Neurological Sciences and Neurophysiology

ORIGINAL ARTICLE
Year
: 2020  |  Volume : 37  |  Issue : 4  |  Page : 176--182

The association between causes and electrophysiology in myoclonus: When and why electrophysiology?


Meral E Kiziltan, Aysegül Gündüz, M Hazal Ser, S Naz Yeni, Çigdem Özkara, Veysi Demirbilek, Cengiz Yalçınkaya, Günes Kızıltan 
 Department of Neurology, Cerrahpasa School of Medicine, Istanbul University-Cerrahpasa, Istanbul, Turkey

Correspondence Address:
Aysegül Gündüz
Department of Neurology, Cerrahpasa School of Medicine, Istanbul University-Cerrahpasa, Istanbul
Turkey

Abstract

Objective: We aimed to identify the possible causes of myoclonus and related electrophysiological features in a cohort of young patients. Patients and Methods: We performed a retrospective analysis of all clinical and electrophysiological recordings of patients who had myoclonus and were under 60 years between 2005 and 2018. The clinical data included age at examination, gender, age at onset, and main neurological or systemic findings, underlying cause and electrophysiological features constituted surface electromyogram, long-loop reflexes, electroencephalography, and somatosensory-evoked potentials. Results: In the study period, we identified 155 patients with myoclonus. Myoclonus was most commonly related with epilepsy and movement disorders. Myoclonus with epilepsy was the leading cause between 10 and 30 years, whereas myoclonus with movement disorders was more common after 30 years. In our country, SSPE was an important cause of myoclonus under the 20 years of age. We identified cortico-subcortical subtype followed by cortical, cortical reflex, subcortical-basal ganglia, subcortical-brainstem and propriospinal subtypes, and correlated these subtypes with relevant disorders. Conclusion: The electrophysiological classification provides anatomical subtyping, which is favorable in diagnosing certain cases such as brainstem and propriospinal myoclonus. Certain characteristics such as reflex nature of myoclonus and accompanying features narrow the list of differentials and help in making the diagnosis.



How to cite this article:
Kiziltan ME, Gündüz A, Ser M H, Yeni S N, Özkara &, Demirbilek V, Yalçınkaya C, Kızıltan G. The association between causes and electrophysiology in myoclonus: When and why electrophysiology?.Neurol Sci Neurophysiol 2020;37:176-182


How to cite this URL:
Kiziltan ME, Gündüz A, Ser M H, Yeni S N, Özkara &, Demirbilek V, Yalçınkaya C, Kızıltan G. The association between causes and electrophysiology in myoclonus: When and why electrophysiology?. Neurol Sci Neurophysiol [serial online] 2020 [cited 2021 Mar 5 ];37:176-182
Available from: http://www.nsnjournal.org/text.asp?2020/37/4/176/305393


Full Text



 Introduction



Myoclonus is characterized by brief, rapid, irregular, and involuntary jerks. Myoclonus is classified according to the origin, time course, triggering factors, or other clinical features.[1],[2] Although its recognition is on clinical grounds, electrophysiology helps confirmation of its presence, differentiates it from other involuntary movements and determines its anatomical origin.[3] In a case with myoclonus, finding the underlying etiology is challenging because the list of differential diagnosis is long.

Polymyogram, evoked potentials, and jerk-locked averaging are the main electrophysiological investigations, which are used in the analysis of myoclonus.[4] A recent study identified cortical myoclonus as the most frequent one followed by subcortical, spinal, and peripheral myoclonus, respectively.[5] Functional myoclonus also constituted a huge group in this case series. In a cohort of clinically suspected cases of cortical myoclonus, electrophysiology confirmed the presence of cortical myoclonus in only half of the cases.[6] Hallett indicates that the presence or absence of certain electrophysiological findings such as C reflex cannot be used as an evidence of cortical myoclonus in all cases, although clinical neurophysiological testing does provide the gold standard in the diagnosis.[7]

Zutt et al . proposed an algorithm in 2015, in which the first step is to confirm the myoclonus and to determine its anatomical substrate. In any case of spinal or peripheral myoclonus, they directly recommended to perform specific tests, whereas in cortical myoclonus starting with detailed biochemistry is crucial.[8]

In this study, we aimed to identify the electrophysiological findings of myoclonus and possible causes in a cohort of young patients under the age of 60 years.

 Patients and Methods



Patients

We performed a retrospective analysis of all recordings, which were performed in our electrophysiology laboratory for the evaluation of involuntary movements between 2005 and 2018. A single neurophysiologist (MEK) did all recordings and two neurophysiologists (MEK, AG) reviewed them. Among these recordings, we have identified the patients with myoclonus under the age of 60 years. There was no lower limit of age, which was only limited by patient co-operation. We have included all the information regarding clinical, etiological, and electrophysiological findings. Patients who had limited information regarding clinical or etiological findings were excluded from the analysis.

The Institutional Review Board approved the study.

Methods

Clinical and electrophysiological data of patients were retrieved from the medical records. Clinical data included age at examination, gender, age at onset, and main neurological or systemic finding and underlying cause. The findings in neuroimaging and when provided, genetic analysis was also noted.

The diagnosis and classification of the epilepsy was based on the ILAE criteria.[9],[10] The diagnosis of movement disorders was done according to the clinical criteria.[11] Myoclonus, which clearly manifested after use of any drugs, was classified as medication-related. The diagnosis of functional movement disorder was also based on the clinical criteria.[12],[13]

Electrophysiological analyses were done using Ag-AgCl surface electromyography (EMG) recording electrodes (Neuropack Σ-MEB-5504K, Nihon Kohden Corporation, Tokyo, Japan) according to the standard techniques. These tests included surface EMG of appropriate muscles (maximum eight channels depending on the clinical findings), long-loop reflexes (LLRs), somatosensory-evoked responses (SEP), and electroencephalography (EEG) in some.

Myoclonus was described according to the previous reports.[1],[5] The categorization of myoclonus was done in accordance with the previous report, in which the criteria were adapted from Caviness 2003, Zutt et al. 2017 and Kızıltan et al . 2018.[1],[5],[14]

Polymyographic analysis

First, we identified the muscles that showed involuntary movements by a thorough clinical examination and placed the recording electrodes on these muscles up to eight. While recording by surface electromyography, the presence and characteristics of myoclonus were examined during rest, during loading, when hands were out-stretched or were performing a task (drinking coffee from a small cup), or after auditory and somatosensory stimulation. Filter settings were 10 Hz to 2 KHz.

Electroencephalography

EEG was recorded according to 10–20 international system using 28 electrodes with simultaneous recordings over muscles with involuntary movements. High-cut filter and time constant were 70 Hz and 0.3 s, respectively.

Long-loop reflexes

The recordings were done over abductor pollicis brevis muscle after a stimulus on the median nerve at wrist during rest and performance of a slight (approximately 10%–25% of maximum) contraction. The stimulus duration and intensity were 0.2 ms and 20–30 mA, respectively. The recordings were repeated twenty times. The sensitivity was 100 μV/division. Low-cut and high-cut filters were 2 and 2000 Hz.

Somatosensory-evoked potentials

The recordings were done over parietal cortices “C3” and “C4” with reference electrodes over Fz areas according to the 10–20 international system after stimulating the median nerve at wrist. Sensory threshold was determined, and stimulus intensity was set at three times the sensory threshold. Stimulus duration was 0.2 ms. The sensitivity was 5 μV. The activity was filtered between 10 and 100 Hz. Recordings were averaged 200 times.

Statistical analysis

Mean age, gender, etiology, main clinical finding, electrophysiological subtype of myoclonus, mean duration of bursts, presence of C reflex, amplitude of C reflex, and abnormal findings in brainstem reflexes were determined in the whole group. Burst duration was categorized: 30–50, 50–100, >100 ms.

Data analyses were performed using the SPSS software version 20 statistical package (SPSS Inc., Chicago, IL, USA).

 Results



In the study period, we have identified the recordings of 189 patients, which fulfilled the inclusion criteria. Among 189 patients, five patients were excluded because detailed electrophysiological investigations could not be done due to limited co-operation (age under 2 years or severe mental retardation). Twenty-nine patients were excluded due to the lack of the clinical information. The remaining 155 patients were included in the study and etiology, age, and type of myoclonus were analyzed. [Table 1] presents demographics.{Table 1}

The number of patients under 10 or higher than 50 years of age was lower. The percentage of male and female patients were similar among groups (P = 0.695).

Etiological factors and causes of admission

The causes were quite heterogeneous. Five big categories were remarkable: (i) myoclonus with epilepsy, (ii) myoclonus with a movement disorder, (iii) myoclonus in the course of other neurological disorders, (iv) myoclonus in systemic disorders, and (v) myoclonus after the use of medications. Among them, the major group was epilepsy (n = 55), which comprised progressive myoclonic epilepsy (PME, n = 26), juvenile myoclonic epilepsy (JME, n = 13), focal epilepsy (n = 10), unspecified epilepsy (n = 9), epilepsia partialis continua (n = 3), and Lennox-Gastatut syndrome (n = 1). Myoclonus was the major complaint on admission in PME and JME with other seizure types. In other types of epilepsies, myoclonus was incidentally determined when polymyogram was indicated to analyze clinically evident involuntary movements. In the focal epilepsy group, there were patients with temporal and frontal epilepsies followed by occipital epilepsies.

The second group included movement disorders; however, they were mostly genetically determined movement disorders. They were DYT-11 (n = 4), action myoclonus-renal failure (AM-RF, n = 4), ataxia telangiectasia like disorder (ATLD, n = 2), Niemann Pick type C (NP-C, n = 2), Wilson disease (n = 1), Gaucher disease (n = 1), and cerebrotendinous xanthomatosis (CTX, n = 1).

Among these disorders, myoclonus was the leading symptom in only DYT-11 and AM-RF. We also determined myoclonus in five patients with essential tremor, in five patients with cervical dystonia and in one patient with Tourette syndrome. In those patients, myoclonus was an adjunct to the other types of movement disorders and incidental during the analysis for other types of involuntary movements. We determined functional myoclonus in eight patients.

Other neurological disorders included subacute sclerosing panencephalitis (SSPE), post-hypoxic injury, posttraumatic injury, immune disorders, and early-onset cognitive disorders.

Epilepsy with myoclonus was most frequently observed in patients with ages between 10 and 30 years, whereas SSPE was only seen under 20 years of age. Myoclonus with movement disorders was evenly distributed among age groups [Table 1].

Subtypes of myoclonus according to electrophysiological findings

Based on the electrophysiological findings, the most commonly-identified subtype of myoclonus was the corticosubcortical subtype followed by cortical, subcortical-basal ganglia, subcortical-brainstem, and propriospinal subtypes [Table 2].{Table 2}

In cortical and corticosubcortical subtypes, burst durations were 30–50 or 50–100 ms. In subcortical-basal ganglia subtypes, burst durations were 50–100 ms. In subcortical-brainstem and propriospinal subtypes, burst durations were >100 ms. We also mentioned burst durations according to etiologies, because there were specific and interesting findings in some of the disorders such as PMEs or SSPEs.

Electrophysiological subtypes of myoclonus and other electrophysiological features according to major etiologies

In PME, all patients had cortical reflex myoclonus, which was triggered by somatosensory stimuli and seizures other than myoclonus [Figure 1]a. Short-duration positive and negative myoclonic discharges (duration <100 ms) were generalized, but was predominant on the distal parts of extremities with. C reflex and high-amplitude LLRs were present in all and giant SEPs were recorded in ten patients [Figure 1]b. All patients with JME had both positive and negative myoclonic discharges. Both types of discharges were <70 ms and affecting purely distal parts. None of them had C reflex or giant SEPs and only three had high-amplitude LLRs. Cortical origin was only established by EEG recording in some of the patients. We observed a similar pattern with short-duration myoclonic discharges and EEG findings but no C reflex or giant SEPs in a patient with glutamic acid decarboxylase antibody-related encephalitis. Patients with focal epilepsy also had positive and negative myoclonic discharges shorter than <60 ms. None had C reflex or high-amplitude LLRs. The discharges in epilepsia partialis continua were short-duration myoclonic discharges, themselves, cortical in origin, and were previously described elsewhere.[15] Three patients with unspecified epilepsy had cortical myoclonus (giant SEPs in two and C reflex in one).{Figure 1}

In patients without overt EEG findings correlated with EMG discharges, we classified the myoclonus as corticosubcortical subtype. Therefore, some of the patients with JME and most patients with focal epilepsies had corticosubcortical subtype because of other findings suggesting hyperexcitability of the cortex such as EEG findings and other types of seizures.

In AM-RF, there were short-duration positive and negative myoclonic discharges; however, the myoclonic bursts were time-related with spike activity on EEG suggesting cortical origin [Figure 1]c. They did not have C reflex. SEPs were low-amplitude, probably because of polyneuropathy.

Subcortical-basal ganglia subtype was common in patients with movement disorders such as Wilson disease, CTX, NP-C [Figure 1]d, and DYT11. The characteristics of myoclonus was short-duration positive and negative discharges which appeared during action or posture with lower amplitude relative to the cortical and probable corticosubcortical types without time-locked EEG findings, C reflex, or giant SEPs.

All patients with Valproic acid or lithium-related myoclonus had subcortical subtype.

There was subcortical-brainstem reticular myoclonus in patients with SSPE, post-hypoxic encephalopathy, or after traumatic brain injury, in primary hyperekplexia and in ATLD. In two siblings with hyperekplexia, auditory stimulation triggered very long-duration bursts with caudal spread, a startle response. A similar burst pattern was also obtained after the electrical stimulation of the median nerve at wrist. They did not have other types of myoclonus or C reflex, seizures, or other types of movement disorders. One patient with ATLD who had ataxia and oculomotor apraxia had short-duration positive myoclonic discharges. However, auditory stimulation triggered long-duration (>500 ms) spasm-like activity on the facial and neck muscles in both similar to patients with hyperekplexia. They did not have C reflex.

There were multiple types of myoclonus in cases with post-hypoxic injury, posttraumatic injury, and SSPE. Posthypoxic encephalopathy, however, simultaneously had myoclonus with cortical origin and also had long-duration, spasm-like discharges on facial, truncal, or lower extremity muscles which formed semi-rhythmic trains suggesting segmental origin. There were nine patients with SSPE. Five patients had very short-duration and high-amplitude positive myoclonic discharges. In three, the bursts were predominant on the distal parts of upper extremities forming trains. In all, negative myoclonus accompanied the positive discharges. In two, long duration and spasm-like activity accompanied short-duration bursts. None had C reflex, however, two patients had high-amplitude LLRII suggesting cortical origin. In four, we only recorded semi-rhythmic trains of very long duration (100–1000 ms), spasm-like discharges on facial, truncal or extremity muscles [Figure 1]e and [Figure 1]f suggesting brainstem reticular origin. All discharges were present spontaneously and increased in the intensity during posture or action.

Propriospinal myoclonus was classified as functional disorder in all cases based on the clinical criteria.

 Discussion



In this study, we have shown the features of myoclonus according to the different etiologies. According to our findings, epilepsy syndromes and genetic movement disorders mostly caused myoclonus among patients under the age of 60 years.

Despite the clinical significance, age at onset did not provide an easy diagnosis in cases with myoclonus. Degenerative conditions, which were seen in older ages,[14] were uncommon in this young population, whereas epilepsy and genetic movement disorders were common causes in the younger population.

The use of the electrophysiological subtyping facilitates the diagnosis in patients with subcortical-brainstem or propriospinal myoclonus.[3],[8] Electrophysiological subtyping narrowed the diagnostic investigations in subcortical-brainstem, brainstem reticular, and propriospinal subtypes of myoclonus. Because the underlying etiologies are quite limited in those subtypes in our cohort and in the literature. For example, in the presence of axial jerks with propriospinal origin, functional myoclonus is of primary importance. Incongruence, inconsistent patterns of bursts, and distractibility provided the evidence of functional origin in cases reported here. When we encounter a patient with brainstem reticular myoclonus, we consider primary and secondary causes of hyperekplexia. Neuroimaging to show the integrity of brainstem is generally indicated. The genetic causes are appraised if there are appropriate clinical findings without structural lesions.

For other subtypes, the causes were miscellaneous and making a diagnosis requires clinical features and other electrophysiology methods. We did not use the classification of epileptic, essential, and physiological myoclonus because we had disagreements about grouping the epileptic and essential ones. There were causes of cortical myoclonus other than epilepsies such as posthypoxic injury, AM-RF, or SSPE. The essential one is mainly considered myoclonus dystonia in the modern era. Therefore, in our opinion, the classification of epileptic, essential, etc., is outdated.

Cortical myoclonus is defined as the myoclonus, which originates in the sensorimotor cortex. The configuration of myoclonus is generally short duration and high-amplitude. In clinical practice, an EEG correlate in routine EEG investigation or jerk-locked back-averaging is required to consider a discharge as the cortical myoclonus. However, we know back averaging may be negative in more than half of the patients with cortical myoclonus.[3] Cortical reflex myoclonus is cortical myoclonus that is triggered by several modalities of stimulus and is characterized by the presence of C reflex or presence high-amplitude SEPs.

Actually, there are different types of epilepsy syndromes, in which cortical myoclonus is a part of the syndrome under the age of 10.[16],[17] In our cohort, there were no cases of infantile or childhood myoclonic epilepsies, probably for several reasons. First, the diagnosis was based on the specific clinical and EEG findings, and further electrophysiological tests were not required in these disorders. Second, a very young age is a disadvantage for the polymyographic analysis. Between 10 and 30 years of age, there were patients with PME syndromes who had cortical reflex myoclonus. Although there was not a patient in this cohort, one should remember that Huntington disease may lead to cortical reflex myoclonus even without development of chorea.[18],[19] In literature, Gaucher disease has also been described mimicking PME.[20] However, we here detected a patient with Gaucher disease who had myoclonus similar to subcortical subtype. Although myoclonic discharges in JME is short duration and high-in amplitude very similar to those seen in PME, there were no reflex properties such as C reflex or giant SEPs, which were high-amplitude only in a certain proportion of the JME patients.[21] In our opinion, myoclonus in JME is quite probably cortical in origin; however, we were only able to show it in some using EEG findings. This is because one may not record a myoclonus during a routine EEG. Second, we did not use jerk-locked back-averaging which was a limitation.

AM-RF is characterized by cortical myoclonus triggered by the action and renal involvement related to a genetic defect. Although movement or stimulus-activated myoclonus is not specific for an etiology and action myoclonus has been reported in Unverricht-Lundborg disease or other types of PME, or corticobasal degeneration, in any case of movement activated cortical myoclonus, investigations should include screening of proteinuria and renal functions. The electrophysiological clues for the AM-RF are the absence of giant SEPs or C reflex and the presence of polyneuropathy.

Cortical tremor and myoclonus syndrome also develops between 10 and 30 years of age.[22] There was one case in this cohort who had tremor and myoclonus; however, lacked high-amplitude SEPs or accompanying other seizures.

Regarding probable corticosubcortical or subcortical type-basal ganglia, we have identified metabolic disorders such as WD, CTX, NP-C, mitochondrial mutations, movement disorders such as DYT11, focal epilepsies, or use of medications. These are generally inherited conditions. Myoclonus is rare.[23] Although the electrophysiological studies are limited, it is mostly related with subcortical type-basal ganglia.[23],[24],[25] Interestingly, in patients with myoclonus-predominant phenotype of CTX, no tendon xanthomas are seen as in our case.[25] The presence of cataracts in the juvenile period should suggest CTX. In NP-C, cortical myoclonus has also been described.[26] In myoclonus-dystonia syndrome (DYT11), no cortical correlates in jerk-locked back-averaging or no high-amplitude LLRs and SEPs.[27]

Zutt et al . proposed an algorithm in 2015.[28] According to that algorithm, the recognition of spinal or peripheral myoclonus reduces the list of differential diagnosis. Although acquired metabolic disorders such as liver or renal failure or use of medications[29] may be the cause of myoclonic jerks in all age groups, the number was very low in our cohort. We think that the low number of patients with acquired systemic disorders is related with the low number of referrals due to the low level of disability experienced by patients or misdiagnosis as tremor. Functional myoclonus was only responsible for 5% of the entire cohort in this study, in contradiction to Zutt et al . 2017,[5] in which functional myoclonus was more common. The reason may be attributed to low referral of pseudo seizures since they were diagnosed using video EEG in our clinic. A second cause is that we did not include patients with facial involuntary movements. Electrophysiological findings in our study were not able to differentiate JME or other epilepsies with possible corticosubcortical myoclonus from other causes. As we mentioned, the major problem in these types of disorders is the less frequent occurrence of discharges impeding jerk-locked back-averaging. The interictal EEG findings were also less frequent and mild in these epilepsy syndromes. There was no stimulus sensitivity; however, we did not evaluate photic sensitivity, which is a known triggering factor in JME. Interestingly, the cases with infectious causes (such as Whipple disease or viral disorders) or autoimmune encephalitis were low in our cohort. Besides the limitations mentioned above, the retrospective nature of the study was the main limitation.

 Conclusion



In any case with myoclonus, electrophysiology provides a way of classification and a contribution to major clinical findings to identify the underlying etiology. Electrophysiological subtyping of myoclonus is especially helpful in certain cases such as brainstem reticular myoclonus or propriospinal myoclonus. In other subtypes, certain characteristics such as reflex nature of myoclonus and accompanying features may help in making the diagnosis.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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