Differentiation of conversive sensory loss and malingering by P300 in a modified oddball task

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1 Pain p Website publication January NeuroReport, () WE applied the methodology of evoked potentials (EP) to reveal the functional level of abnormality in a patient with circumscribed complete anaesthesia due to conversion disorder. EP components related to sensory and perceptual processing of both innocuous electrical and noxious laser stimuli were normal. However, a P00 component indicating cognitive processing failed to appear when using a modified oddball task with rare stimuli applied to the anaesthetic right hand. P00 was present with this paradigm stimulating the healthy left hand, as well as in a malingerer a healthy subject who was instructed to feign the same deficit. These results suggest cognitive deficits underlying sensory loss as conversion symptom which can be differentiated from malingering by use of P00. Key words: Conversion disorder; Hysteria; Laser evoked potentials; Malingering; Oddball task; P00; Sensory deficit; Somatosensory evoked potentials Introduction The methodology of evoked potentials (EPs) recorded from the human electroencephalogram (EEG) is established for the objective documentation of sensory deficits in a variety of pathologies of the peripheral and central nervous system pathways. Normal EPs are typically present if the patient s complaint is of non-organic origin, as in cases of sensory deficits associated with hysteria and malingering. However, clinical features and psychological processes of hysterical sensory deficits are clearly different from malingering and feigning a disease where there is a voluntary attempt to deceive the physician. In the present study we applied conventional short- and long-latency somatosensory evoked potentials (SEP) and pain-relevant laser evoked potentials (LEP) in a patient with a complete sensory loss at the right distal arm due to hysteria. Furthermore, we designed a modified oddball paradigm in order to elicit a cognitive P00 component of event related potentials (ERP) after stimuli applied to the affected hand. Results were compared with those of a healthy subject performing the same task with the instruction to feign a sensory loss. A second oddball experiment was performed in the patient to control elicitation of a P00 related to stimulation of the non-affected left hand. The P00, first described by Sutton et al. with auditory stimuli and a little later with somatosensory stimuli by Desmedt et al., is often referred to as an endogenous ERP component reflecting Differentiation of conversive sensory loss and malingering by P00 in a modified oddball task Jürgen Lorenz, CA Klaus Kunze and Burkhart Bromm Institute of Physiology and Neurological Department, University Hospital Eppendorf, Martinistrasse, D-0 Hamburg, Germany CA Corresponding Author psychological processing, in contrast to exogenous components, like the N0, which primarily vary with physical stimulus attributes. Prior to elicitation of P00, or sometimes overlapping it, there is a group of long-latency EP characterized by negative-to-positive ongoing components (N P) with maximum over Cz (vertex). These potentials can be found with some differences in any sensory modality and also depend upon attention. It was suggested that this group of components reflects a spatio-temporal overlap of exogenous and endogenous activity related to conscious perception of the eliciting stimulus that occurs before an actively controlled categorization and extraction of behaviorally relevant information is performed which is indexed by the purely cognitive P00. LEPs also belong to long-latency vertex EPs, distinct from a P00 wave,, and were applied in the patient based on sound evidence that LEPs can reliably indicate selective damage of small afferents and projection pathways of the nociceptive system which are not adequately assessed by standard electrical SEP which represents a measure of large diameter afferents and mechanoreceptive function.,0 The aim of the present study was to use the different EP components, short-latency SEP (N0) and long-latency SEP/LEP and oddball-p00, as measures of early sensory, late perceptual and modality-unspecific cognitive information processing, respectively. In this way, we attempted to characterize the functional stage of information processing which might be disturbed in sensory loss due to hysteria. According to current psychiatric Rapid Science Publishers Vol No January

2 J. Lorenz, K. Kunze and B. Bromm p terminology the ancient term hysteria covers various psychogenic symptomatologies, including somatization, conversion and dissociative disorders., As within this classification of hysterias the present patient best complied with a condition described as conversion disorder (i.e. loss of physical function which suggests physical disorder but is the expression of an underlying psychological process such as conflict or need), we will use this term subsequently. An important criterion for conversion disorder is that the symptom is not under voluntary control. We, therefore, wanted to know whether this condition in fact can be neurophysiologically contrasted with malingering using the modified oddball paradigm designed to indicate volitional processes by ERP correlates appearing independently of behavioral responses. Materials and Methods A -year-old engineer suffered a whiplash trauma of head and neck in a car accident years ago. There was a light commotio in the acute phase from which the patient recovered without persistence of symptoms or complaints. Two years later he presented with a complete sensory loss in the right arm distal to the elbow and with inability to actively move the right arm. The patient claimed that he had suffered several injuries and burns due to the lack of pain sensation in his right arm. After consulting various physicians he presented in our hospital for further diagnostics and treatment. Neurological examination, needle EMG, EEG and imaging techniques failed to reveal any consistent abnormality. Neuropsychological testing yielded deficits in attention and shortterm memory. Psychiatric examination revealed the presence of a chronic conversion disorder with sensory deficit. In a first session standard electrical median nerve SEP (0. ms duration, innocuous intensity above motor threshold, trials) and pain relevant laser evoked potentials (LEP) according to our standardized paradigm were recorded: 0 brief CO -laser stimuli (wavelength 0. m, 0 ms duration, mm beam diameter) were applied to the back of affected right and non-affected left hands using intensities of and 0 W, which are above normal pain threshold. The random sequence of stimuli at variable intervals (0 0 s) also included 0 electrical median nerve stimuli in order to elicit late SEP. Each side was tested twice with the following sequence: affected control control affected. In a second session the patient was given a series of 00 non-painful electrical shocks to both thumbs (Fig. ). Rare stimuli (0%) occurred at the right affected digit and frequent stimuli (0%) were applied FIG.. Modified oddball paradigm with electrical stimuli: a healthy subject feigning a sensory loss in his right hand and the patient with conversive sensory loss in his right hand were given a random sequence of frequent and rare stimuli to left and right thumbs, respectively. Each stimulus had to be verbally reported by the words left and right indicating the stimulated side. A second block was performed in the patient with frequent and rare stimuli only applied to the healthy left hand. Vol No January

3 p Evoked potentials in conversive sensory loss to the left digit. Interstimulus intervals were fixed at s. Each stimulus had to be verbally reported with the words right or left to indicate the side being stimulated. A healthy control subject instructed to feign a sensory loss at his right hand performed the same task. It was expected that both the patient and the healthy subject would not respond to right digit stimulations. Feigning, however, would involve the active witholding of a response to the unpredictable rare stimulus with the consequence of processing it as a so-called no-go event, which is known to elicit a P00. A second block in the patient was performed with both rare and frequent stimulations within his healthy left hand using thumb and little finger, respectively, again with the task instruction to report the stimulated side after each stimulus presentation. For early components, EEG was recorded within Hz from an electrode placed over the hand area of primary somatosensory cortex contralateral to stimulation ( cm behind C of the 0-0 system) using a frontal reference (Fz). The N0 response was compared with height corrected normative data from our laboratory. In the laser and oddball paradigm EEG was recorded within 0. 0 Hz from Fz, Cz and Pz, referenced against linked earlobes. A vertical electrooculogram (EOG) served to exclude trials contaminated by ocular artefacts from averaging. Normal LEP, analyzed from Cz lead, was stated when side-to-side differences in the peak-to-peak amplitude did not exceed 0% according to the formula: (higher value lower value)/higher value 00 (%). P00 was evaluated from the Pz lead. Results Latency and amplitude of the N0 response of right short-latency median nerve SEP (Fig., top st graph) were well reproducible and normal according to our normative data. The long-latency SEP and pain-relevant LEP in response to stimuli applied to the affected hand did not differ from responses obtained after stimulating the healthy left hand (top nd and rd graphs). Thus, sensory perception was intact up to the stage of secondary processing at which long-latency vertex EPs are elicited, both in the mechanoreceptive and nociceptive modality. As expected the healthy subject ( malingerer ) exhibited a substantial no-go P00 component in response to rare stimuli at a site where he feigned to be insensitive (Fig., medium st graph). The presence of P00 is, as usual, best verified in the difference wave form (bottom) when the ERP after frequent (medium, thin line) is subtracted from the ERP after rare (thick line) events. In contrast, the patient failed to show a P00 after rare stimuli applied to his affected right thumb (medium and bottom nd graphs). However, a P00 was clearly elicited in the patient when he performed the task with both rare and frequent stimuli applied to fingers of the normal left hand (medium and bottom rd graphs). This result was reproducible in two repeated blocks. Thus, the circumscribed somaesthetic deficit in the patient cannot be explained by disturbed sensory perception, irrespective of whether noxious or innocuous stimuli were used. A lack of P00 points to a cognitive disturbance and was the only neurophysiological sign of abnormality to correlate with the absence of subjective experience after any stimuli applied to the right hand. Discussion Consistent with the results of other studies,,, we found normal sensory perceptual components of evoked potentials in a patient with sensory deficits related to conversion disorder. If the somatosensory modality is involved, as in the present case, one should generally consider the possibility of selective damage to the nociceptive system, e.g. due to anterolateral spinal tract or lateral brain stem lesions, which typically yield normal standard electrical SEPs, but which can be verified by abnormal laser evoked potentials (LEPs).,0 Abnormal LEPs in the presence of normal electrical SEPs were also recently shown in a case of monosegmental dorsal root lesion due to cervical spondylosis. Although these latter pathologies could be reasonably ruled out in this patient based on clinical presentation, the present finding that conversive sensory loss exhibits normal LEPs suggests their use as supplement diagnostic to verify the non-organic nature of this condition. In the second set of experiments, we showed for the first time to our knowledge that conversive sensory loss can be neurophysiologically distinguished from malingering, the latter being operationally characterized by a healthy subject who voluntarily simulated the same sensory deficit as the patient. The applied modification of the typical oddball paradigm was explicitly designed to elicit a P00 response that would occur without any overt behavioral response. Non-compliant performance would have been easily detected because the task involved continuous reply to stimuli from the normal body part, interrupted only by occasional stimuli to the abnormal body part. These sudden events, however, required active witholding of an answer which unavoidably elicited a no-go P00, as seen in the malingering healthy subject of our study. Absence of a P00 after stimuli applied to the affected hand of the patient points to the absence of volun- Vol No January

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5 Evoked potentials in conversive sensory loss p tary control of the symptom which is, therefore, more adequately described as cognitive deficit indicated by the inability to utilize the available sensory perceptual information for the purpose of adequate behavior. However, clear elicitation of a P00 when the task only involved stimulations to the patient s healthy left hand excludes a general cognitive or attention deficit to account for the lack of the P00 after right hand stimulation. Towle et al. applied a standard oddball task with visual stimuli to differentiate non-organic from organic visual deficits by use of the P00 component. They detected a P00 in two malingerers and one patient with conversive blindness. The P00 amplitudes were clearly smaller than in healthy controls, but there was no difference between malingering and conversion. The fact that we found a clear difference between these two conditions is obviously due to our use of a modified P00 design with greater control of task compliance. Subjects in the study of Towle et al. were instructed to mentally count rare events and a malingerer could have adopted any strategy to distract himself from the task which would have been undetected by the experimenters. Furthermore, conversive blindness does not allow P00 generation from healthy body parts to be controlled within the same stimulus modality, in contrast to our patient with a circumscribed somaesthetic loss. Finally, one should also keep in mind that patients who comply with diagnostic criteria of conversion disorder or other hysterical disorders may also malinger. Conclusion A sensory loss as symptom of conversion disorder appears with normal sensory perceptual function and may result from a cognitive deficit characterized by the inability to functionally link intact perception of stimuli from the affected body part with adequate behavior. The psychological processes of this abnormality can be neurophysiologically distinguished from malingering by the absence of a cognitive P00 component after stimulation of the affected body part using the paradigm presented here. References. Chiappa KH. Evoked Potentials in Clinical Medicine, nd edn. New York: Raven Press, 0.. Howard JE and Dorfman LJ. J Clin Neurophysiol, ().. Sutton S, Baren M, Zubin J et al. Science 0, ().. Desmedt JE, Debecker J and Manil J. Bull Acad Roy Méd Belg, ().. Hillyard SA and Kutas M. Am Rev Psychol, ().. Näätänen R. Behav Brain Sci, 0 (0).. Towell AD and Boyd SG. Electroencephalogr Clin Neurophysiol, ().. Siedenberg R and Treede RD. Electroencephalogr Clin Neurophysiol 00, 0 ().. Bromm B, Frieling A and Lankers J. Electroencephalogr Clin Neurophysiol 0, (). 0. Bromm B and Treede RD. Rév Neurol, ().. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders (DSM IV). Washington: APA,.. Mai FM. Can J Neurol Sci, 0 0 ().. Pfefferbaum A, Ford JM, Weller BJ and Kopell BS. Electroencephalogr Clin Neurophysiol 0,.. Levy R and Behrman J. Electroencephalogr Clin Neurophysiol, 00 0 (0).. Towle VL, Sutcliffe E and Sokol S. Arch Ophthamol 0, 0 ().. Lorenz J, Hansen HC, Kunze K et al. J Neurol Neurosurg Psychiatry, 0 0 (). Received October ; accepted November FIG.. Top: Normal short-latency somatosensory evoked potentials (SEP) and no side-to-side differences of long-latency SEP and pain-relevant laser evoked potentials (LEP) in the patient with complete conversive sensory loss in the right hand. Middle and bottom: A healthy subject (left) feigning a sensory loss for right digit stimulation produces a no-go P00 related to active witholding of a response, best verified in the difference wave form resulting from substraction of the ERP to frequent stimuli from ERP to rare stimuli (bottom). P00 failed to appear in the patient after rare stimuli applied to the right digit with conversive sensory loss, but was present after rare stimuli applied to the healthy left digit. Vol No January