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Giant SEPs and SEP-recovery function in Unverricht–Lundborg disease
Clinical Neurophysiology 124 (2013) 1013–1018
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Giant SEPs and SEP-recovery function in Unverricht–Lundborg disease E. Visani a, L. Canafoglia a, D. Rossi Sebastiano a, P. Agazzi a,b, F. Panzica a, V. Scaioli a, C. Ciano a, S. Franceschetti a,⇑ a b
Department of Neurophysiology, Fondazione IRCCS Istituto Neurologico Carlo Besta, Milan, Italy Neurocentro, Ospedale Civico di Lugano, Switzerland
a r t i c l e
i n f o
Article history: Accepted 17 November 2012 Available online 29 December 2012 Keywords: SEP recovery cycle Giant SEPs Myoclonus Unverricht–Lundborg disease
h i g h l i g h t s In progressive myoclonic epilepsy 1A patients with giant SEPs had a delayed SEP recovery (SEP-R). The delayed SEP-R affected the ‘‘gigantisms’’ of the conditioned SEPs, but not the evoked response. Cortico-subcortical circuitries may contribute to both the gigantism and the delayed SEP-R.
a b s t r a c t Objective: To evaluate the relationship between sensory hyperexcitability as revealed by giant SEPs and the SEP recovery function (SEP-R) in a series of patient with progressive myoclonic epilepsy of Unverricht–Lundborg type, identified as epilepsy, progressive myoclonic 1A (EPM1A), MIM #254800. Methods: We evaluated SEPs by applying median nerve stimuli and SEP-R using paired stimuli at interstimulus intervals (ISIs) of between 20 and 600 ms in 25 patients and 20 controls. The SEPs were considered ‘‘giant’’ if the N20P25 and P25N33 amplitudes exceeded normal mean values by +3SD. Results: During the paired-stimulus protocol, the SEPs elicited by the second stimulus (S2) were detectable at all ISIs but consistently suppressed in the 13 patients with giant SEPs reflecting a significantly delayed SEP-R. Maximal suppression roughly corresponded to the plateau of a broad middle latency (>100 ms) wave pertaining to the S1 response. Conclusions: The cortical processing dysfunction generating giant SEPs in EPM1A patients consistently combines with a long-lasting suppression of hyperexcitability that leads to a delayed giant SEP-R without obstructing the response to incoming stimuli. Significance: The delayed SEP-R is not due to true inhibition but the suppression of aberrant hyper-synchronisation sustaining giant SEPs. A broad middle latency SEP component adds a significantly suppressive effect. This suggests that cortico-subcortical circuitries contribute to both the gigantism and the delayed SEP-R. Ó 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Unverricht–Lundborg disease, the most common form of progressive myoclonus epilepsy (PME) in Europe, is due to mutations of the cystatin B (CSTB) gene on chromosome 21q22.3 (Pennacchio et al., 1996), and has been labelled as EPM1A (epilepsy, progressive myoclonic 1A, MIM #254800). It presents with the typical phenotype of PME, including myoclonus, epileptic seizures and progressive neurological dysfunction, and is associated with neurophysiological markers of hyperexcitable sensory-motor
⇑ Corresponding author. Address: Department of Neurophysiology, Fondazione IRCCS Istituto Neurologico Carlo Besta, Via Celoria 11, I-20133 Milan, Italy. Tel.: +39 0223942301; fax: +39 0223942560. E-mail address: [email protected] (S. Franceschetti).
cortex (Karhu et al., 1994; Kälviäinen et al., 2008) including enhanced somatosensory evoked potentials (SEPs), facilitated long loop reflexes, and the presence of EEG correlates of the myoclonus indicating its cortical origin. The enlarged amplitude of SEPs that certainly indicate increased excitability in response to incoming stimuli can open a window for evaluating the characteristics of stimulus elaboration and the balance between excitatory and inhibitory events occurring in the sensorimotor cortex of PME patients. The SEP recovery cycle (SEP-R) is a marker of primary somatosensory cortex excitability because the response to the first stimulus starts the activation of intra-cortical inhibitory and excitatory circuits that interfere with the response to the subsequent stimulus. When paired stimuli are delivered at different inter-stimulus intervals (ISIs), the SEP-R is a function of the inter-stimulus intervals (Ugawa et al., 1991; Hamada et al.,
1388-2457/$36.00 Ó 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clinph.2012.11.011
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2002). Moreover, evidences obtained in healthy subjects suggested that recovery of different SEP components occurs with a different time-course resulting from varying excitatory and inhibitory balance at different ISIs within a complex circuitry (Hoshiyama and Kakigi, 2002). The SEP-R has been evaluated in central and peripheral nervous system disorders, including myotonic dystrophy (Mochizuki et al., 2001), dystonia (Frasson et al., 2001; Tamura et al., 2008), amyotrophic lateral sclerosis and Parkinson’s disease (Machii et al., 2003), and children with migraine (Valeriani et al., 2005; Vollono et al., 2010). Ugawa et al. (1991) studied patients with various neurological disorders, including nine who had an unspecified form of PME: they did not find a clear correlation between the presence of giant SEPs and an abnormal SEP-R, but three myoclonic patients showed a delay in the recovery of the N20–P25 and P25–N33 components. This finding is in line with those of Shibasaki et al. (1994) who found a depressed recovery of the N33 component in 4 out of 5 patients with PME. Mochizuki et al. (2003) found a long-lasting suppression of the P25–N33 component in 4 patients with PME in comparison with normal subjects and patients with Parkinson’s disease. However, in all of the studies of PME patients, the correlation between SEP-R and the presence of giant SEPs remained unclear. The aim of this study was to investigate SEP-R in a series of EPM1A patients, and assess the relationship between SEP-R and SEP amplitude. Enlarged SEPs are a typical marker of the cortical dysfunction associated with PME, and the suppressive effect of giant SEPs could significantly interfere with stimulus processing; moreover, a better understanding of the pathophysiology of the excitatory/inhibitory events occurring in these patients may contribute to our understanding of the mechanisms of cortical dysfunction.
2. Methods 2.1. Study population The study involved 25 right-handed EPM1A patients (14 women; mean disease duration 33.7 ± 14.9 years) and 20 righthanded controls (14 women, 29.5 ± 9.7 years). Table 1 summarises the clinical and electrophysiological characteristics of the patients. EPM1A was diagnosed in the presence of a typical electroclinical presentation and biomolecular findings of a dodecamer expansion of the cstb gene (Virtaneva et al., 1997). The mean disease duration was 12.4 ± 2.6 years and the mean age of the patients was 33.7 ± 14.9 years. Their myoclonus rating scale scores (Magaudda et al., 2004) ranged from 2 to 5 (mean: 2.7 ± 0.9), and they were all receiving quite similar anti-epileptic treatment. All of the subjects participating in the study (or their parents) gave their written informed consent. Our Ethics Committee approved the study, which was performed in accordance with the Declaration of Helsinki.
Table 1 Patients’ main clinical and neurophysiological characteristics. No., Gender, Age (yrs)
Age at onset (yrs)
Treatment
Myoclonus rating
LLR
SEP
1, F, 19
11
2
Facilitated
Giant
2, M, 17 3, M, 25
12 16
2 2
n.e. Normal
Giant Giant
4, 5, 6, 7,
11 10 10 10
VPA, TPM, BZP VPA, PRM VPA, LTG, LEV VPA, BZP VPA, BZP, PIR VPA, BZP, PIR VPA, LTG, BZP, PIR VPA, TPM, BZP VPA, TPM, PB, LEV, BZP VPA VPA, PB, BZP VPA, BZP VPA, BZP, PIR VPA, BZP, PIR VPA VPA, BZP, PB, ZNS VPA, BZP VPA, TPM VPA, TPM, BZP VPA, PB, BZP VPA, LEV, BZP VPA VPA, LEV VPA, LEV VPA, LEV
2 2 2 4
Normal Normal Normal Normal
Normal Normal Giant Giant
3
Normal
Normal
4
Facilitated
Giant
2 3 3 2 3 2 4
Normal Facilitated Facilitated Normal Normal Facilitated Normal
Normal Normal Normal Normal Giant Normal Normal
3 2 3
Normal Normal Facilitated
Normal Normal Giant
5 3
Normal Facilitated
Normal Giant
2 2 2 3
Facilitated Facilitated Normal Normal
Giant Giant Giant Giant
F, 25 F, 25 F, 26 M, 20
8, F, 37
12
9, F, 48
11
10, M, 34 11, M, 40 12,F, 33 13, M, 25 14, M, 40 15, F, 16 16, M, 66
14 17 13 11 9 11 15
17, F, 65 18, M, 46 19, M, 41
18 15 16
20, F, 46 21, F, 61
9 14
22, 23, 24, 25,
12 12 12 9
F, 22 F, 22 M, 18 F, 21
Treatments: VPA = valproate; TPM = topiramate; LTG = lamotrigine; LEV = levetiracetam; PB = phenobarbital; ZNS = zonisamide; PIR = piracetam; BZP = benzodiazepine. Simplified myoclonus rating (0–5; Magaudda et al., 2004). LLR = long loop reflex; n.e. = not evaluated; SEP = somatosensory evoked potential.
frequency was set at 1 Hz. At least one hundred sweeps were averaged for each trial. The latencies of N20, P25 and N33 were measured on the C30 electrode referred to Fz (Mochizuki et al., 2001), and those of P14 and N30 on Fz referred to A2. We measured the peak-to-peak amplitudes of N20P25, P25N33 and P14N30 cortical components, as well as the latency and area under the main late negative wave (latency 100–200 ms after stimulus) on C30 and C40 referred to A2. 2.3. Definition of giant SEPs Cortical SEPs were considered ‘‘giant’’ when the amplitudes of the N20P25 and P25N33 components both exceeded the mean value + 3SD the normative laboratory values obtained in healthy subjects (12.3 lV for N20P25, and 8.6 lV for P25N33).
2.2. SEP recordings 2.4. SEP recovery cycle Upper limb SEPs were elicited by electrically stimulating the right median nerve at the wrist at an intensity that was just above the motor threshold (visible and stable twitching of the thenar eminence) using Ag–AgCl electrodes positioned over C30 , C40 (2 cm posterior to the C3 and C4 positions of the standard international 10–20 system), Fz and the earlobe ipsilateral to the stimulus (A2). The signals were recorded using the Galileo Mizar System (EBNeuro, Florence, Italy) at a sampling frequency of 4 kHz, and digitally filtered off line (band pass 5–2000 Hz). The stimulation
The paired stimulation was based on pairs of identical stimuli with different ISIs (20, 50, 80, 100, 120, 150, 180, 200, 250, 300, 400, 500 and 600 ms), which were delivered every 1.5 s with a pause of 3 min before changing ISI, and SEP-R was measured on the basis of the amplitudes of N20P25 and P25N33. In order to extract the waveform evoked by the second stimulus (S2) of each pair, the responses evoked by the single stimulus protocol (SEP1) were subtracted from those elicited by the paired stimulus
E. Visani et al. / Clinical Neurophysiology 124 (2013) 1013–1018
protocol. The amplitude of the response to S2 of each ISI was evaluated on the subtracted waveform and compared with the amplitude of the (un-subtracted) response to the first stimulus (S1), which was assumed to be 100% (Höffken et al., 2010; Lenz et al., 2011).
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Regardless of the amplitude of the cortical components, amplitude and latencies of P14 were normal in all of the patients and controls (Table 2). The patients with giant SEPs had significantly larger N20P25, P25N33 and P14N30 amplitudes than those without giant SEPs, whereas there was no statistical difference in amplitude between the controls and the patients without giant SEPs (Table 2).
2.5. Statistical analysis 3.2. SEPs and clinical features The clinical data, latencies and amplitudes of conventional SEPs were compared between the patients with and without giant SEPs and the controls using one-way analysis of variance (ANOVA) with Bonferroni’s correction for multiple comparisons. Pearson’s correlation analysis was used to evaluate the relationships between SEP amplitude and time to recovery of the response to the second stimulus. In order to evaluate the SEP-R, repeated measure analysis of variance (RM-ANOVA) was used to identify the effect of withingroup factors (ISI) and the groups (between factors). The sphericity assumption was evaluated using Mauchley’s test and the degree of freedom was adjusted by means of the Greenhouse-Geisser correction when appropriate.
3. Results 3.1. Unconditioned SEPs Table 1 summarises the clinical and neurophysiological findings characterizing the study population. Thirteen of the 25 patients (52.0%) had giant SEPs (N20P25 13.3–38.9 lV; P25N33 11.3– 67.3 lV).
Table 2 Mean (±SD) amplitudes and latencies of unconditioned SEPs and ANOVA results. Healthy subjects
ULD patients with giant SEPs
N20 Latency (ms) Amplitude (lV)
18.5 ± 0.9 1.5 ± 1.3
20.9 ± 2.1* 2.6 ± 1.4
P25 Latency (ms) Amplitude (lV)
25.3 ± 3.5 3.5 ± 1.9
26 ± 2.2 5.5 ± 2.2
26.2 ± 2.5 18.8 ± 8.2*,**
N33 Latency (ms) Amplitude (lV)
33.8 ± 5.6 0.6 ± 1.4
36.2 ± 4.2 2.8 ± 1.8
35.3 ± 3.6 10.2 ± 10*,**
N20–P25 Amplitude (lV)
5.1 ± 2.4
7.5 ± 2.2
22 ± 8.5*,**
P25–N33 Amplitude (lV)
2.9 ± 1.9
8.3 ± 2.9
29 ± 17.9*,**
P14 Latency (ms) Amplitude (lV)
13.7 ± 0.8 1.2 ± 0.9
14.4 ± 0.7 0.2 ± 1.5
14.3 ± 1.1 1.2 ± 1.2
N30 Latency (ms) Amplitude (lV)
30.2 ± 3.3 2.6 ± 1.4
29.4 ± 2.3 3.6 ± 2.3
27.4 ± 2.3 7.4 ± 2.8*,**
P14–N30 Amplitude (lV)
3.8 ± 1.6
3.5 ± 1.3
8.2 ± 3.2*,**
N100 Area under the curve (C30 ) Area under the curve (C40 ) *
ULD patients without giant SEPs
20.3 ± 2* 3.3 ± 2.5*
–
153.7 ± 84.6
459.2 ± 246.5**
–
68.5 ± 43.7
268.1 ± 228.8**
p < 0.01 vs healthy subjects. ** p < 0.01 vs ULD patients without giant SEPs.
There were no significant relationships between SEP amplitude and the severity of myoclonus or treatment with specific drugs (e.g. benzodiazepines). Facilitated LLR was more common in the patients with giant SEPs (46.1% vs 25%), but this association was not significant (Chi-square test). The amplitudes of the cortical SEP components negatively correlated with the age of the patients (R = 0.402, p = 0.048 and R = 0.415, p = 0.042 for N20P25 and P25N33; Pearson correlation), whereas we did not find any significant correlation in healthy subjects. 3.3. SEP recovery cycle The early SEP components (N20P25, P25N33) were clearly detectable and measurable starting from an ISI of 20 ms in all of the patients. Fig. 1A shows the mean SEP-R values in the different groups. RM-ANOVA revealed a significant main effect of groups for N20P25 (F = 8.6, p = 0.001), P25N33 (F = 22.1, p < 0.001) and P14N30 (F = 4.9; p = 0.013). Post hoc tests showed significant differences between the patients with giant SEPs and the controls (N20P25: p = 0.002, P25N33: p < 0.001, P14N30: p = 0.011), between the patients without giant SEPs and the controls (N20P25: p = 0.013, P25N33: p = 0.003), and between the patients with and without giant SEPs (P25N33: p = 0.040). The post hoc test of the ISIs showed that N20P25 amplitude was significantly reduced in the patients with giant SEPs at ISIs ranging from 150 to 400 ms, as was P25N33 amplitude at ISIs ranging from 20 to 300 ms. In patients without giant SEPs the N20P25 amplitude was significantly reduced at ISIs 300 ms and the P25N33 amplitude at ISIs 20, 50, 120, 180 and 250 ms. The latencies of the conditioned responses remained unchanged at the different ISIs. Fig. 1B and C shows examples of the responses to paired stimuli in two representative patients without and with giant SEPs at 120 ms ISI. The association between the SEP amplitude and SEP-R was performed by means of correlation analysis performed at different ISIs. In EPM1A patients, the Pearson correlation indicated that the amplitudes of N20P25 and P25N33 evoked by the first stimulus of the pair inversely correlated with recovery at ISIs 120 (R = 0.530, p = 0.006; R = 0.523, p = 0.007), 150 (R = 0.624, p = 0.001; R = 0.609, p = 0.001), 180 (R = 0.547, p = 0.005; R = 0.553, p = 0.004) and 200 (R = 0.599, p = 0.002; R = 0.533, p = 0.006) while healthy subjects did not show any significant relationship. In order to verify whether the period of decreased amplitude of the giant responses corresponded to a reduced ability of the somatosensory cortex to respond to incoming stimuli, we analysed the SEP waveform in the whole ISI range in detail. The early SEP waveform did not show any distortion, and at ISIs > 250 ms, the conditioned responses in the patients with giant SEPs, already exceeded the threshold of +3SD of the normative values considered as defining SEP gigantism (Fig. 2). 3.4. Middle-latency SEP components The unconditioned SEP waveforms of all of the patients showed evidence of an abnormal middle-latency complex that consistently included a prominent broad negative wave. This broad wave had a
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Fig. 1. (A) SEP-R profiles of N20P25 and P25N33 showing a significant recovery delay in patients with giant SEPs. Note the clearly suppressive effect and the U-shaped form in the range of ISIs of between 100 and 250 ms giving rise to delayed SEP-R that was more obvious in the patients with giant SEPs. (B and C) Representative SEPs obtained at ISI 120 ms in a patient without and with giant SEPs.
Fig. 2. Relationship between the mean amplitude of the response to the first stimulus (S1, circles) and second stimulus (S2, triangles) during paired stimulus in patients without (A) and with giant SEPs (B). The line indicates the threshold for the definition of a giant response (mean + 3SD of the control subjects’ response). Note that, despite the delayed recovery of the giant response, the patients with giant SEPs show a clearly detectable response to S2 at all ISIs.
latency ranging from 114.5 to 204.8 ms (149.4 ± 22.1 ms), and was obvious on both C30 and C40 , although consistently larger on C30 (Table 2, Fig. 3). It was significantly larger in the patients with giant SEPs, but also presents in the patients with normal amplitude early SEP components. In the patients with giant SEPs, the area of the
long negative wave measured on C30 positively correlated with that of N20P25 (R = 0.821, p = 0.001) and P25N33 (R = 0.832, p < 0.001) and, during SEP-R, the greatest decline in P25N33 in response to S2 occurred at ISIs ranging from 120 to 180 ms. At these ISIs, the recovery of this SEP component inversely correlated with
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Fig. 3. Broad middle latency components of SEPs in a representative patient with giant SEPs recorded on contralateral (A) and ipsilateral (B) derivation. Note the suppressive effect on S2 concomitant with the appearance of the middle-latency negative component (arrow).
that of the area under the negative late component (R = 0.629; R = 0.578; R = 0.580; p < 0.05 for ISIs of 120, 150 and 180 ms). 4. Discussion Our data concerning SEP-R in EPM1A patients suggest that the dysfunction in cortical sensory processing responsible for giant SEPs consistently combines with a long-lasting depression of excitability that leads to a significant delay in the recovery of conditioned SEPs. 4.1. Relationship between unconditioned SEPs and the recovery profile Previous information obtained by evaluating SEP-R in patients with cortical myoclonus often relates to a few subjects and partially conflicts with our findings. Ugawa et al. (1991) did not find any consistent relationship between SEP amplitude and the recovery function in three PME patients with enlarged SEPs, but an accelerated recovery of P25N33 suggested a less suppressive effect in one patient, and Ugawa et al. (1996) found a less suppressive effect in a range of 20–150 ms in three PME patients, but a delayed recovery of N20P25 and P25N33 components. Shibasaki et al. (1994), found an almost immediate recovery followed by a less excitable phase in patients with positive myoclonus, and increased suppression of the response to the first stimulus in patients with negative myoclonus. In our relatively large case series of EPM1A patients, we consistently found a highly significant relationship between the amplitude of the early components of the SEPs evoked by the first stimulus and the time to recover of the response to the second. This was indicated by the different profile of the recovery function of SEPs as well by the significant relationship between the amplitude of the response to the first stimulus of the pair and the percentage of recovery at ISIs between 120 and 200 ms SEPs found in individual patients. The delayed SEP-R affected the ‘‘gigantisms’’ of the conditioned SEPs, but not the occurrence of the evoked response. The early components of the conditioned SEPs were clearly detectable and measurable in all of the patients starting from an ISI of 20 ms, without any significant wave distortion or change in latencies suggesting a ‘‘jittered’’ response. This indicates that the delayed SEP-R does not reflect a depressed ability of the sensory cortex to respond to incoming stimuli, but indicates a failure of the neuronal pool to return to the original pathological synchronisation, which takes about 500–600 ms to restore. Therefore, whatever the post-excitatory mechanism eliciting the giant responses, the delayed recovery cannot be considered a truly suppressive period but a weakening in the pathological network generating giant SEPs.
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The pathophysiology of giant cortical SEPs is still uncertain, as is the nature of post-excitatory phenomena. The neuronal populations responsible for normal SEPs probably also generate giant N20P25 and P25N33 (Karhu et al., 1994), whereas other components may arise from complex behaviors of the neuronal network and the synchronous activation of cortical areas beyond the primary somatosensory cortex. This would be in line with the data reported by Valeriani et al. (1997), who evaluated giant SEPs in three subjects with cortical myoclonus of unknown cause and found that the early components had the same tangential dipole found in normal subjects, whereas the later components had a different dipole orientation. Moreover, when mapping the distribution of SEP components in four PME patients of various etiologies, Ikeda et al. (1995) found two distinct components of giant SEPs (radial P25–N35 and tangential P30–N30). In our EPM1A patients with giant SEPs, the consistent presence of a giant N30 component suggests the involvement of cortical areas other than the sensory cortex, including the supplementary motor area (Desmedt and Bourguet, 1985), or sensory-motor interactions. Additionally, circuitry rearrangement leading to hyperexcitability may also activate sources that are ‘‘inactive’’ in normal SEPs, thus contributing to the ‘‘giant’’ components (Ragazzoni et al., 1999). However, the variety of clinical conditions associated with giant SEPs makes difficult to compare the data. Shibasaki et al. (1985) postulated that giant SEPs (the P25N33 component) at least partially arise from the same pathophysiological mechanism as the myoclonus-related cortical spike. The hypothesis of a relationship between giant SEPs and epileptic transients in PME patients may also suggest that the post-excitatory inhibition that typically occurs after interictal EEG transients can also play a role in the long inhibitory phenomenon delaying the recovery of giant SEPs. Moreover, Ikeda et al. (1995) found that the enhanced components of giant SEPs were suppressed immediately after a seizure and assumed that they were markers of a lack of inhibitory mechanism. Post-excitatory inhibition in the epileptic network is frequently due to membrane hyperpolarisation caused by intrinsic properties (e.g. sustained ionic currents activated by calcium or sodium entry) and synaptic mechanisms (mainly due to GABAa and GABAb inhibition) (see review by Prince and Connors, 1986). The long-lasting attenuation of the N20P25 and P25N33 amplitudes of conditioned responses in patients with giant SEPs could be explained by enhanced membrane hyperpolarisation. However, the presence of intrinsic hyperpolarizing mechanisms is not supported by specific evidence and synaptic-mediated hyperpolarisation seems to be unlikely because evidence obtained in EPM1A patients (Manganotti et al., 2001; Canafoglia et al., 2010) and models of EPM1A (Franceschetti et al., 2007; Buzzi et al., 2012) suggesting a decreased rather than increased GABA effectiveness. A possible modulation of SEP-R played by serotonin cannot be excluded, since evidence obtained in rat sensory cortex suggested a significant role of this neurotransmitter in modulating paired pulse facilitation/depression (Torres-Escalante et al., 2004) and some data suggest changes in serotonergic neurotransmission occurring in EPM1A patients (Pranzatelli et al., 1995) and in the mouse model (Vaarmann et al., 2006). We observed that the giant early SEPs were consistently followed by a complex of late waves, which is difficult to compare with physiological middle SEP components. This waveform always included a large and broad negative wave with a latency of >100 ms, whose amplitude correlated with those of N20P25 and P25N33 in the patients with giant-SEPS. At the time of maximum amplitude (plateau) of this negative wave, the conditioned SEP was maximally depressed, which could hypothetically account for the roughly U-shaped profile of the SEP-R function. A similar wave with less amplitude was also observed in the EPM1A patients
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without giant SEPs, thus possibly accounting for their slightly delayed SEP-R. This suggests that the late SEP component may represents a sustained depolarization that is sufficient to suppress the temporarily abnormal local synchronisation that significantly occurs in EPM1A patients with giant early SEP components and, albeit to a lesser extent, in those with non-giant early SEPs components. In this setting, the interaction between post-excitatory hyperpolarisation and a late ‘‘giant’’ depolarising component may well account for a delayed SEP-R. Even if the asymmetrical amplitude of the late negative wave suggests that its generation includes a local depolarising phenomenon occurring in the sensory cortex, its bilateral representation indicates the involvement of subcortical loops. This observation is in line with the hypothesis of enhanced thalamo-cortical circuitry in patients with cortical myoclonus. Indeed, an aberrant activation of complex thalamocortical loops may generate both abnormal depolarization and ensuing after-depolarization mechanisms accounting for giant SEPs components and subsequent inhibition. 4.2. Relationship of giant SEPs and their recovery profiles with clinical features In our EPM1A patients, the amplitude of the giant SEP components inversely correlated with the patient age. Kobayashi et al. (2011) has described specific attenuation of the N40 component in two Unverricht–Lundborg patients during long-term followup. Although we did not find the consistent presence of N40, our data substantially agree with the idea of progressive attenuation of giant SEPs as a marker of hyperexcitability in this disease that has a tendency to stabilize late (Genton, 2006). Various studies have evaluated the effect of anticonvulsant drugs on SEPs and SEP-R (Hitomi et al., 2006; Vollono et al., 2010). Our patients were all treated with one or more drugs, but we did not find any relationship between SEP amplitudes and recovery parameters and specific drugs (e.g. benzodiazepine). Moreover, as in a previous study (Canafoglia et al., 2004) and in line with the observation of Ikeda et al. (1995), we did not find any clear relationship between the presence of giant SEPs or SEPR profiles and the functional severity of the myoclonus. This suggests that giant SEPs and the ensuing complex of excitatory-inhibitory events significantly reflect changes in cortical excitability but does not significantly affect the dysfunctioning motor circuitries sustaining myoclonic jerks. References Buzzi A, Chikhladze M, Falcicchia C, Paradiso B, Lanza G, Soukupova M, et al. Loss of cortical GABA terminals in Unverricht–Lundborg disease. Neurobiol Dis 2012;47:216–24. Canafoglia L, Ciano C, Panzica F, Scapoli V, Zucca C, Agazzi P, et al. Sensorimotor cortex excitability in Unverricht–Lundborg disease and Lafora body disease. Neurology 2004;63:2309–15. Canafoglia L, Ciano C, Visani E, Anversa P, Panzica F, Viri M, et al. Short and long interval cortical inhibition in patients with Unverricht–Lundborg and Lafora body disease. Epilepsy Res 2010;89:232–7. Desmedt JE, Bourguet M. Color imaging of parietal and frontal somatosensory potential fields evoked by stimulation of median or posterior tibial nerve in man. Electroencephalogr Clin Neurophysiol 1985;62:1–17. Franceschetti S, Sancini G, Buzzi A, Zucchini S, Paradiso B, Magnaghi G, et al. A pathogenetic hypothesis of Unverricht–Lundborg disease onset and progression. Neurobiol Dis 2007;25:675–85. Frasson E, Priori A, Bertolasi L, Mauguiere F, Fiaschi A, Tinazzi M. Somatosensory disinhibition in dystonia. Mov Disord 2001;16:674–82. Genton P. Unverricht–Lundborg disease (PME1). Rev Neurol (Paris) 2006;162: 819–26. Hamada Y, Otsuka S, Okamoto T, Suzuki R. The profile of the recovery cycle in human primary and secondary somatosensory cortex: a magnetoencephalography study. Clin Neurophysiol 2002;113:1787–93. Hitomi T, Ikeda A, Matsumoto R, Kinoshita M, Taki J, Usui K, et al. Generators and temporal succession of giant somatosensory evoked potentials in cortical reflex
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Cortical reactivity in progressive myoclonus epilepsy. Electroencephalogr Clin Neurophysiol 1994;90: 93–102. Kobayashi K, Matsumoto R, Kondo T, Kawamata J, Hitomi T, Inouchi M, et al. Decreased cortical excitability in Unverricht–Lundborg disease in the long-term follow-up: a consecutive SEP study. Clin Neurophysiol 2011;122:1617–21. Lenz M, Höffken O, Stude P, Lissek S, Schwenkreis P, Reinersmann A, et al. Bilateral somatosensory cortex disinhibition in complex regional pain syndrome type I. Neurology 2011;77:1096–101. Machii K, Ugawa Y, Kokubo Y, Sasaki R, Kuzuhara S. Somatosensory evoked potential recovery in Kii amyotrophic lateral sclerosis/parkinsonism-dementia complex (Kii ALS/PDC). Clin Neurophysiol 2003;114:564–8. Magaudda A, Gelisse P, Genton P. Antimyoclonic effect of levetiracetam in 13 patients with Unverricht–Lundborg disease: clinical observations. Epilepsia 2004;45:678–81. Manganotti P, Tamburin S, Zanette G, Fiaschi A. Hyperexcitable cortical responses in progressive myoclonic epilepsy: a TMS study. Neurology 2001;57:1793–9. Mochizuki H, Hanajima R, Kowa H, Motoyoshi Y, Ashida H, Kamakura K, et al. Somatosensory evoked potential in myotonic dystrophy. Clin Neurophysiol 2001;112:793–9. Mochizuki H, Machii K, Terao Y, Furubayashi T, Hanajima R, Enomoto H, et al. Recovery function of and effects of hyperventilation on somatosensory evoked high-frequency oscillation in Parkinson’s disease and myoclonus epilepsy. Neurosci Res 2003;46:485–92. Pennacchio LA, Lehesjoki AE, Stone NE, Willour VL, Virtaneva K, Miao J, et al. Mutations in the gene encoding cystatin B in progressive myoclonus epilepsy (EPM1). Science 1996;271:1731–4. Pranzatelli MR, Tate E, Huang Y, Haas RH, Bodensteiner J, Ashwal S, et al. Neuropharmacology of progressive myoclonus epilepsy: response to hydroxyL-tryptophan. Epilepsia 1995;36:783–91. Prince DA, Connors BW. Mechanisms of interictal epileptogenesis. Adv Neurol 1986;44:275–99. Ragazzoni A, Ferri R, Di Russo F, Del Gracco S, Barcaro U, Navona C. Giant somatosensory evoked potentials in different clinical conditions: scalp topography and dipole source analysis. Electroencephalogr Clin Neurophysiol Suppl 1999;49:81–9. Shibasaki H, Ikeda A, Nagamine T, Mima T, Terada K, Nishitani N, et al. Cortical reflex negative myoclonus. Brain 1994;117:477–86. Shibasaki H, Yamashita Y, Neshige R, Tobimatsu S, Fukui R. Pathogenesis of giant somatosensory evoked potentials in progressive myoclonic epilepsy. Brain 1985;108:225–40. Tamura Y, Matsuhashi M, Lin P, Ou B, Vorbach S, Kakigi R, et al. Impaired intracortical inhibition in the primary somatosensory cortex in focal hand dystonia. Mov Disord 2008;23:558–65. Torres-Escalante JL, Barral JA, Ibarra-Villa MD, Pérez-Burgos A, Góngora-Alfaro JL, Pineda JC. 5-HT1A, 5-HT2, and GABAB receptors interact to modulate neurotransmitter release probability in layer 2/3 somatosensory rat cortex as evaluated by the paired pulse protocol. 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Giant SEPs and SEP-recovery function in Unverricht–Lundborg disease E. Visani a, L. Canafoglia a, D. Rossi Sebastiano a, P. Agazzi a,b, F. Panzica a, V. Scaioli a, C. Ciano a, S. Franceschetti a,⇑ a b
Department of Neurophysiology, Fondazione IRCCS Istituto Neurologico Carlo Besta, Milan, Italy Neurocentro, Ospedale Civico di Lugano, Switzerland
a r t i c l e
i n f o
Article history: Accepted 17 November 2012 Available online 29 December 2012 Keywords: SEP recovery cycle Giant SEPs Myoclonus Unverricht–Lundborg disease
h i g h l i g h t s In progressive myoclonic epilepsy 1A patients with giant SEPs had a delayed SEP recovery (SEP-R). The delayed SEP-R affected the ‘‘gigantisms’’ of the conditioned SEPs, but not the evoked response. Cortico-subcortical circuitries may contribute to both the gigantism and the delayed SEP-R.
a b s t r a c t Objective: To evaluate the relationship between sensory hyperexcitability as revealed by giant SEPs and the SEP recovery function (SEP-R) in a series of patient with progressive myoclonic epilepsy of Unverricht–Lundborg type, identified as epilepsy, progressive myoclonic 1A (EPM1A), MIM #254800. Methods: We evaluated SEPs by applying median nerve stimuli and SEP-R using paired stimuli at interstimulus intervals (ISIs) of between 20 and 600 ms in 25 patients and 20 controls. The SEPs were considered ‘‘giant’’ if the N20P25 and P25N33 amplitudes exceeded normal mean values by +3SD. Results: During the paired-stimulus protocol, the SEPs elicited by the second stimulus (S2) were detectable at all ISIs but consistently suppressed in the 13 patients with giant SEPs reflecting a significantly delayed SEP-R. Maximal suppression roughly corresponded to the plateau of a broad middle latency (>100 ms) wave pertaining to the S1 response. Conclusions: The cortical processing dysfunction generating giant SEPs in EPM1A patients consistently combines with a long-lasting suppression of hyperexcitability that leads to a delayed giant SEP-R without obstructing the response to incoming stimuli. Significance: The delayed SEP-R is not due to true inhibition but the suppression of aberrant hyper-synchronisation sustaining giant SEPs. A broad middle latency SEP component adds a significantly suppressive effect. This suggests that cortico-subcortical circuitries contribute to both the gigantism and the delayed SEP-R. Ó 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Unverricht–Lundborg disease, the most common form of progressive myoclonus epilepsy (PME) in Europe, is due to mutations of the cystatin B (CSTB) gene on chromosome 21q22.3 (Pennacchio et al., 1996), and has been labelled as EPM1A (epilepsy, progressive myoclonic 1A, MIM #254800). It presents with the typical phenotype of PME, including myoclonus, epileptic seizures and progressive neurological dysfunction, and is associated with neurophysiological markers of hyperexcitable sensory-motor
⇑ Corresponding author. Address: Department of Neurophysiology, Fondazione IRCCS Istituto Neurologico Carlo Besta, Via Celoria 11, I-20133 Milan, Italy. Tel.: +39 0223942301; fax: +39 0223942560. E-mail address: [email protected] (S. Franceschetti).
cortex (Karhu et al., 1994; Kälviäinen et al., 2008) including enhanced somatosensory evoked potentials (SEPs), facilitated long loop reflexes, and the presence of EEG correlates of the myoclonus indicating its cortical origin. The enlarged amplitude of SEPs that certainly indicate increased excitability in response to incoming stimuli can open a window for evaluating the characteristics of stimulus elaboration and the balance between excitatory and inhibitory events occurring in the sensorimotor cortex of PME patients. The SEP recovery cycle (SEP-R) is a marker of primary somatosensory cortex excitability because the response to the first stimulus starts the activation of intra-cortical inhibitory and excitatory circuits that interfere with the response to the subsequent stimulus. When paired stimuli are delivered at different inter-stimulus intervals (ISIs), the SEP-R is a function of the inter-stimulus intervals (Ugawa et al., 1991; Hamada et al.,
1388-2457/$36.00 Ó 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clinph.2012.11.011
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2002). Moreover, evidences obtained in healthy subjects suggested that recovery of different SEP components occurs with a different time-course resulting from varying excitatory and inhibitory balance at different ISIs within a complex circuitry (Hoshiyama and Kakigi, 2002). The SEP-R has been evaluated in central and peripheral nervous system disorders, including myotonic dystrophy (Mochizuki et al., 2001), dystonia (Frasson et al., 2001; Tamura et al., 2008), amyotrophic lateral sclerosis and Parkinson’s disease (Machii et al., 2003), and children with migraine (Valeriani et al., 2005; Vollono et al., 2010). Ugawa et al. (1991) studied patients with various neurological disorders, including nine who had an unspecified form of PME: they did not find a clear correlation between the presence of giant SEPs and an abnormal SEP-R, but three myoclonic patients showed a delay in the recovery of the N20–P25 and P25–N33 components. This finding is in line with those of Shibasaki et al. (1994) who found a depressed recovery of the N33 component in 4 out of 5 patients with PME. Mochizuki et al. (2003) found a long-lasting suppression of the P25–N33 component in 4 patients with PME in comparison with normal subjects and patients with Parkinson’s disease. However, in all of the studies of PME patients, the correlation between SEP-R and the presence of giant SEPs remained unclear. The aim of this study was to investigate SEP-R in a series of EPM1A patients, and assess the relationship between SEP-R and SEP amplitude. Enlarged SEPs are a typical marker of the cortical dysfunction associated with PME, and the suppressive effect of giant SEPs could significantly interfere with stimulus processing; moreover, a better understanding of the pathophysiology of the excitatory/inhibitory events occurring in these patients may contribute to our understanding of the mechanisms of cortical dysfunction.
2. Methods 2.1. Study population The study involved 25 right-handed EPM1A patients (14 women; mean disease duration 33.7 ± 14.9 years) and 20 righthanded controls (14 women, 29.5 ± 9.7 years). Table 1 summarises the clinical and electrophysiological characteristics of the patients. EPM1A was diagnosed in the presence of a typical electroclinical presentation and biomolecular findings of a dodecamer expansion of the cstb gene (Virtaneva et al., 1997). The mean disease duration was 12.4 ± 2.6 years and the mean age of the patients was 33.7 ± 14.9 years. Their myoclonus rating scale scores (Magaudda et al., 2004) ranged from 2 to 5 (mean: 2.7 ± 0.9), and they were all receiving quite similar anti-epileptic treatment. All of the subjects participating in the study (or their parents) gave their written informed consent. Our Ethics Committee approved the study, which was performed in accordance with the Declaration of Helsinki.
Table 1 Patients’ main clinical and neurophysiological characteristics. No., Gender, Age (yrs)
Age at onset (yrs)
Treatment
Myoclonus rating
LLR
SEP
1, F, 19
11
2
Facilitated
Giant
2, M, 17 3, M, 25
12 16
2 2
n.e. Normal
Giant Giant
4, 5, 6, 7,
11 10 10 10
VPA, TPM, BZP VPA, PRM VPA, LTG, LEV VPA, BZP VPA, BZP, PIR VPA, BZP, PIR VPA, LTG, BZP, PIR VPA, TPM, BZP VPA, TPM, PB, LEV, BZP VPA VPA, PB, BZP VPA, BZP VPA, BZP, PIR VPA, BZP, PIR VPA VPA, BZP, PB, ZNS VPA, BZP VPA, TPM VPA, TPM, BZP VPA, PB, BZP VPA, LEV, BZP VPA VPA, LEV VPA, LEV VPA, LEV
2 2 2 4
Normal Normal Normal Normal
Normal Normal Giant Giant
3
Normal
Normal
4
Facilitated
Giant
2 3 3 2 3 2 4
Normal Facilitated Facilitated Normal Normal Facilitated Normal
Normal Normal Normal Normal Giant Normal Normal
3 2 3
Normal Normal Facilitated
Normal Normal Giant
5 3
Normal Facilitated
Normal Giant
2 2 2 3
Facilitated Facilitated Normal Normal
Giant Giant Giant Giant
F, 25 F, 25 F, 26 M, 20
8, F, 37
12
9, F, 48
11
10, M, 34 11, M, 40 12,F, 33 13, M, 25 14, M, 40 15, F, 16 16, M, 66
14 17 13 11 9 11 15
17, F, 65 18, M, 46 19, M, 41
18 15 16
20, F, 46 21, F, 61
9 14
22, 23, 24, 25,
12 12 12 9
F, 22 F, 22 M, 18 F, 21
Treatments: VPA = valproate; TPM = topiramate; LTG = lamotrigine; LEV = levetiracetam; PB = phenobarbital; ZNS = zonisamide; PIR = piracetam; BZP = benzodiazepine. Simplified myoclonus rating (0–5; Magaudda et al., 2004). LLR = long loop reflex; n.e. = not evaluated; SEP = somatosensory evoked potential.
frequency was set at 1 Hz. At least one hundred sweeps were averaged for each trial. The latencies of N20, P25 and N33 were measured on the C30 electrode referred to Fz (Mochizuki et al., 2001), and those of P14 and N30 on Fz referred to A2. We measured the peak-to-peak amplitudes of N20P25, P25N33 and P14N30 cortical components, as well as the latency and area under the main late negative wave (latency 100–200 ms after stimulus) on C30 and C40 referred to A2. 2.3. Definition of giant SEPs Cortical SEPs were considered ‘‘giant’’ when the amplitudes of the N20P25 and P25N33 components both exceeded the mean value + 3SD the normative laboratory values obtained in healthy subjects (12.3 lV for N20P25, and 8.6 lV for P25N33).
2.2. SEP recordings 2.4. SEP recovery cycle Upper limb SEPs were elicited by electrically stimulating the right median nerve at the wrist at an intensity that was just above the motor threshold (visible and stable twitching of the thenar eminence) using Ag–AgCl electrodes positioned over C30 , C40 (2 cm posterior to the C3 and C4 positions of the standard international 10–20 system), Fz and the earlobe ipsilateral to the stimulus (A2). The signals were recorded using the Galileo Mizar System (EBNeuro, Florence, Italy) at a sampling frequency of 4 kHz, and digitally filtered off line (band pass 5–2000 Hz). The stimulation
The paired stimulation was based on pairs of identical stimuli with different ISIs (20, 50, 80, 100, 120, 150, 180, 200, 250, 300, 400, 500 and 600 ms), which were delivered every 1.5 s with a pause of 3 min before changing ISI, and SEP-R was measured on the basis of the amplitudes of N20P25 and P25N33. In order to extract the waveform evoked by the second stimulus (S2) of each pair, the responses evoked by the single stimulus protocol (SEP1) were subtracted from those elicited by the paired stimulus
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protocol. The amplitude of the response to S2 of each ISI was evaluated on the subtracted waveform and compared with the amplitude of the (un-subtracted) response to the first stimulus (S1), which was assumed to be 100% (Höffken et al., 2010; Lenz et al., 2011).
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Regardless of the amplitude of the cortical components, amplitude and latencies of P14 were normal in all of the patients and controls (Table 2). The patients with giant SEPs had significantly larger N20P25, P25N33 and P14N30 amplitudes than those without giant SEPs, whereas there was no statistical difference in amplitude between the controls and the patients without giant SEPs (Table 2).
2.5. Statistical analysis 3.2. SEPs and clinical features The clinical data, latencies and amplitudes of conventional SEPs were compared between the patients with and without giant SEPs and the controls using one-way analysis of variance (ANOVA) with Bonferroni’s correction for multiple comparisons. Pearson’s correlation analysis was used to evaluate the relationships between SEP amplitude and time to recovery of the response to the second stimulus. In order to evaluate the SEP-R, repeated measure analysis of variance (RM-ANOVA) was used to identify the effect of withingroup factors (ISI) and the groups (between factors). The sphericity assumption was evaluated using Mauchley’s test and the degree of freedom was adjusted by means of the Greenhouse-Geisser correction when appropriate.
3. Results 3.1. Unconditioned SEPs Table 1 summarises the clinical and neurophysiological findings characterizing the study population. Thirteen of the 25 patients (52.0%) had giant SEPs (N20P25 13.3–38.9 lV; P25N33 11.3– 67.3 lV).
Table 2 Mean (±SD) amplitudes and latencies of unconditioned SEPs and ANOVA results. Healthy subjects
ULD patients with giant SEPs
N20 Latency (ms) Amplitude (lV)
18.5 ± 0.9 1.5 ± 1.3
20.9 ± 2.1* 2.6 ± 1.4
P25 Latency (ms) Amplitude (lV)
25.3 ± 3.5 3.5 ± 1.9
26 ± 2.2 5.5 ± 2.2
26.2 ± 2.5 18.8 ± 8.2*,**
N33 Latency (ms) Amplitude (lV)
33.8 ± 5.6 0.6 ± 1.4
36.2 ± 4.2 2.8 ± 1.8
35.3 ± 3.6 10.2 ± 10*,**
N20–P25 Amplitude (lV)
5.1 ± 2.4
7.5 ± 2.2
22 ± 8.5*,**
P25–N33 Amplitude (lV)
2.9 ± 1.9
8.3 ± 2.9
29 ± 17.9*,**
P14 Latency (ms) Amplitude (lV)
13.7 ± 0.8 1.2 ± 0.9
14.4 ± 0.7 0.2 ± 1.5
14.3 ± 1.1 1.2 ± 1.2
N30 Latency (ms) Amplitude (lV)
30.2 ± 3.3 2.6 ± 1.4
29.4 ± 2.3 3.6 ± 2.3
27.4 ± 2.3 7.4 ± 2.8*,**
P14–N30 Amplitude (lV)
3.8 ± 1.6
3.5 ± 1.3
8.2 ± 3.2*,**
N100 Area under the curve (C30 ) Area under the curve (C40 ) *
ULD patients without giant SEPs
20.3 ± 2* 3.3 ± 2.5*
–
153.7 ± 84.6
459.2 ± 246.5**
–
68.5 ± 43.7
268.1 ± 228.8**
p < 0.01 vs healthy subjects. ** p < 0.01 vs ULD patients without giant SEPs.
There were no significant relationships between SEP amplitude and the severity of myoclonus or treatment with specific drugs (e.g. benzodiazepines). Facilitated LLR was more common in the patients with giant SEPs (46.1% vs 25%), but this association was not significant (Chi-square test). The amplitudes of the cortical SEP components negatively correlated with the age of the patients (R = 0.402, p = 0.048 and R = 0.415, p = 0.042 for N20P25 and P25N33; Pearson correlation), whereas we did not find any significant correlation in healthy subjects. 3.3. SEP recovery cycle The early SEP components (N20P25, P25N33) were clearly detectable and measurable starting from an ISI of 20 ms in all of the patients. Fig. 1A shows the mean SEP-R values in the different groups. RM-ANOVA revealed a significant main effect of groups for N20P25 (F = 8.6, p = 0.001), P25N33 (F = 22.1, p < 0.001) and P14N30 (F = 4.9; p = 0.013). Post hoc tests showed significant differences between the patients with giant SEPs and the controls (N20P25: p = 0.002, P25N33: p < 0.001, P14N30: p = 0.011), between the patients without giant SEPs and the controls (N20P25: p = 0.013, P25N33: p = 0.003), and between the patients with and without giant SEPs (P25N33: p = 0.040). The post hoc test of the ISIs showed that N20P25 amplitude was significantly reduced in the patients with giant SEPs at ISIs ranging from 150 to 400 ms, as was P25N33 amplitude at ISIs ranging from 20 to 300 ms. In patients without giant SEPs the N20P25 amplitude was significantly reduced at ISIs 300 ms and the P25N33 amplitude at ISIs 20, 50, 120, 180 and 250 ms. The latencies of the conditioned responses remained unchanged at the different ISIs. Fig. 1B and C shows examples of the responses to paired stimuli in two representative patients without and with giant SEPs at 120 ms ISI. The association between the SEP amplitude and SEP-R was performed by means of correlation analysis performed at different ISIs. In EPM1A patients, the Pearson correlation indicated that the amplitudes of N20P25 and P25N33 evoked by the first stimulus of the pair inversely correlated with recovery at ISIs 120 (R = 0.530, p = 0.006; R = 0.523, p = 0.007), 150 (R = 0.624, p = 0.001; R = 0.609, p = 0.001), 180 (R = 0.547, p = 0.005; R = 0.553, p = 0.004) and 200 (R = 0.599, p = 0.002; R = 0.533, p = 0.006) while healthy subjects did not show any significant relationship. In order to verify whether the period of decreased amplitude of the giant responses corresponded to a reduced ability of the somatosensory cortex to respond to incoming stimuli, we analysed the SEP waveform in the whole ISI range in detail. The early SEP waveform did not show any distortion, and at ISIs > 250 ms, the conditioned responses in the patients with giant SEPs, already exceeded the threshold of +3SD of the normative values considered as defining SEP gigantism (Fig. 2). 3.4. Middle-latency SEP components The unconditioned SEP waveforms of all of the patients showed evidence of an abnormal middle-latency complex that consistently included a prominent broad negative wave. This broad wave had a
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Fig. 1. (A) SEP-R profiles of N20P25 and P25N33 showing a significant recovery delay in patients with giant SEPs. Note the clearly suppressive effect and the U-shaped form in the range of ISIs of between 100 and 250 ms giving rise to delayed SEP-R that was more obvious in the patients with giant SEPs. (B and C) Representative SEPs obtained at ISI 120 ms in a patient without and with giant SEPs.
Fig. 2. Relationship between the mean amplitude of the response to the first stimulus (S1, circles) and second stimulus (S2, triangles) during paired stimulus in patients without (A) and with giant SEPs (B). The line indicates the threshold for the definition of a giant response (mean + 3SD of the control subjects’ response). Note that, despite the delayed recovery of the giant response, the patients with giant SEPs show a clearly detectable response to S2 at all ISIs.
latency ranging from 114.5 to 204.8 ms (149.4 ± 22.1 ms), and was obvious on both C30 and C40 , although consistently larger on C30 (Table 2, Fig. 3). It was significantly larger in the patients with giant SEPs, but also presents in the patients with normal amplitude early SEP components. In the patients with giant SEPs, the area of the
long negative wave measured on C30 positively correlated with that of N20P25 (R = 0.821, p = 0.001) and P25N33 (R = 0.832, p < 0.001) and, during SEP-R, the greatest decline in P25N33 in response to S2 occurred at ISIs ranging from 120 to 180 ms. At these ISIs, the recovery of this SEP component inversely correlated with
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Fig. 3. Broad middle latency components of SEPs in a representative patient with giant SEPs recorded on contralateral (A) and ipsilateral (B) derivation. Note the suppressive effect on S2 concomitant with the appearance of the middle-latency negative component (arrow).
that of the area under the negative late component (R = 0.629; R = 0.578; R = 0.580; p < 0.05 for ISIs of 120, 150 and 180 ms). 4. Discussion Our data concerning SEP-R in EPM1A patients suggest that the dysfunction in cortical sensory processing responsible for giant SEPs consistently combines with a long-lasting depression of excitability that leads to a significant delay in the recovery of conditioned SEPs. 4.1. Relationship between unconditioned SEPs and the recovery profile Previous information obtained by evaluating SEP-R in patients with cortical myoclonus often relates to a few subjects and partially conflicts with our findings. Ugawa et al. (1991) did not find any consistent relationship between SEP amplitude and the recovery function in three PME patients with enlarged SEPs, but an accelerated recovery of P25N33 suggested a less suppressive effect in one patient, and Ugawa et al. (1996) found a less suppressive effect in a range of 20–150 ms in three PME patients, but a delayed recovery of N20P25 and P25N33 components. Shibasaki et al. (1994), found an almost immediate recovery followed by a less excitable phase in patients with positive myoclonus, and increased suppression of the response to the first stimulus in patients with negative myoclonus. In our relatively large case series of EPM1A patients, we consistently found a highly significant relationship between the amplitude of the early components of the SEPs evoked by the first stimulus and the time to recover of the response to the second. This was indicated by the different profile of the recovery function of SEPs as well by the significant relationship between the amplitude of the response to the first stimulus of the pair and the percentage of recovery at ISIs between 120 and 200 ms SEPs found in individual patients. The delayed SEP-R affected the ‘‘gigantisms’’ of the conditioned SEPs, but not the occurrence of the evoked response. The early components of the conditioned SEPs were clearly detectable and measurable in all of the patients starting from an ISI of 20 ms, without any significant wave distortion or change in latencies suggesting a ‘‘jittered’’ response. This indicates that the delayed SEP-R does not reflect a depressed ability of the sensory cortex to respond to incoming stimuli, but indicates a failure of the neuronal pool to return to the original pathological synchronisation, which takes about 500–600 ms to restore. Therefore, whatever the post-excitatory mechanism eliciting the giant responses, the delayed recovery cannot be considered a truly suppressive period but a weakening in the pathological network generating giant SEPs.
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The pathophysiology of giant cortical SEPs is still uncertain, as is the nature of post-excitatory phenomena. The neuronal populations responsible for normal SEPs probably also generate giant N20P25 and P25N33 (Karhu et al., 1994), whereas other components may arise from complex behaviors of the neuronal network and the synchronous activation of cortical areas beyond the primary somatosensory cortex. This would be in line with the data reported by Valeriani et al. (1997), who evaluated giant SEPs in three subjects with cortical myoclonus of unknown cause and found that the early components had the same tangential dipole found in normal subjects, whereas the later components had a different dipole orientation. Moreover, when mapping the distribution of SEP components in four PME patients of various etiologies, Ikeda et al. (1995) found two distinct components of giant SEPs (radial P25–N35 and tangential P30–N30). In our EPM1A patients with giant SEPs, the consistent presence of a giant N30 component suggests the involvement of cortical areas other than the sensory cortex, including the supplementary motor area (Desmedt and Bourguet, 1985), or sensory-motor interactions. Additionally, circuitry rearrangement leading to hyperexcitability may also activate sources that are ‘‘inactive’’ in normal SEPs, thus contributing to the ‘‘giant’’ components (Ragazzoni et al., 1999). However, the variety of clinical conditions associated with giant SEPs makes difficult to compare the data. Shibasaki et al. (1985) postulated that giant SEPs (the P25N33 component) at least partially arise from the same pathophysiological mechanism as the myoclonus-related cortical spike. The hypothesis of a relationship between giant SEPs and epileptic transients in PME patients may also suggest that the post-excitatory inhibition that typically occurs after interictal EEG transients can also play a role in the long inhibitory phenomenon delaying the recovery of giant SEPs. Moreover, Ikeda et al. (1995) found that the enhanced components of giant SEPs were suppressed immediately after a seizure and assumed that they were markers of a lack of inhibitory mechanism. Post-excitatory inhibition in the epileptic network is frequently due to membrane hyperpolarisation caused by intrinsic properties (e.g. sustained ionic currents activated by calcium or sodium entry) and synaptic mechanisms (mainly due to GABAa and GABAb inhibition) (see review by Prince and Connors, 1986). The long-lasting attenuation of the N20P25 and P25N33 amplitudes of conditioned responses in patients with giant SEPs could be explained by enhanced membrane hyperpolarisation. However, the presence of intrinsic hyperpolarizing mechanisms is not supported by specific evidence and synaptic-mediated hyperpolarisation seems to be unlikely because evidence obtained in EPM1A patients (Manganotti et al., 2001; Canafoglia et al., 2010) and models of EPM1A (Franceschetti et al., 2007; Buzzi et al., 2012) suggesting a decreased rather than increased GABA effectiveness. A possible modulation of SEP-R played by serotonin cannot be excluded, since evidence obtained in rat sensory cortex suggested a significant role of this neurotransmitter in modulating paired pulse facilitation/depression (Torres-Escalante et al., 2004) and some data suggest changes in serotonergic neurotransmission occurring in EPM1A patients (Pranzatelli et al., 1995) and in the mouse model (Vaarmann et al., 2006). We observed that the giant early SEPs were consistently followed by a complex of late waves, which is difficult to compare with physiological middle SEP components. This waveform always included a large and broad negative wave with a latency of >100 ms, whose amplitude correlated with those of N20P25 and P25N33 in the patients with giant-SEPS. At the time of maximum amplitude (plateau) of this negative wave, the conditioned SEP was maximally depressed, which could hypothetically account for the roughly U-shaped profile of the SEP-R function. A similar wave with less amplitude was also observed in the EPM1A patients
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without giant SEPs, thus possibly accounting for their slightly delayed SEP-R. This suggests that the late SEP component may represents a sustained depolarization that is sufficient to suppress the temporarily abnormal local synchronisation that significantly occurs in EPM1A patients with giant early SEP components and, albeit to a lesser extent, in those with non-giant early SEPs components. In this setting, the interaction between post-excitatory hyperpolarisation and a late ‘‘giant’’ depolarising component may well account for a delayed SEP-R. Even if the asymmetrical amplitude of the late negative wave suggests that its generation includes a local depolarising phenomenon occurring in the sensory cortex, its bilateral representation indicates the involvement of subcortical loops. This observation is in line with the hypothesis of enhanced thalamo-cortical circuitry in patients with cortical myoclonus. Indeed, an aberrant activation of complex thalamocortical loops may generate both abnormal depolarization and ensuing after-depolarization mechanisms accounting for giant SEPs components and subsequent inhibition. 4.2. Relationship of giant SEPs and their recovery profiles with clinical features In our EPM1A patients, the amplitude of the giant SEP components inversely correlated with the patient age. Kobayashi et al. (2011) has described specific attenuation of the N40 component in two Unverricht–Lundborg patients during long-term followup. Although we did not find the consistent presence of N40, our data substantially agree with the idea of progressive attenuation of giant SEPs as a marker of hyperexcitability in this disease that has a tendency to stabilize late (Genton, 2006). Various studies have evaluated the effect of anticonvulsant drugs on SEPs and SEP-R (Hitomi et al., 2006; Vollono et al., 2010). Our patients were all treated with one or more drugs, but we did not find any relationship between SEP amplitudes and recovery parameters and specific drugs (e.g. benzodiazepine). Moreover, as in a previous study (Canafoglia et al., 2004) and in line with the observation of Ikeda et al. (1995), we did not find any clear relationship between the presence of giant SEPs or SEPR profiles and the functional severity of the myoclonus. This suggests that giant SEPs and the ensuing complex of excitatory-inhibitory events significantly reflect changes in cortical excitability but does not significantly affect the dysfunctioning motor circuitries sustaining myoclonic jerks. References Buzzi A, Chikhladze M, Falcicchia C, Paradiso B, Lanza G, Soukupova M, et al. Loss of cortical GABA terminals in Unverricht–Lundborg disease. Neurobiol Dis 2012;47:216–24. Canafoglia L, Ciano C, Panzica F, Scapoli V, Zucca C, Agazzi P, et al. Sensorimotor cortex excitability in Unverricht–Lundborg disease and Lafora body disease. Neurology 2004;63:2309–15. Canafoglia L, Ciano C, Visani E, Anversa P, Panzica F, Viri M, et al. Short and long interval cortical inhibition in patients with Unverricht–Lundborg and Lafora body disease. Epilepsy Res 2010;89:232–7. Desmedt JE, Bourguet M. Color imaging of parietal and frontal somatosensory potential fields evoked by stimulation of median or posterior tibial nerve in man. Electroencephalogr Clin Neurophysiol 1985;62:1–17. Franceschetti S, Sancini G, Buzzi A, Zucchini S, Paradiso B, Magnaghi G, et al. A pathogenetic hypothesis of Unverricht–Lundborg disease onset and progression. Neurobiol Dis 2007;25:675–85. Frasson E, Priori A, Bertolasi L, Mauguiere F, Fiaschi A, Tinazzi M. Somatosensory disinhibition in dystonia. Mov Disord 2001;16:674–82. Genton P. Unverricht–Lundborg disease (PME1). Rev Neurol (Paris) 2006;162: 819–26. Hamada Y, Otsuka S, Okamoto T, Suzuki R. The profile of the recovery cycle in human primary and secondary somatosensory cortex: a magnetoencephalography study. Clin Neurophysiol 2002;113:1787–93. Hitomi T, Ikeda A, Matsumoto R, Kinoshita M, Taki J, Usui K, et al. Generators and temporal succession of giant somatosensory evoked potentials in cortical reflex
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