalized from the onset are
typically seen in the idiopathic generalized epilepsies:
myoclonic seizures are typically accompanied by polyspike
or polyspike-and-wave sustained discharges, and petit mal
seizures by sustained spike-and-dome discharges very similar
to those seen interictally (Browne et al. 1974).
Ictal discharges that are focal at the onset may remain so
for the duration of the seizure, and in most of these cases,
clinically one sees a simple partial seizure of one sort or the
other. In cases when there is spread to a more or less focal
area contralaterally a complex partial seizure is often seen,
and in those cases when the generalization involves the
entire cortex, a grand mal seizure is the typical accompaniment.
Although in some cases in which there was some preceding
interictal epileptiform activity the ictal discharges
may resemble the interictal ones, in most cases ictal discharges
are morphologically different (Blume et al. 1984;
Geiger and Horner 1978). In general, the ictal activity is
rhythmic and may occur at any frequency, from delta to
beta; rarely, instead of primarily a change in frequency, one
may see a change in amplitude, namely an ‘electrodecremental’
pattern in which the seizure is accompanied
only by a paroxysmal loss of amplitude.
Remarkably, in the case of partial seizures, the surface
EEG may remain normal, even in the face of indisputable
seizures. In the case of simple partial seizures (Cockerell et al.
1996; Devinsky et al. 1989; Seshia and McLachlan 2005) this
is seen in the majority; however, with complex partial seizures
it is noted in only a very small percentage (Lieb et al. 1976).
PERIODIC COMPLEXES
Periodic complexes generally consist of one or more sharp
waves combined with one or more slow waves that recur
on a regular basis, at intervals ranging from 1 to 15 seconds,
often on a background of generalized slowing.
Although they may begin with a focal predominance, they
fairly soon become generalized and synchronous, often
with a frontal prominence. Such periodic complexes are
often associated with myoclonus and are classically seen in
such disorders as subacute sclerosing panencephalitis
(Cobb 1966; Cobb and Hill 1950) and Creutzfeldt–Jakob
disease (Aguglia et al. 1987; Burger et al. 1972; Chiofalo et al.
1980; Levy et al. 1986; Steinhoff et al. 1996). Importantly,
although almost all patients with Creutzfeldt–Jakob disease
eventually develop periodic complexes (Browne et al.
1986) (generally within the first 3 months [Levy et al.
1986]), these may be absent (Bortone et al. 1994; Zochodne
et al. 1988), and this appears to be particularly the case with
new-variant Creutzfeldt–Jakob disease (Will et al. 1996).
Periodic complexes have also been noted in various other
conditions, including herpes simplex viral encephalitis
(Upton and Gumpert 1970), post-anoxic encephalopathy
(Hockaday et al. 1965), Alzheimer’s disease and diffuse
Lewy body disease (Doran and Larner 2004; Tschampa et al.
2001), and baclofen intoxication (Hormes et al. 1988).
Triphasic waves constitute a specific kind of periodic
complex. These are slow waves that, as the name indicates,
possess a triphasic morphology. They typically occur in a
generalized, bilaterally synchronous fashion, often with a
frontal predominance, either singly, in an isolated fashion,
or in longer bursts. Although they are classically associated
with hepatic encephalopathy (Karnatze and Bickford 1984;
Summerskill et al. 1956), they may be seen in other types of
metabolic delirium (Fisch and Klass 1988) such as that of
uremia, hypercalcemia, hypoglycemia, hypernatremia, or
hyponatremia, and have also been noted in a toxic delirium
secondary to lithium (Kaplan and Birbeck 2006).
Periodic lateralized epileptiform discharges (PLEDS) constitute
another specific form of periodic complex. These
consist of lateralized epileptiform discharges (either spikes
or sharp waves) that occur with a fairly regular periodicity,
varying from once every half a second to every 5 seconds
(Chatrian et al. 1964; Markand and Daly 1971). As described
in two large studies (Garcia-Morales et al. 2002; Gurer et al.
2004), in most cases PLEDS are associated with lesions
affecting the cortex. Although subcortical white matter
lesions may also be responsible, this is far less common; the
most common lesions are infarctions, followed by tumors,
abcesses, and subdural hematomas. Other conditions include
Creutzfeldt–Jakob disease, herpes simplex encephalitis,
and post-anoxic encephalopathy and, rarely, alcohol withdrawal
(Chu 1980). In most cases, PLEDS occur during the
acute onset of these underlying lesions and then gradually
remit; in a small minority of cases, however, they may persist
chronically. Clinically, most patients will also have
seizures (Baykan et al. 2000); importantly, however, with