Deep brain stimulation for treatment-refractory obsessive-compulsive disorder: psychopathological and neuropsychological outcome in three cases

OBJECTIVE

Investigation of deep brain stimulation (DBS) as a last-resort treatment alternative to capsulotomy in treatment-refractory obsessive-compulsive disorder (OCD).

METHOD

Prospective single-case based design with evaluation of DBS impact on emotions, behaviour, personality traits and executive function in three patients with OCD.

RESULTS

Two patients experienced sustained improvement of OCD symptoms with DBS. Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) dropped 12 points and 23 points to baseline and Y-BOCS self-rating scale (Y-BOCS-SRS) and Profile of Mood States (POMS) for depression and tension decreased with increasing stimulation amplitude. Total Maladjustment Score on the Brief Psychiatric Rating Scale reduced with 44 and 59% to baseline. Reduction in psychopathology was sustained under continuous stimulation. No deleterious impact of DBS on neuropsychological testing or personality traits measured on a self-rated personality inventory was detected.

CONCLUSION

These preliminary findings demonstrate that DBS may have important therapeutic benefits on psychopathology in OCD. No harmful side-effects were detected during follow-up (33/33/39 months, respectively).

Effect of the serotonin agonist 8-OH-DPAT on the sensorimotor system of the rat

8-Hydroxy-2-(di-n-propylamino)-tetralin hydrobromide (8-OH-DPAT, 2 mg/kg) is used to induce perseverative behavior in rats in a T-maze as a model for obsessive-compulsive disorder (OCD). Using the open-field test, radiant heat test, and the test with von Frey filaments, we examined whether alterations in sensorimotor functioning could contribute to the perseverative tendencies in this model by measuring differences in left versus right hind paw reactions after 8-OH-DPAT administration (2 mg/kg, sc). Also, the effect of repeated 8-OH-DPAT administration on sensorimotor functioning was tested every third day. 8-OH-DPAT administration induced a significantly decreased sensorimotor performance in the open-field test, an increased threshold for noxious thermal stimulation (increased withdrawal latency, WL, and decreased elevation time, ET) in the radiant heat test, and a decreased nociceptive threshold for mechanical stimulation in the test with von Frey filaments. All changes in sensorimotor functioning were similar for left and right hind paws suggesting that, these changes as measured with the tests in the present study, are not likely to contribute to the perseverative behavior of rats in a T-maze. Further, repeated administration of 8-OH-DPAT had no effect in the radiant heat test and the test with the Frey filaments, but produced a tolerance effect in the open-field test.

Thalamic stimulation in neuropathic pain: 27 years later

An overview is given of CNS mechanisms which are behind the beneficial effects of VPL-VPM thalamic stimulation in the treatment of neuropathic pain. Further research in this field is urgently needed and the recent possibility to combine Deep Brain Stimulation with positron emission tomography (PET) will certainly help to unravel the brain circuitry implicated in stimulation-produced analgesia. Brain stimulation is an artificial way to activate nervous tissue that is reversible and, when correctly applied, has few complications. The clinical results warrant a continued dissemination of brain stimulation as a treatment in well selected cases of neuropathic pain.

Targeting the basal ganglia for deep brain stimulation in Parkinson's disease

The revitalization of surgery for Parkinson's disease (PD) has fueled discussion about the best methodology to define the target. Placement of electrodes for deep brain stimulation (DBS) requires the usual stereotactic technique but the argument is mainly centered on whether or not microrecording neuronal activity is necessary. We compared the accuracy of calculating the coordinates X (medio-lateral) and Y (rostro-caudal) considered by the classic stereotactic method, i.e., definition of the AC-PC intercomissural line by MRI and a digitized version of the Schaltenbrand's atlas, with final electrode placement according with microrecording and microstimulation in 21 patients. For both the globus pallidum internum (GPi) (n = 21) and the subthalamic nucleus (STN) (n = 36) there was, respectively, a 43% and 45% mismatching of more than 3 mm between the theoretic coordinates and the final site of electrode location. This applies to both the X and Y planes. Accuracy was not improved in patients (n = 11) in whom the bilateral procedure was undertaken in a single day. We conclude that proper electrode positioning of the STN and GPi requires fine electrophysiologic assessment.

Electrical stimulation in anterior limbs of internal capsules in patients with obsessive-compulsive disorder

Chronic electrical stimulation instead of bilateral capsulotomy was done in four selected patients with long-standing treatment-resistant obsessive-compulsive disorder. In three of them beneficial effects were observed.

[The placebo effect: classes of explanation]

The placebo effect is a frequent phenomenon in medicine, but very little is known about its mechanisms. An overview is given of the different classes of explanation of the placebo effect in analgesia and in particular the role of endogenous opioids, classical conditioning and expectations. Then the question is raised which are the properties of placebo for which a theory has to provide answers in order to be coherent. These properties are, between others, the efficacy of placebo in a variety of conditions, in individuals with different personality characteristics, etc. Finally, the difficulty of observing individual placebo is emphasized and problems concerning the diagnostic and therapeutic use of placebo are mentioned.

[Image-guided surgery]

Initially, stereotactic surgery was developed to treat functional brain diseases only. The localisation of targets was based on stereotactic atlases and radiographs. The introduction of computer based imaging techniques, such as CT and MRI, have offered the possibility to "see" anomalies and to approach them stereotactically. The working principle of such a procedure consists of three steps. It is assumed that brain tissue does not move with respect to the skull. 1. Acquisition of the images and their registration with the patient, usually based on a series of reference points (fiducials) that belong to a stereotactic localizer attached to the base ring of the stereotactic frame, 2. planning and simulation of the surgical intervention, mostly based on two-dimensional (2D) images resliced along arbitrary directions, and 3. intra-operative guidance of the instruments mounted onto the stereotactic frame. This procedure has continuously been updated by new image acquisition techniques, 3D visualization and frameless stereotaxy. 1. PET, MRI and angiography (DSA, MRA) have been used in addition to CT. Moreover, such images of different modalities can automatically be fused without using a stereotactic frame or other artificial fiducials. 2. 3D images for surgery planning have become available, a feature that has proved to be very useful for the cerebral blood vessels. 3. The stereotactic frame can be replaced by a robot arm or an optical guidance system. Registration of the instruments with the patient is then performed by using markers in the cranial bone or on the scalp, or by means of intra-operative images such as radiographs or video images. Recently the use of registered video images has resulted in a number of experiments with "improved reality" and telesurgery. The same working principle has shown to be useful for bone and bone related surgery. As in brain surgery, the prerequisites of rigidity and immobility with respect to a reference are satisfied. Because bone structures are rigid and can easily be outlined in CT images, 3D graphical as well as stereolithographic representations can be produced for the purpose of planning and even for navigation. Unfortunately, for most organs or soft tissue the above conditions of inflexibility and fixed position with respect to a reference are not fulfilled. Real time imaging, partly due to the introduction of the "open" MR scanner, may offer a solution. It can be expected that interventional diagnostic imaging will become increasingly important in the future, also for neurosurgery.

Production of tumor necrosis factor in spinal cord following traumatic injury in rats

Production of tumor necrosis factor (TNF) in the spinal cord following traumatic injury has been studied. In these experiments, the level of TNF was examined in the homogenate of the spinal cord, cerebrospinal fluid (CSF) and serum (n = 56). TNF could be detected in the injured spinal cord but not in the normal spinal cord. The TNF level increased in the spinal cord after the injury. At the lesion site, a maximal TNF concentration was observed 1 h after the injury, and the TNF concentration remained at this level until 8 h after the injury. Thereafter, it decreased gradually. However, TNF still could be detected 72 h after the injury. No TNF could be detected in the CSF and serum, collected from rats both with and without spinal cord injury (SCI). This study thus suggests that TNF is produced locally in the spinal cord following traumatic injury, and this TNF production is caused by the injury. The present results also demonstrate that TNF production is an acute and rapid reaction in the spinal cord following traumatic injury.

The use of magnetic resonance angiography in stereotactic neurosurgery

The authors discuss the advantages and disadvantages of the use of magnetic resonance (MR) angiography images in stereotactic neurosurgery. Current computer programs designed to assist the neurosurgeon in the planning of stereotactic neurosurgical interventions use intraarterial digital subtraction angiography images to visualize the blood vessels. Magnetic resonance angiography is a recent technique with a number of advantages over the digital subtraction method: it is less invasive and less prone to complications; it provides truly three-dimensional data sets that can be viewed from any direction; and it can visualize both stationary and flowing tissues with the same imaging device and localizer frame. Although digital subtraction images are still superior in contrast and vascular detail, state-of-the-art high-resolution MR angiography sequences provide sufficient vascular detail for planning surgery. Contrast-enhanced MR angiography images were acquired using adapted gradient-echo sequences to compensate for flow-induced distortions; postacquisition distortion correction was not necessary. Five methods to integrate and inspect a possible trajectory in the MR angiography data are discussed. Initial clinical experience with eight patients led to the conclusion that MR angiography is a valuable imaging modality that can be integrated reliably into a stereotactic neurosurgery planning procedure.

The consumption of fentanyl is increased in rats with nociceptive but not with neuropathic pain

The question whether opioids relieve neuropathic pain remains a controversial issue. Experimental as well as clinical studies report contradictory results. This study investigated the consumption of fentanyl, a short-acting opioid, in rats with neuropathic pain, induced by partial sciatic nerve injury. The experiment consisted of a drug choice procedure in which the animals could choose between a solution containing 0.008 mg/ml of fentanyl and a highly palatable sweet solution. It was reasoned that if opioids have an analgesic effect in neuropathic pain, this will reinforce the intake of fentanyl more so in rats with neuropathic pain than in pain-free controls. This protocol was previously already used by Colpaert et al. (1982) in a rat model of chronic pain of nociceptive origin, namely polyarthritis. No significant differences were found in the relative oral intake of the fentanyl solution in mononeuropathic and pain-free control rats. In contrast, rats with nociceptive pain, adjuvant monoarthritis, drank significantly more of the fentanyl solution than did control rats. These data give experimental support for the clinical findings that opioids have a poor analgesic effect in neuropathic pain.

Protocol for the clinical functionality assessment of a workstation for stereotactic neurosurgery

The objective of this study is to establish a protocol for the technical and clinical evaluation of a workstation for the planning of stereotactic neurosurgical interventions that has been developed in the framework of a joint European research project. Although several such workstations have been proposed before, they lacked the final and most important step, that of clinical validation. They failed to rigorously prove that their product was useful. The authors present a new method that is applicable to the evaluation of a wide range of medical technologies. Their protocol basically assesses the clinical relevance of the user requirements that are at the root of the development of the new technology. The evaluation consists of two stages. During functional specification, iterative prototyping is used to establish the clinical requirements and to assure the quality of the final product. A case study design is used in a second stage that assesses the clinical usability. A before-after study gives a first indication of cost effectiveness and improvement of health care quality.

[Pain of autonomic nervous system origin]

Reflex sympathetic dystrophy (algodystrophy) is a descriptive term used to designate a clinical phenomenology, which appears after a trauma, often a minor one; it is thought that the sympathetic nervous system plays an important role in its physiopathology. The same applies to sympathetic maintained pain appearing after a lesion of a peripheral nerve (causalgia). There is no agreement as to which are the criteria necessary to make the diagnosis and as to what is the physiopathology of these syndromes. Research in this field is very active today.

How does the stereotactic workstation help the neurosurgeon?

At the KUL University of Leuven a workstation for the planning of neurosurgical stereotactic procedures has been developed. Its benefits are illustrated in three exemplary cases. The CT and/or MR images, acquired under stereotactic conditions, are transmitted via a PACS network (picture archiving and communication systems) directly to the stereotactic workstation in the operating theater. Target and entry point can be accurately defined on zoomed images. The trajectory can be checked and modified on all registered data sets and on resliced images along any plane. Maximum intensity projection of magnetic resonance angiography data sets along any arbitrary direction show the relative position of the blood vessels and the trajectory. During the preceding 32 months 29 patients were operated on using the stereotactic workstation. Postoperatively no new neurological deficit was observed in any of these patients. The workstation improves patient safety and increases the accuracy of neurosurgical stereotactic operations, because it helps the neurosurgeon to avoid blood vessels and/or important functional areas.

On the problem of geometric distortion in magnetic resonance images for stereotactic neurosurgery

In this paper, we discuss the issue of geometric distortion in magnetic resonance (MR) images used to plan stereotactic neurosurgical interventions. We analyze the process for the case of Fourier transform imaging and demonstrate that spatial misregistrations are fundamentally due to two causes: deviations of the magnetic field from its ideal value and blood flow. This enables us to relate the causes of geometric distortion to the MR imaging system, the patient and the stereotactic localizer frame. Based on the general model, we propose model refinements and discuss methods for the quantification and correction of all causes. The results of our calculations and experiments indicate that, using the proposed corrections, MRI and MR angiography should be considered valuable and reliable acquisition modalities for the planning of stereotactic neurosurgical interventions.

Therapeutic stereotactic procedures on the thalamus for pain

Thalamotomy and electrical stimulation of a thalamic target as treatment for persistent pain are discussed. Thalamotomy is only rarely performed these days according to a questionnaire, given to some colleagues, about the type and the number of operations they performed in the years 1984, 1985 and 1986. The need for stimulation in the periventricular or periaqueductal grey for nociceptive pain is decreasing due to the advent of intraspinal and intraventricular administration of opioids. Nowadays medial and lateral ventro-posterior thalamic nuclei are frequently stimulated for treatment of deafferentation pain. Of 36 patients with deafferentation pain, 22 initially had benefit from this stimulation, but long-term success was only achieved in 11 (30%) of them. It was a general trend that the patients with an initial high pain relief score obtained the best long-term results.

What can the neurosurgeon offer in peripheral neuropathic pain?

Neurosurgery has much to offer in the treatment of peripheral neuropathic pain but selection of the best procedure for a given patient remains problematic: planning of the treatment must be based on an analysis of the pathophysiological mechanism in the given case but the identification of this mechanism is often difficult. Available procedures are: 1) Nerve repair, neurolysis and nerve relocation; 2) Interventions on the sympathetic nervous system; 3) Neurostimulation; 4) Intraspinal morphine; 5) Ablative lesions. Neurosurgeons have, or should have, the necessary neuroscience background and microsurgical skills to be important partners of the team caring for patients with peripheral neuropathic pain.

Further evidence for myelinated as well as unmyelinated fibre damage in a rat model of neuropathic pain

A mononeuropathy, produced by ligation of the sciatic nerve in rats, has recently been proposed as an animal model of experimental pain and pain-related disorders (hyperalgesia and allodynia). We investigated quantitatively the morphological changes in myelinated and unmyelinated fibres of the sciatic nerves 2 weeks after ligation in rats exhibiting allodynia to thermal stimulation. There was a marked reduction in the number of large myelinated fibres distal to the ligature (711 +/- 34 compared with 5315 +/- 230 in normal nerves). We also found a significant loss of small myelinated fibres (2429 +/- 109 compared with 3197 +/- 308 in normal nerves), the remaining fibres of this type showing pathological properties. Finally, ultrastructural evidence of damage to unmyelinated fibres was found. The typical pattern of large clusters of normal unmyelinated axons was no longer present within most regions of the nerve. There was a significant reduction in the size of the unmyelinated fibres (0.41 micron +/- 0.15 compared with 0.71 micron +/- 0.08 in normal nerves), together with a twofold increase in their number per cluster. Hypotheses about the mechanism of thermal allodynia in this pain model therefore must take into account the fact that all fibre classes show pathological changes.

Subcortical aphasia from a thalamic abscess

A patient is reported who was treated successfully for a left thalamic abscess that resulted in subcortical aphasia. A SPECT scan showed large areas of hypoperfusion in the cortex of the left hemisphere. At follow up after seven months there was marked improvement in the language disorder and the cortical hypoperfusion. It is suggested that aphasia in patients with subcortical lesions results from secondary cortical dysfunctions. The evidence is confined to patients with stroke lesions. The possible implications of this case on current theories of pathophysiological mechanisms, in particular the ischaemic penumbra theory and the cortical diaschisis theory, are briefly discussed. Cortical diaschisis may be the appropriate explanation in this patient.

Deep brain stimulation in the treatment of chronic pain in man: where and why?

In current clinical practice, two brain structures are stimulated for the relief of chronic pain, namely the somatosensory thalamic nuclei (VPL-VPM) and the periventricular and periaqueductal gray matter (PVG-PAG). Whereas stimulation of the VPL-VPM is almost exclusively used for the treatment of deafferentation pain, stimulation of the PVG-PAG is mostly used in cases of nociceptive pain. We present our results of VPL-VPM stimulation in 36 patients with deafferentation pain. Initial pain relief was obtained in 61% of patients. To-day, after a mean follow-up of more than 4 years, 30% are still pain free. This success rate was found to be lower than the mean reported success rate of 57%, based on a survey of the world literature. Upon reviewing the literature, it was apparent that the reported success rates vary considerably between different authors. Some tentative explanations are given for this large discrepancy in success rate. The mechanisms by which electrical stimulation of the VPL-VPM suppresses deafferentation pain remain to be elucidated. Recent clinical and experimental findings suggest that a dopaminergic mechanism might be involved.

A prototype medical workstation for computer-assisted stereotactic neurosurgery

We have developed a prototype display workstation for use in stereotactic neurosurgery. Patient image data from computed tomography, magnetic resonance imaging, and digital subtraction angiography are acquired with the stereotactic frame in place and subsequently transferred to the workstation for further processing. Target points may be identified on any image type and probe trajectories defined. Any point or line indicated on one set of images may be transferred immediately to other images, to determine, for instance, safe avascular probe paths. We present some general outlines for the use of computers for stereotactic neurosurgery and discuss the different components of the current system. Finally, we make some suggestions as to further developments.

Suppression of the hindlimb flexor reflex by stimulation of the medial hypothalamus and thalamus in the rat

Pentobarbital-anesthetized rats received electrical hindpaw stimulation every 10 s to elicit a maximal hindlimb withdrawal reflex. The integrated EMG response in the ipsilateral tibialis anterior was sampled by a computer which also controlled the timing of electrical stimuli applied to the brain. A suppression of the evoked flexor activity was obtained with currents below 0.05 mA for stimuli applied in the medial hypothalamic region. A second effective site was located in the paraventricular area of the thalamus. The suppression had an onset latency of 30 ms, increased over a period of 500 ms and was followed by a postinhibitory facilitation (rebound). When the noxious electrical shocks were given over prolonged periods (140 s) the suppression of the flexor reflex was seen to outlast the central stimulation by more than 100 s. Intravenous injection of naloxone or methysergide failed to reverse the effects of the brain stimuli. It is suggested that the hypothalamic induced inhibition of withdrawal reflexes is functionally meaningful in view of the incompatibility between these reflexes and the locomotor behavior which is part of the behavioral responses (i.e. fight or flight) controlled by this area.

Intrathecal baclofen for long-term treatment of spasticity: a multi-centre study

Twenty eight patients with severe, intractable spasticity have been treated by chronic intrathecal administration of baclofen. An implantable programmable drug-administration-device (DAD) was used with a permanent intrathecal catheter. Infusion of 50 to 800 micrograms/day of baclofen completely abolished spasticity. Follow-up was up to two years. Therapeutic effect was documented by clinical assessment of tone, spasms and reflexes and by electrophysiological recordings of mono- and polysynaptic reflex activity. Complications and untoward side-effects of the procedure were few. This procedure is recommended for spasticity of spinal origin refractory to physiotherapy and oral medication. It is a preferable alternative to ablative surgical intervention.

Myelotomies for chronic pain

The literature on myelotomy for the treatment of chronic pain was reviewed and a total of 635 published cases scrutinized. Two main modes of myelotomy can be distinguished 1) a longitudinal commissural section tuned to the segmental pain level and 2) a focused central lesion, irrespective of considerations of the metameric pain distribution, mainly carried out at a high cervical level. Of the longitudinal commissural myelotomy, a posteriorly restricted and a complete type can moreover be discerned. The pain relief decays with time after myelotomy of any kind. Central myelotomy scores better than complete commissural section for malignant pain in a statistically significant manner but its superiority over posterior commissurotomy cannot be statistically proven. Except of a girdle-shaped hypo-algesia, which is expected after the section of the decussating spinothalamic fibers in a complete commissurotomy, other--irregular--patterns of hypo-algesia have been observed, especially after central myelotomy. This unusual lesion, provoking unusual hypo-algesia patterns, together with phenomena like a preserved sharp-blunt-discrimination within the hypo-algesic area, points at a different sensory channel that might be severed in a central myelotomy as compared with an anterolateral chordotomy or a complete commissurotomy. This hypothesis is matched with recent physiological evidences.

The automated measurement of hindlimb flexor reflex of the rat as a substitute for the tail-flick assay

A new method for the study of stimulation-produced analgesia is introduced. The hindlimb flexor EMG, in response to noxious electrical stimulation of the paw, is used as an indirect index of analgesia induced by electrical stimulation at different brain sites. In the lightly anesthetized rat (pentobarbital 10-20 mg/kg/h) such responses are quite stable and easily evoked at intertrial intervals as short as 10 s without habituation. The high repetition rate allows a rapid determination of the threshold current intensities or the brain stimuli required to suppress the hindlimb withdrawal. The test is objective, avoids skin damage and sensitization and can be performed semi-automatically when implemented with a small PC.

Angiographic localizer for the BRW stereotactic system

Preliminary experience with a newly constructed angiographic localizer system for use in stereotactic neurosurgery is reported. This localizer ring, mounted on the BRW head ring, allows for the transformation of target points with known stereotactic coordinates (e.g., visible on computerized tomography scans) onto angiograms, as well as the determination of stereotactic coordinates of a set of points (e.g., arteriovenous malformations) indicated on at least two angiograms.

Angiographic localizer ring for the BRW stereotactic system

An accessory locating device to the existing BRW stereotactic system is presented. It can be used as a reference device to locate angiographic data with respect to the BRW stereotactic system. Hence, the projection of target points onto angiograms, visible on CT scans, are easily calculated, as well as the stereotactic coordinates of a set of points (e.g., AVM) indicated on at least two angiograms. As a final result integrated images of cerebral blood vessels and an outline of tumor lesions can be generated using more sophisticated computer equipment.

Histochemical changes of substance P, FRAP, serotonin and succinic dehydrogenase in the spinal cord of rats with adjuvant arthritis

Various histochemical changes were found in spinal segments L4-L5 of rats with adjuvant arthritis, predominantly 30 days after inoculation. A slight to marked increase of substance P immunoreactivity occurred in laminae I, II and X. FRAP activity was enhanced in lamina II. Serotonin immunoreactivity was heavier in laminae I, VIII and IX in a few animals. The intensity of the histoenzymological reaction for succinic dehydrogenase increased in certain laminae VIII and X neurons. At day 15 of the disease the increase of substance P and FRAP activities was chiefly restricted to the medial portion of the superficial dorsal horn. There was a significant positive correlation between the scratching behaviour of arthritic rats and the substance P immunoreactivity in laminae X and I. If one accepts that scratching is pain-related, the data are consistent with a possible role of substance P in the chronic pain associated with adjuvant arthritis. They leave undetermined the significance of the other histochemical changes.

ERPs associated with decisions based on slowly increasing warm and painful stimuli

The event-related potential (ERP), produced by subjects who were experiencing a gradually increasing heat stimulus, was recorded 3.5 sec before and 1 sec after a motor response. Subjects responded when the stimulus was 'just warm,' 'just at prickling pain' or 'above the temperature of just prickling pain.' Two control procedures mimicked the readiness potential (RP) and motor potential (MP) experiments. The results showed that the steadily rising negative MP was modified to a positive wave before the decision of 'warm.' During the 'pain' and 'above pain' conditions, the pre-decision slow wave was less negative than the MP but less positive than that of the 'warm' condition. A temperature control condition confirmed that rising temperature alone did not produce these results. Principal components analysis confirmed these differences by producing factors which were interpreted as representing the ERPs of the conditions of 'warm,' 'pain,' 'above pain,' RP and MP. It was evident from pilot studies that uncontrolled electrical activity from the skin potential response (SPR) was contaminating the results. SPR forced all ERP curves in the positive direction and several attempts to eliminate the SPR from the ERP data failed. While the SPR was produced differentially across conditions and across the experimental session, the smaller the variability in the electro-oculogram (EOG) recorded from above and below the orbits, the smaller was the SPR contamination that we recorded in the ERP. Therefore, we selected subjects for this experiment who showed a stable orbital EOG.

Suppression of C-fibre discharges upon repeated heat stimulation may explain characteristics of concomitant pain sensations

Nociceptors with unmyelinated axons were recorded from the superficial radial nerves of 7 volunteers. A sequence of uniform radiant heat stimuli of 18 s duration, starting from an individually adjusted adapting temperature were used to raise the skin surface temperature by 6 degrees C to a painful level (41-43 degrees C). These stimuli followed each other at 3 different interstimulus intervals of 35 s, 70 s and 105 s, occurring in a random order. The subjects were asked to track the time course of the stimulus-evoked sensation by manipulating the length of a light bar. Adaptation and stimulus temperatures were chosen to induce sensations of heat and/or pain. All nociceptors studied responded to these stimuli with a phasic response of 3-5 s duration, often followed by a low frequency tonic discharge, lasting as long as the stimulus. No discharges were seen in interstimulus periods. Discharge rates during the phasic responses were linearly related to interval duration, whereas tonic discharges were not influenced by the preceding interval. In parallel readings of pain responses were lower up to the 10th second of the stimulus after short rather than after long intervals. These results indicate that the suppression of C-fibre nociceptor discharges during repetitive stimulation may explain concomitant reductions in the magnitude of human pain sensations.

The suggested mechanisms of chronic pain and the rationale of neurosurgical treatment

Knowledge about the pathogenesis of certain human diseases and how to treat them, has in general been advanced by the availability of a reasonably similar disease counterpart somewhere else in the animal kingdom. Unfortunately, a model of chronic pain is difficult to come by because of the inherent problem of defining animal pain, and it is only in recent years that such models have been proposed, and a study of them has started. The purpose of the present short review has been therefore to summarize some of the major developments in this field in such a way as to assist the reader in deciding just how exciting prospects in chronic pain research for the future might be.

C nociceptor activity in human nerve during painful and non painful skin stimulation

Percutaneous recordings from more than one hundred single C fibres have been performed in the radial nerve of conscious human subjects. All these fibres belong to the poly-modal C nociceptor group, being excited by mechanical and thermal and also by chemical stimulation. Conduction velocities showed a monophasic distribution with a mean value of 0.86 m/s (SD: 0.17). The mechanical threshold, measured with von Frey hairs, varied between 2.3 and 13.1 g. The receptive field was circular or elliptical; for 33 units the mean axes were 6 mm and 7 mm. Mechanically evoked C fibre discharge even up to more than 10 spikes/s was not necessarily accompanied by pain sensation. Nettle sting evoked an irregular C fibre discharge (maximum 10 spikes/s) accompanied by a pricking and burning sensation; the sensation of itch which was sometimes reported, was not correlated with the discharge frequency. C fibre activation by a chemical irritant (paint remover) also evoked an irregular discharge (maximum 3 to 6 spikes/s), accompanied by pricking and burning pain sensation. The C threshold for radiant heat usually lay below the subject's pain threshold. Increasing skin temperature produced increasing neural firing rate. The mean spike frequency rarely exceeded two spikes/s even with stimuli evoking strong heat pain. The occurrence of subjective heat pain response could be as well predicted from th C fibre spike frequency as from the skin temperature. It is concluded that nociceptive C input provoked by thermal or chemical stimuli correlates well with pain sensation. However, similar C input provided by mechanical stimulation which activates also A beta mechanoreceptors, did not necessarily produce pain sensation.

Latencies of chemically evoked discharges in human cutaneous nociceptors and of the concurrent subjective sensations

Percutaneous recordings from single nociceptive A delta- and C-fibers have been performed from the superficial radial nerves of conscious human subjects. Nociceptors were tested with a chemical irritant substance, which induced a burning sensation when applied to the intact skin. A comparison of the onset of spike discharges in nociceptors and the onset of the subjective burning sensation indicated that under the conditions of our experiments summation of input from nociceptors is needed in order to induce pain sensations. In particular, our results indicate spatial summation.