Neurophysiologie Clinique/Clinical Neurophysiology (2009) 39, 117—121 ORIGINAL ARTICLE/ARTICLE ORIGINAL Asymmetric POSTS associated with unilateral EEG abnormalities POSTS asymétriques associés à des anomalies EEG unilatérales S. Aybek , M. Maeder-Ingvar , P.-A. Despland , A.-O. Rossetti ∗ Service de neurologie, université de Lausanne, CHU Vaudois, Lausanne, Switzerland Received 4 September 2008; accepted 6 January 2009 Available online 29 January 2009 KEYWORDS POSTS; Asymmetric; EEG abnormalities; Non-REM sleep; Physiological MOTS CLÉS POSTS ; Asymétriques ; Anomalies EEG ; Summary Objective. — Positive occipital sharp transient of the sleep (POSTS) are considered a normal variant of non-REM sleep EEG. We describe a small series of patients with asymmetric POSTS and ipsilateral abnormal EEG findings. Methods. — Over a period of 30 weeks, we prospectively observed five consecutive subjects with strictly unilateral POSTS associated with ispilateral electrographic abnormalities. They represent 0.4% of all EEG performed over the same time lapse (5/1130), including inpatients, outpatients and long-term monitoring. Results. — Four women and one boy suffering from epileptic seizures (aged 7—76 years old) had unilateral POSTS, occurring only on the right side, during light sleep. They also presented ipsilateral epileptiform abnormalities. Conclusion. — The fact that POSTS were asymmetric and found only on the same side as the abnormalities raises the question whether these transients should still be considered physiological or could be interpreted at times as markers of underlying electrical abnormalities, pointing to an increased cortical excitability on the more active side. Although larger samples are needed to confirm our preliminary results, this case study questions the interpretation of POSTS as a uniformly normal variant. © 2009 Elsevier Masson SAS. All rights reserved. Résumé But. — Les positive occipital sharp transient of the sleep (POSTS) sont considérés comme une variante de la norme de l’électroencéphalogramme du sommeil non REM. Une petite série de patients présentant des POSTS asymétriques associés à des anomalies EEG ispilatérales est décrite. Patients et méthode. — Sur une période de 30 semaines, cinq patients consécutifs ont présenté des POSTS strictement unilatéraux associés à des anomalies électrographiques ipsilatérales, ∗ Corresponding author. Service de neurologie, CHUV, BH-07, center hospitalier, 1011 Lausanne, Switzerland. E-mail address: andrea.rossetti@chuv.ch (A.-O. Rossetti). 0987-7053/$ – see front matter © 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.neucli.2009.01.001 118 Sommeil non-REM ; Physiologique S. Aybek et al. représentant 0,4 % des enregistrements EEG effectués dans ce même temps (5/1130), y compris les patients hospitalisés, ambulatoires et les enregistrements de longue durée. Résultats. — Quatre femmes et un garçon ayant présenté une crise comitiale (âgés de sept à 76 ans) ont présenté des POSTS unilatéraux sur l’hémisphère droit lors du sommeil léger. Des anomalies épileptiformes ipsilatérales ont été observées. Conclusion. — Le fait que les POSTS soient asymétriques et seulement présents du côté où les anomalies épileptiformes ont été notées pose la question de savoir si les POSTS doivent toujours être considérés comme physiologiques ou devraient être interprétés parfois comme marqueurs d’une anomalie électrique sous-jacente, suggérant une excitabilité corticale accrue du côté plus actif. Bien qu’un échantillon plus large soit nécessaire pour confirmer nos résultats préliminaires, cette observation remet en question l’interprétation des POSTS comme une variante de la norme. © 2009 Elsevier Masson SAS. Tous droits réservés. Introduction Positive occipital sharp transients of sleep (POSTS) were first identified on EEG in 1950 [4] and have been accepted as being part of physiological sleep [3,11]. POSTS appear as positive waves in the occipital region, occurring during all non-REM sleep stages; they are reported more often in subjects showing occipital lambda waves during the waking state, raising the possibility that POSTS might be related to the fixation of visual information [8]. They may have a shifting predominance [11]. POSTS are found on routine EEG including sleep in 50% [4] to 80% [2] of adult subjects, more often in young and middleage adults, with a decline of prevalence after age 70 [12]; they are infrequently seen in infancy and early childhood and usually appear between 6—12 years, becoming more prominent in teenagers. A female preponderance has been reported [9]. There are rare descriptions of pathological conditions associated with POSTS, such as fainting spells [11], dysthymic disorder [7], headaches [10], and behavioural disorders with increased impulsivity [5]. In all these cases, there was no mention of asymmetrical POSTS. To the best of our knowledge, the description of unilateral POSTS as a potential marker of abnormal EEG has not been reported yet. We describe here a small series of patients that we recently observed with asymmetric POSTS and ipsilateral abnormal EEG findings. Methods and patients’ description Over a period of 30 weeks (October 2006—May 2007), we prospectively observed five consecutive subjects with asymmetric POSTS associated with ispilateral electrographic abnormalities on routine EEG. Asymmetric POSTS were defined when occurrence and persistence clearly predominated on one side over the entire recording. This occurred in 0.4% of routine EEG performed over the same time lapse (5/1130), including inpatients, outpatients, and longterm monitoring. In these five patients, EEG were digitally recorded over 20—30 minutes, during wakefulness and light sleep, by registered technologists, using 21 or 23 electrodes arranged according to the International 10—20 System (in selected cases with anterior temporal leads). Standard settings during acquisition included high-frequency filter at 70 Hz, low-frequency filter at 0.5 Hz, notch filter at 50 Hz. The tracings were interpreted on longitudinal and transverse bipolar, as well as average reference montages; these could be reformatted ad libitum during the reading. Patients’ histories are described below and clinical findings are summarized in Table 1. An EEG example of patient 5 can be seen in Fig. 1. All patients showed alpha rhythm during wakefulness, with no significant asymmetry (i.e., no side had an amplitude exceeding the double of the contralateral side), and symmetrical sleep transients (spindles, vertex waves, and K complexes). One of them also showed lambda waves during wakefulness, retaining the asymmetry. Patient 1 A 76-year-old woman, known for hypertension, had a sudden loss of contact with asymmetric facial rhythmic movements on the left side, and then displayed tonic-clonic movements of the left hemibody, followed by drowsiness. A left-sided weakness was observed in the emergency room, and an acute right middle cerebral artery (MCA) stroke was visualized on the brain CT scan. Her wakefulness EEG showed an 8—10 Hz alpha activity with intermittent right temporal rhythmic theta slowing. During sleep, symmetric spindles and K waves were observed, and asymmetric POSTS, only on the right hemisphere, were noted. She was diagnosed with an early symptomatic seizure. Patient 2 A 7-year-old boy developed 4 years earlier nocturnal generalized tonic seizures with cyanosis and sialorrhea. His neurological examination and brain MRI were within normal limits. The EEG showed right frontocentral epileptiform discharges. He was diagnosed with cryptogenic frontal lobe epilepsy. A follow-up EEG confirmed right frontocentral interictal epileptiform discharges and right-sided POSTS during sleep. EEG during wakefulness revealed a 7—8 Hz, symmetrical alpha activity. Asymmetric POSTS associated with unilateral EEG abnormalities Table 1 119 Clinical and EEG findings for the five patients. Patient Age (y) Gender Diagnosis Background EEG POSTS 1 76 F Right MCA stroke with early ipsilateral complex partial seizure Alpha 8—10 Hz (R: 20 ␮V, L: 20 ␮V) Right temporal rhythmic theta slowing PS: not performed Right-sided 2 7 M Cryptogenic frontal lobe epilepsy Right-sided 3 28 F Symptomatic epilepsy 4 20 F Provoked seizures after BDZ withdrawal 5 24 F Symptomatic epilepsy Probable Multiple sclerosis Alpha 7—8 Hz (R: 50 ␮V, L: 50 ␮V) Rare right frontocentral sharp waves during sleep PS: no driving response Alpha 10 Hz (R: 40 ␮V, L: 50 ␮V) Right posterior theta with a pseudorythmic aspect and superimposed sharp waves Lambda waves only on the right hemisphere PS: asymmetric (right) photic driving at low frequencies Alpha 10 Hz (R: 30 ␮V, L: 40 ␮V), frontosagittal sharp waves more prominent on the right PS: not performed Alpha 11 Hz (R: 60 ␮V, L: 50 ␮V) (2005: right central and posterior sharp theta ) PS: not peformed Right-sided Right-sided Right-sided MCA: middle cerebral artery; GTC: generalized tonic-clonic seizure; BDZ: benzodiazepines; PS: photic stimulation. Figure 1 EEG during drowsiness (patient 5) showing right posterior epileptiform transients (A), and POSTS occurring only on the right (B, C). Bipolar, longitudinal montage (A and C), average referential montage (B), 30 mm/s, 100 ␮V/cm, LFF 0,5 Hz, HFF 70 Hz. 120 Patient 3 A 28-year-old woman, known for depression, presented a sudden loss of consciousness with a tonic phase and a postictal confusion lasting 30 min, suggesting a generalized convulsive seizure. She reported a lack of sleep in the preceding weeks. She was on paroxetine, and had chronic moderate alcohol consumption. Her neurological examination and the lumbar puncture were normal. Brain MRI was normal. Two EEG recorded within 72 h showed 10 Hz alpha rhythm, right occipital brief rhythmic theta, with intermixed sharp transients and ipsilateral lambda waves; photic driving was asymmetric (right more than left). During drowsiness, POSTS were seen on the right. Three months later, she had a similar episode, and the diagnosis of symptomatic epilepsy was made. Patient 4 A 20-year-old woman, known for a psychotic disorder, experienced several generalized convulsive seizures since the age of 19, either following acute alcohol consumption or benzodiazepine withdrawal. She was on quetiapine. Neurological examination and her contrast enhanced CT scan performed after a new episode were normal. Her wakefulness EEG showed a 10 Hz alpha rhythm, with rare frontosagittal sharp waves more prominent on the right. Her sleep EEG revealed right-sided POSTS. Patient 5 A 24-year-old woman experienced two secondarily generalized seizures preceded by a feeling of déjà vu over 3 years. She also described a numbness of her right leg for 3 months. Her examination demonstrated bilateral ataxia and right leg hypoesthesia. Brain MRI disclosed multiple nonenhancing periventricular white matter lesions and her lumbar CSF analysis oligoclonal bands. A diagnosis of probable Multiple Sclerosis with symptomatic partial seizure with secondary generalization was made. Her EEG showed an 11 Hz alpha rhythm with right-sided POSTS during somnolence. Her previous EEG (2005) demonstrated right central and posterior sharp theta activity. Discussion S. Aybek et al. suggest that POSTS might be rarely associated with epileptiform or other focal abnormalities. The fact that POSTS were asymmetric and found only on the same side as the abnormalities appears intriguing, and raises the question whether these transients should still be always considered physiological or could be interpreted, in some rare instances, as a marker of underlying electrical abnormalities. The asymmetry we noted concerned the frequency of occurrence of POSTS and not only an asymmetry in amplitude, which could depend on skull thickness, as it was described for the alpha rhythm [6]. These findings are somewhat similar to asymmetrical photic driving: although generally representing a normal response, an asymmetrical pattern may point to an increased cortical excitability on the active side [1]. Of note, the POSTS did not always co-localize with the epileptiform abnormalities; furthermore, apart from situations occurring in parenchymal brain defects, epileptiform abnormalities are generally surface negative or tangential, but not surface positive. This suggests that in our patients, POSTS might be simply a marker of cortical excitability. Although we don’t have a control group (the other EEGs were not performed in every patient during sleep), and despite our small sample size and the relatively short recording time of each EEG, limiting the generalizability of our observations, both gender were represented and the age range was wide (7—76 years). Moreover, a hypnogram would have been better to observe POSTS, since the proportion of POSTS can vary according to the sleep cycles. Abnormalities were located on the right hemisphere in all patients; this might be a mere coincidence or suggest that the non-dominant hemisphere could be more likely to produce POSTS associated to abnormal EEG findings. Our retrospective design did not allow having information on patients’ handedness, which would have been interesting to correlate with POSTS side. Only prospective inclusions, ideally with left-handed patients and left-sided POSTS will elucidate this issue, together with a control group of nonepileptic patients. Only larger, prospective observations will be able to confirm our preliminary findings and elucidate these aspects. Hopefully, the inclusion of patients with focal cortical dysplasia could help clarify the association of POSTS with cortical excitability. It would be also important to determine the prevalence of asymmetric POSTS without associated EEG abnormalities; we are currently performing a prospective analysis to answer to this question. Conflicts of Interest POSTS are typically described as a normal non-REM sleep EEG variant. A systematic analysis [10] of 1627 consecutive sleepEEG recordings found no association between the occurrence of high-frequency POSTS (≥ three positive waves of 5—7 Hz) and EEG abnormalities. Indeed, the authors observed that the incidence of POSTS was lower in abnormal EEG and higher in recordings with minor or no abnormalities. When analyzing EEG indications, they found that POSTS were encountered more often when the patient suffered from headaches or fainting than in subjects with epilepsy or brain infarction, hypothesizing that normal recordings were more likely to occur in the former two conditions. 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