Address correspondence to Professor Richard Macdonell, MD FRACP, Director of Neurology, Department of Neurology, Austin Health, Studley Road, Heidelberg, Victoria 3084, Australia. E-mail: email@example.com
Purpose: We used transcranial magnetic stimulation (TMS) to investigate whether there were any characteristic cortical excitability changes in progressive myoclonic epilepsy (PME) compared to juvenile myoclonic epilepsy (JME).
Methods: Six patients with PME were studied. Motor threshold (MT) at rest and recovery curve analysis using paired-pulse stimulation at a number of interstimulus intervals (ISIs) was determined. Results were compared to those of 9 patients with chronic refractory JME and 10 with chronic well-controlled JME.
Results: PME showed a marked increase in cortical excitability at all the long ISIs (p < 0.01), compared to refractory JME (effect sizes ranging from 1.4 to 1.9) and well-controlled JME (effect sizes ranging from 2.0 to 2.4). Significant differences at the short ISIs 2–5 ms were seen only on comparison with the well-controlled group (p < 0.05, effect size 0.6, 0.7). There were no significant differences in MTs of PME compared to either JME groups.
Conclusion: Our findings demonstrate specific differences in cortical excitability using TMS between PME and those with JME, particularly at long latencies in the paired-pulse paradigm, implicating a role for γ-aminobutyric acid (GABA)B-mediated networks.
Progressive myoclonic epilepsy (PME) is a syndrome consisting of a group of disorders characterized by myoclonic and tonic–clonic seizures, associated with progressive neurologic dysfunction, especially ataxia (Berkovic et al., 1986). In the early stages, distinguishing PME from other forms of idiopathic generalized epilepsy (IGE) with prominent myoclonic seizures, especially juvenile myoclonic epilepsy (JME), can be difficult. It may be years before the initial effectiveness of antiepileptic drugs (AEDs) diminishes in patients with PME, seizures become more frequent, and the inevitable neurologic decline becomes apparent.
Reflex or action myoclonus and exaggerated responses to afferent stimuli in the form of giant somatosensory evoked potentials (Ikeda et al., 1995; Mima et al., 1997) or long latency responses to peripheral nerve stimulation (So et al., 1989; Berkovic et al., 1991) are some of the main clinical and neurophysiologic characteristics of patients with PME. These abnormalities have been attributed to hyperexcitability of the motor cortex (Shibasaki et al., 1985). JME is a disorder characterized by prominent myoclonic seizures, which also indicates increased motor cortical excitability (Grunewald & Panayiotopoulos, 1993). Although increased motor cortical excitability is a feature of both PME and JME, the pathophysiologic mechanisms underlying this are most likely different. It could be that PME has unique patterns of alterations in motor cortical excitability that can be used to distinguish it from other forms of IGE.
Patients were recruited through the Epilepsy Clinic at Austin Health in Melbourne. This is a tertiary referral center for the follow-up of patients diagnosed with epilepsy. The diagnosis of epilepsy and its subsyndrome was made by at least two experienced epileptologists on the basis of the clinical history, imaging, and electroencephalography (EEG) findings. Only patients with a confirmed diagnosis of either PME or JME were included.
Six patients with PME were studied (three female; mean age 39 years, range 32–54 years). Patients included three diagnosed with Unverricht Lundborg disease (ULD) with mutations in CSTB, the gene encoding cystatin B; two with action myoclonus with renal failure syndrome (AMRF) with mutations in SCARB2 (16); and one with myoclonus epilepsy with ragged-red fiber syndrome (MERFF) with a mitochondrial DNA mutation (A8344G). All were taking multiple AEDs; all had rare tonic–clonic seizures but active action myoclonus (Table 1).
Table 1. Summary of clinical characteristics of patients in each group
GTCS: 2 ± 1 per year Frequent myoclonic jerks Frequent action myoclonus Fragmentary myoclonus at rest
VPA, CBZ, PRM, LEV, PTC, ZNS
17 ± 8
14 ± 6
GTCS: 4 ± 2 per year Frequent myoclonic jerks on awakening
VPA, LEV, TPM
16 ± 5
14 ± 5
VPA, LEV, TPM
The results were compared to those of 9 patients with chronic refractory JME (four female; mean age 29 years, range 21–44 years), 10 patients with well-controlled JME on treatment (six female; mean age 28 years, range 22–42 years) (Table 1). Patients were considered refractory if they continued to have ongoing seizures for at least 2 years despite trials of three different AEDs, on therapeutic doses. This included myoclonic and generalized tonic–clonic seizures. Patients were considered well controlled if they had not had seizures of any type for the preceding 2 years while receiving the same dose of AED (Kwan & Brodie, 2000).
Thirty-two healthy control subjects (twenty female; mean age 31 years, range 16–73 years) without a history of seizures or other neurologic conditions were used as controls.
Transcranial magnetic stimulation
Each subject (patients and nonepilepsy controls) had a single TMS test performed on the dominant hemisphere using the testing paradigm described in detail in our previous reports (Badawy et al., 2007). Briefly, this involved the use of a flat circular 9-cm diameter magnetic coil (14-cm external diameter) with the center of the coil positioned over the vertex to stimulate the contralateral motor cortex and recording the motor-evoked potential (MEP) from the abductor pollicis brevis muscle. Intracortical excitability was studied by paired stimulation at various interstimulus intervals (ISIs) using a Bistim module to connect two stimulators to the coil.
In each experimental session the following parameters were recorded: (1) Motor threshold (MT): defined as the lowest level of stimulus intensity that produced an MEP in the target muscle of peak-to-peak amplitude >100 μV on ≥50% of 10 trials (Rossini et al., 1994) and (2) Cortical recovery curves using paired pulse stimulation: to construct the long ISI recovery curves using paired stimuli in 50 ms increments at ISIs of 50–400 ms and the short ISI recovery curves at 2, 5, 10, and 15 ms ISIs.
To avoid any effect of diurnal variation in cortical excitability, all studies were performed in the middle of the day (between noon and 2 p.m.). All patients had 7–9 h of uninterrupted sleep the night before the test and the results were analyzed only after a minimum of 2 weeks of tonic–clonic seizure freedom on either side of the test was confirmed. No patient had a tonic–clonic seizure during the TMS study.
The study protocols were approved by the Austin Health Human Research Ethics Committee and written informed consent was obtained from all individuals.
A one-way analysis of variance (ANOVA) was used. Each test had a between-subjects factor “group” as follows: PME versus refractory and well-controlled JME.
The same analysis was repeated comparing PME with nonepilepsy control subjects.
Post hoc analysis with pair-wise paired t test and Bonferroni correction was used to compare all significant interactions.
The effect size (d) of therapy was calculated for the significant results (MT and each ISI) using the formula:
Effect size 0.2 was considered small, 0.5 was considered medium, and ≥0.8 was considered large (Cohen, 1969).
Table 2 presents the MT values for the dominant hemisphere for each subject group. There were no significant intragroup or intergroup differences in mean MT between PME and either the JME group or nonepilepsy control subjects. However, it was noted that the MT value was lowest in the PME group.
Table 2. Motor threshold (mean ± SD) for the dominant hemisphere in PME, refractory, and well-controlled JME and nonepilepsy control subjects
MT (stimulus intensity %)
54.7 ± 5.9
55.3 ± 9.7
57.6 ± 6.8
Nonepilepsy control subjects
56.9 ± 6.4
Cortical recovery curves
PME compared to JME and controls
Figure 1 presents the short and long ISI recovery curves for each subject group. PME showed a marked increase in cortical excitability at all the long ISIs (50–400 ms) (p < 0.01), compared to the refractory JME group (effect sizes ranging from 1.4 to 1.9) and the well-controlled JME and nonepilepsy control groups (effect sizes ranging from 2.0 to 2.4).
Significant differences at the short ISIs of 2–5 ms were seen on comparison of PME with the well-controlled group with JME (p < 0.05; effect sizes 0.6 and 0.7, respectively) and nonepilepsy control subjects (p < 0.01; effect sizes 1.0 and 1.2). At short ISIs the PME group did not differ from the refractory JME group.
JME compared to controls
Refractory JME showed an increase in cortical excitability at the short ISIs of 2 and 5 ms (p < 0.05; effect size 0.8) and long ISIs of 100–400 ms (p < 0.05; effect sizes ranging from 1 to 1.6) compared to nonepilepsy controls. The well-controlled JME cohort showed a much smaller increase in cortical excitability compared to nonepilepsy controls and was only observed at the long ISIs 150, 250, and 300 ms ISIs (p < 0.05; effect sizes 0.5–0.9).
In the present study we show increased cortical excitability in patients with PME compared to patients with chronic refractory and well-controlled JME and nonepilepsy control subjects. Previous studies using afferent stimulation by light (Rubboli et al., 1999) and by peripheral sensory stimulation (Reutens et al., 1993; Manganotti et al., 2001) have also shown evidence of increased cortical excitability; the paired-pulse paradigm used here extends this work allowing inferences as to the mechanisms.
The most striking finding was a marked reduction in long intracortical inhibition (facilitation of the test response at all the long ISIs) in PME compared to all the other groups. Short intracortical inhibition was also reduced but to a lesser extent, and did not differ from that seen in refractory JME. Patients with refractory JME also showed reduced short and long intracortical inhibition compared to controls, but the effect size was smaller than that seen in PME. This could underlie the difference in myoclonus between the two cohorts. All our PME patients had active action myoclonus, fragmentary myoclonus at rest, as well as frequent bilateral myoclonic seizures. In contrast our refractory JME cohort had bilateral myoclonic seizures, with no myoclonus at rest and only rare myoclonus on action.
Short intracortical inhibition is thought to reflect activity in the GABAA-mediated circuits in the motor cortex (Kujirai et al., 1993; Inghilleri et al., 1996; Boroojerdi, 2002; Ziemann, 2003) and long intracortical inhibition to indicate activity in GABAB-mediated circuits (Mott & Lewis, 1994; Ziemann et al., 1998; Valzania et al., 1999). Our findings would thus imply a specific pattern of altered GABAergic (particularly GABAB) mediated inhibition in PME compared to JME, which cannot be attributed to the effect of AEDs alone as the patients with chronic refractory JME were also taking multiple AEDs. It is also well known that cerebellar pathology exists in PME (Lance, 1986), which could result in thalamic and/or motor cortical excitability from the loss of Purkinje cell inhibition. The findings of the current study are in accord with those of previous reports investigating short (ISIs 1–4 ms) (Manganotti et al., 2001) and long recovery curves (ISIs 50–150 ms) (Valzania et al., 1999) in patients with PME compared to nonepilepsy controls, which also demonstrated motor cortex hyperexcitability in PME.
Other authors have found that the alteration of cortical inhibition in patients with cortical myoclonus also affects interhemispheric connectivity, as evidenced by the loss of interhemispheric inhibition in these patients, and correlated this with myoclonus (Hanajima et al., 2001). Similarly, in the current study, patients had rare tonic–clonic seizures, so the reduction of short and long intracortical inhibition also appears to be related to the presence of myoclonus. This is consistent with previous reports and may imply that this pattern of reduced motor cortical inhibition facilitates the occurrence of myoclonus irrespective of tonic–clonic seizure activity (Lefaucheur, 2006).
There was no difference in MT in the current study in patients with PME compared to both groups with longstanding JME and consistent with previous reports (Reutens et al., 1993; Valzania et al., 1999; Manganotti et al., 2001) with nonepilepsy controls, although there is a recent report of increased MT in patients with Unverricht-Lundborg disease (Danner et al., 2009). MT mainly reflects neuronal membrane excitability, which largely depends on Na+ channel conductivity (Mavroudakis et al., 1997) and is very sensitive to the effect of most of the currently used AEDs, including the drugs used by patients in our cohort (Ziemann, 2004). It is, therefore, likely that lack of difference in or increased MT between patients with PME and nonepilepsy controls is an effect of medication (Lefaucheur, 2006). It would be interesting to explore cortical excitability changes in drug-naive patients with PME to determine that. This would most likely require prolonged longitudinal studies of patients presenting with newly diagnosed myoclonic seizures due to the current difficulty of distinguishing PME patients at onset.
We wish to thank the neurologists in the Epilepsy Clinics for their help in recruiting the patients and providing input for the study, Dr James MacPherson (Wessex, UK) for analysis of CSTB, Dr Leanne Dibbens (University of Adelaide) for analysis of SCARB2, and Dr. Rosetta Marotta (Melbourne Neuromuscular Research Institute) for analysis of the MERRF mutation. We also thank Dr. Sue Finch and the Statistical Consulting Centre at the University of Melbourne for help with statistical analysis of the results, Dr. Danny Flanagan for his help and continual support, the EEG technicians at Austin Health, and the participants for their time.
We confirm that we have read the Journal’s position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.
None of the authors has any conflict of interest to disclose.