Authors: Martje E. van Egmond MD, Anouk Kuiper MD, Jan Willem J. Elting MD, PhD, Oebele F. Brouwer MD, PhD, Tom J. de Koning MD, PhD and Marina A.J. Tijssen MD, PhD Article first published online: 24 FEB 2015 | DOI: 10.1002/mdc3.12136
Myoclonus can resemble other abrupt movements, such as chorea. The clinical distinction can be challenging, particularly at a young age. Myoclonus is defined as sudden, brief, shock-like involuntary movements caused by muscular contractions or inhibitions. Chorea is a nonpatterned, involuntary movement disorder with continuous movements, variable in speed, unpredictable in timing and direction, and flowing or jerky in appearance. Especially, the jerky components of chorea may be difficult to distinguish from myoclonus.[2-5]
Here, we present the case of a young boy with hyperkinetic movements closely resembling chorea. EEG/electromyography (EMG) recordings showed cortical reflex myoclonus. This case demonstrates that myoclonus in young patients with GOSR2 mutations can mimic chorea.
A Dutch boy presented at the age of 2 years with frequent falls and a clumsy gait. Brain MRI and EMG at the age of 2.5 years were normal. One year later, he developed areflexia and a mild limb and gait ataxia. At the age of 6, multifocal hyperkinetic movements were observed. The family history was negative, his cognitive skills were normal, and he had never had any seizures. The symptoms progressed gradually. At the age of 7, he was referred to our multidisciplinary pediatric movement disorder outpatient clinic. Neurological examination showed multifocal jerky movements, most pronounced in the face and distal part of the upper extremities, and worsening with action and tactile stimuli (see Video 1). There was a mild limb ataxia and his gait was irregular. Tandem walking was hampered by both action myoclonus and ataxia. He had no sensory deficits and no skeletal abnormalities.
EEG/EMG recordings showed multifocal brief muscle contractions with a short burst duration (40–70 ms), supportive of cortical myoclonus. In addition to positive myoclonus, negative myoclonus occurred. Photic stimulation elicited myoclonic jerks and a photoparoxysmal EEG response, with a strict relation between the myoclonus and the EEG, consistent with cortical reflex myoclonus (Fig. 1). Mutation analysis showed a homozygous pathogenic mutation c.430G>T (p.Gly144Trp) in the GOSR2 gene.
Figure 1. EEG/EMG recording showing cortical reflex myoclonus. Cortical activity was recorded by EEG, with 19 channels and a laplacian reference, using a 10 to 20 montage with average reference. Myoclonus was recorded by surface EMG: bilaterally from the biceps brachii muscles and triceps brachii muscles (EMG1-4); wrist flexor and extensor muscles (EMG5-8); and the abductor pollicis brevis muscle on the left side (EMG9). Accelerometer measures were recorded from the back side of both hands (ACC1 and ACC2). The registration revealed multifocal myoclonic jerks with a short burst duration (40–70 ms). Photic stimulation elicited myoclonic jerks and a photoparoxysmal EEG response, with a strict correlation between the myoclonus and the EEG, consistent with cortical reflex myoclonus (amplification, 150 μV/cm).
This case demonstrates that hyperkinetic movements in a young patient with GOSR2 mutation can mimic chorea, which is different from the thus far delineated phenotype of GOSR2 mutations. At the Video Challenge of the Movement Disorder Society Congress 2014, the video of this case was shown and the majority of the expert panel classified the movement disorder as chorea.
Since the first report of the GOSR2 mutation in 2011, 17 cases have been described worldwide,[6-8] including this case, and until now, the clinical phenotype was classified either as progressive myoclonus epilepsy[6, 7] or as Ramsay Hunt syndrome. This case emphasizes the importance of an accurate movement disorder characterization and the utility of neurophysiological studies in order to achieve a correct phenotypic description to guide diagnostic testing.
In clinical practice, an important diagnostic clue for cortical reflex myoclonus is the stimulus sensitivity of the jerks, by tactile, visual, or auditory stimuli. In contrast, chorea is not stimulus sensitive. Typical features of chorea are the constant flowing movements with random distribution and the incorporation into semivoluntary movements. A complicating factor is that abrupt jerks can be part of choreatic movements.[2, 9] This is supported by the fact that the clinical distinction between myoclonus dystonia and benign hereditary chorea can be challenging.[3-5]
In addition to a careful clinical examination, neurophysiological studies can be helpful in the diagnostic workup of rapid, jerky movements. In cortical myoclonus, EMG shows bursts with short duration (<50–70 ms) and sometimes also negative myoclonus can be observed. EEG/EMG coregistration enables jerk-locked back-averaging or coherence analysis to establish whether a cortical spike precedes the myoclonic jerk. In chorea, polymyography shows a random pattern of muscle activation and variable EMG burst length. Furthermore, there is no cortical correlate for the movements. In the case presented here, the EMG bursts were short and the EEG/EMG recordings demonstrated cortical myoclonus. This is in line with previous reports describing neurophysiological studies in patients with GOSR2mutations.[6-8]
The GOSR2 phenotype evolves with age. Most patients present at the age of 2 to 3 years with early-onset ataxia, followed by areflexia, myoclonus, and epilepsy (at the average age of 6.5 years).[6-8] In adolescence, many patients develop scoliosis.[6-8] It was named “North Sea” progressive myoclonus epilepsy. Given that myoclonus and ataxia overshadow relatively mild epilepsy, the phenotype can also be described as Ramsay Hunt syndrome.
In conclusion, our case illustrates that the phenomenology of cortical myoclonus in young children with GOSR2 mutations can closely resemble chorea. Based on the presented phenotype, we recommend (pediatric) movement disorder specialists to consider electrophysiological tests and mutation analysis of GOSR2 in young patients with clinical features resembling chorea, especially if areflexia is also present.