Central poststroke pain: correlation of MRI with clinical pain characteristics and sensory abnormalities

Development and characterization of a hemorrhagic rat model of central post-stroke pain

Stroke is the leading cause of disability in the industrialized world and it is estimated that up to 8% of stroke victims suffer from some form of central post-stroke pain (CPSP). Thalamic syndrome is form of central pain that typically results from stroke in the thalamus. In the present study, we describe the development and characterization of a rat model of thalamic CPSP. This model is based on a hemorrhagic stroke lesion in the ventral posterolateral nucleus of the thalamus, one of the reported causes of thalamic syndrome in humans. Behavioral analysis showed that animals displayed hyperesthesia in response to mechanical pinch stimulation, with sensitivity localized to the hind limb. This response appeared within 7 days of the intra-thalamic hemorrhage. Animals also showed increased thermal sensitivity in the contralateral hind limb. Histopathology indicated the presence of activated microglia adjacent to the core of hemorrhagic lesions in the thalamus. Neutrophils were confined to the hemorrhage core, indicating that they entered in the initial bleed. By 7 days, bands of activated microglia and astrocytes separated the hematoma from surviving neurons at the edge of the lesion. We did not observe any terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positive neurons beyond the immediate hematoma at 1, 3, or 7 days after hemorrhage. Surviving neurons were located in the vicinity of activated microglia and astrocytes at the outer edge of the hematoma. Thus, thalamic hemorrhage produces a confined lesion that destroys the tissue within the initial bleed, with little or no neuron death beyond the hemorrhage core. Surviving neurons surrounded by activated glial cells likely contribute to neuropathic pain in this model. This thalamic hemorrhage model is useful for studying the neuropathology and physiology of thalamic syndrome, and developing therapeutics for central post-stroke pain.

[Central post-stroke pain]

Central post-stroke pain (CPSP) is known since the famous Dejerine-Roussy syndrome and its description has not improved. The subject has however been revived over the last decade thanks to advances in central nervous system imaging with magnetic resonance imaging (MRI), the description of allodynia functional phenomena with fMRI, the study of opioid receptors, and above all, the analysis of pain pathways by laser-evoked potentials. Progress has also occurred in CPSP treatment with motor cortex stimulation, which probably opens a period of neuromodulation of the cortical areas controlling pain. The thalamus plays a prominent role in this disorder of central control of pain.

The characteristics of chronic central pain after traumatic brain injury

Central pain following traumatic brain injury (TBI) has not been studied in depth. Our purpose was to conduct a systematic study of patients with TBI suffering from chronic central pain, and to describe the characteristics of the central pain. Groups were TBI patients with (TBIP) and without central pain (TBINP) and healthy controls. TBI patients with other pain mechanisms were excluded from the study. Participants underwent quantitative somatosensory testing in the painful and pain-free body regions. Thresholds for warmth, cold, heat-pain, touch and graphesthesia were measured and pathologically evoked pain (allodynia, hyperpathia and wind-up pain) evaluated. Chronic pain was mapped and characterized. Chronic pain developed at a relatively late onset (6.6+/-9 months) was almost exclusively unilateral and reported as pricking, throbbing and burning. Although both TBIP and TBINP exhibited a significant reduction in thermal and tactile sensations compared to controls, thermal sensations in the painful regions of TBIP were significantly more impaired than pain-free regions in the same patients (p<0.01) and in TBINP (p<0.01). Painful regions also exhibited very high rates of allodynia, hyperpathia and exaggerated wind-up. The characteristics of the chronic pain resembled those of other central pain patients although TBIP displayed several unique features. The sensory profile indicated that damage to the pain and temperature systems is a necessary but not sufficient condition for the development of chronic central pain following TBI. Neuronal hyperexcitability may be a contributing factor to the chronic pain.

Imaging central pain syndromes

Anatomic, functional, and neurochemical imaging studies have provided new investigative tools in the study of central pain. High-resolution imaging studies allow for precise determination of lesion location, whereas functional neuroimaging studies measure pathophysiologic consequences of injury to the central nervous system. Additionally, magnetic resonance spectroscopy evaluates lesion-induced neurochemical changes in specific brain regions that may be related to central pain. The small number of studies to date precludes definitive conclusions, but the recent findings provide information that either supports or refutes current hypotheses and can serve to generate new ideas.

Lesions limited to the human thalamic principal somatosensory nucleus (ventral caudal) are associated with loss of cold sensations and central pain

Central pain is neuropathic pain resulting from a lesion of the CNS, such as a stroke [poststroke central pain (CPSP)]. Lesions involving the posterior thalamus lead to reduction or loss of sensation and to CPSP, although the responsible nuclei have not been identified. We now examine the hypotheses that thalamic lesions must extend posterior to the ventral caudal nucleus (Vc) and include ventral medial posterior nucleus (VMpo), to result in loss of cold sensibility and CPSP. Patients with small thalamic strokes associated with CPSP were evaluated by atlas-based mapping of magnetic resonance imaging scans, and by somatosensory testing. All lesions involved posterior Vc; two lesions also involved nuclei posterior to Vc, but not VMpo. All patients tested had alterations of cold pain sensation and tactile sensation, as measured by von Frey hairs. Three patients had altered cool sensation, and the patient with the least involvement of Vc had normal cool thresholds, suggesting that a critical volume of Vc must be involved before cool sensation is impaired. Perception of warm was impaired only in lesions involving nuclei posterior to Vc. Heat pain perception was never affected. In a subject with cold allodynia, a single-subject protocol PET study measured the responses to immersion of either hand in a 20 degrees C waterbath. The scan during stimulation of the affected hand was characterized by intense activation of contralateral sensorimotor cortex. Therefore, there are modality-specific subnuclear structures in the posterior thalamus, but lesions of Vc not involving VMpo are sufficient to impair cold sensibility and to produce CPSP.

Somatic Sensation and the Insular-Opercular Cortex: Relationship to Central Pain

We report 5 stroke patients with lesions affecting the insula and parietal operculum sparing the postcentral gyrus (somatosensory cortical area SI); 3 had spontaneous central poststroke pain (CPSP) and 2 did not. All were imaged and underwent quantitative sensory threshold tests, though not all modalities were tested in all subjects. Tactile thresholds were unaltered in all. The patients with CPSP exhibited greatly elevated thresholds for mechanical pain (skinfold pinch), sharpness and thermal sensations; the pain-free patients had distinctly lesser elevations of their skinfold pinch and innocuous and noxious thermal thresholds, and no sharpness deficit. It is therefore suggested that, in the case of similar cortical lesions, the presence or absence of spontaneous pain either modifies the thresholds for some innocuous modalities, or that the degree of deficit of some innocuous modalities determines whether or not central pain occurs.

Pharmacologic Treatment of Central Post-Stroke Pain

OBJECTIVES

Almost 100 years after the first report of the thalamic syndrome, the scientific basis for the treatment of central post-stroke pain (CPSP) is remarkably small. Therefore, the authors aimed to provide evidence-based recommendations for the treatment of CPSP.

METHODS

The authors performed a systematic review of the literature on the pharmacologic treatment of CPSP. All studies and case series were included and evaluated according to their level of evidence. Only CPSP was considered, not other types of central pain.

RESULTS

Amitriptyline and lamotrigine are the only oral drugs proven to be effective in the treatment of CPSP in a placebo-controlled study. IV drugs such as lidocaine, propofol, and ketamine have shown efficacy for short-term control of CPSP, but their application and potential side effects make them unsuitable for long-term treatment. The novel antiepileptic drug gabapentin has been reported to control CPSP in a few patients.

CONCLUSIONS

Amitriptyline, lamotrigine, and gabapentin provide a more favorable efficacy and safety profile than the classic antiepileptic drugs carbamazepine and phenytoin, for which no placebo-controlled evidence of efficacy was found. Clinical trials are urgently needed to optimize pharmacologic treatment of CPSP.

Mechanisms of central neuropathic pain: a combined psychophysical and fMRI study in syringomyelia

The pathophysiology of central pain syndromes is still poorly understood and their treatment remains a major challenge. It has long been suggested that lesions of the spinothalamic pathways are necessary for developing these pain syndromes. The recently proposed thermosensory disinhibition theory suggests that reduction of the inhibition of thermal sensory afferents that affect nociceptive systems may play a major pathophysiological role. Syringomyelia, which is frequently associated with central neuropathic pain, is characterized by a selective or preferential lesion of the spinothalamic tract resulting in thermosensory deficits of various extents and magnitudes. Thus, syringomyelia represents a unique 'pathological model' particularly suited to investigating the relationship between spinothalamic tract dysfunction, thermosensory deficits and pain. Here, we systematically compared the sensory loss (thermal and mechanical), using quantitative sensory testing, between 46 consecutive syringomyelia patients with or without neuropathic pain. We then further investigated the mechanisms of evoked pains in these patients, using functional MRI (fMRI) in a subgroup of patients with cold or brush-evoked allodynia, compared with patients without pain and healthy volunteers. We found no significant difference in the magnitude or extent of sensory deficits between patients with or without neuropathic pain, suggesting that lesions of the spinothalamic pathways are not sufficient for developing central pain. However, a different pattern of sensory deficits was observed between patients with spontaneous pain only (n = 11) and patients with both spontaneous pain and allodynia (n = 20), suggesting that the mechanisms of central pain are not univocal. In patients with spontaneous pain only, the thermal sensory loss was significantly more asymmetrical and there was a direct relationship between the extent of thermosensory deficits (i.e. deafferentation) and the intensity of burning pain. In contrast, patients with allodynia had reduced thermal deficits, in terms of both magnitude and extent. In addition, the sensory deficits were different between patients with cold or tactile allodynia, suggesting distinct pathophysiological mechanisms related to the sub-modalities of allodynia. Our fMRI study further confirmed this, showing that different sub-types of allodynia were associated with distinct patterns of brain activity, which do not necessarily correspond to the 'pain matrix' involved in acute physiological pain. The prefrontal cortex was the only area consistently activated by pathological evoked pains, suggesting that alteration of high-level pain modulatory mechanisms might play a major role in allodynia due to central lesion.

Transient attenuation of neuropathic manifestations in rats following lesion or reversible block of the lateral thalamic somatosensory nuclei

BACKGROUND AND AIMS

Nociceptive behavior in animal models for mononeuropathy has been shown to be altered by spinal tract lesions which suggest a possible supraspinal modulation. The thalamus constitutes a chief center for the processing of nociception. We have, therefore, investigated the effects of transient or permanent blocks of the lateral somatosensory thalamic nuclei (the ventrobasal complex) on the neuropathic manifestations in rats.

METHODS

Different groups of rats (n = 5-6) were subjected to mononeuropathy, following the spared nerve injury model, known to produce sustained heat hyperalgesia and tactile and cold allodynia which peaked about 2 weeks after nerve injury. This was followed by stereotaxic placement of either electrolytic or chemical lesions or implantation of mini osmotic pump for slow release of lidocaine in the ventrobasal complex.

RESULTS

Chronic electrolytic and chemical lesions or reversible block of the lateral somatosensory thalamus produced transient (1-2 weeks) attenuation of neuropathic manifestations along with a persistent decrease of the hot plate latency. The most pronounced effect was observed on heat hyperalgesia, and the least significant and short-lived effect was observed on cold allodynia.

CONCLUSION

We conclude that the lateral somatosensory thalamic complex is involved in the processing of neuropathic manifestations but cannot be considered as an obligatory or exclusive relay center for the neuropathic syndromes.

Allodynia in Relation to Lesion Site in Central Post-Stroke Pain

Seventy-one percent of 122 patients with central post-stroke pain (CPSP) had allodynia that was tactile-, cold-, or movement-evoked. Site of thalamic (and some infratentorial) lesions as revealed by magnetic resonance imaging (MRI) was correlated in some cases with allodynia type and sensory perception threshold testing (QST). Notably, patients with cold allodynia tend to have more dorsally placed thalamic lesions than those without, and those with movement allodynia more anteriorly placed lesions. Suggestions are made for improved correlation.

PERSPECTIVE

Only about half of patients with CPSP have allodynia (pain caused by innocuous stimulation); such stimulation is usually tactile- or cold-evoked or due to activation of stretch receptors (movement). We have found that, in some of our cases, the type of allodynia may depend on lesion location within the thalamus.

Representation of Somatosensory Modalities in Pathways Ascending from the Spinal Anterolateral Funiculus to the Thalamus Demonstrated by Lesions in Man

Patients with cordotomies (16), and brainstem (17) or thalamic (30) infarcts, all except cordotomies verified by magnetic resonance imaging (MRI), have been subjected to quantitative sensory perception threshold testing (QST) for touch (von Frey), mechanical pain, sharpness, innocuous warmth and cold, and heat pain in the maximally affected body area and its unaffected contralateral mirror image region. Some patients were tested twice at widely spaced time intervals; no qualitative differences were found. Results show that all modalities are dissociable from one another by lesions at all levels tested, so that there must be separable representation for each of the six modalities tested. In the lower (crossed symptoms and signs), but not the upper (uncrossed symptoms), deficits for all modalities (except for touch) were more marked than at higher levels. At the level of the thalamus, deficits for innocuous and noxious thermal modalities but not for mechanical pain were recorded in the case of lesions of the principal somatosensory relay nucleus (VPL/Vc), while more medial thalamic lesions resulted in deficits for mechanical pain but not for heat pain or innocuous thermal modalities; there is a marked deficit for sharpness caused by lesions at both thalamic sites.

Functional magnetic resonance imaging and diffusion tensor imaging in a case of central poststroke pain

The role of the lesion location within functional pain systems is not fully understood for central poststroke pain (CPSP) pathogenesis. In a patient with CPSP we used data from both functional magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) for anatomo-functional correlations. Structural MRI showed a small residual cavity confined to the right thalamic ventral posterolateral nucleus and the adjacent posterior arm of the internal capsule. DTI maps showed selective reduction of right sensory thalamocortical fibers. Functional MRI, performed with different thermonociceptive stimuli, showed pain-specific signal changes in the anterior cingulate gyrus (BA 24/32) and in the associative parietal regions (BA 5/7). These findings underline, for CPSP pathogenesis, the role of damage of lateral nociceptive thalamoparietal fibers together with the release of activity of anterior cingulate and posterior parietal regions. In a patient with CPSP, we combined noninvasive neuroimaging techniques (functional and diffusion MRI) to assess the anatomo-functional relationship in CPSP. Our investigations show, for CPSP pathogenesis, the role of damage of lateral nociceptive thalamoparietal fibers together with the release of activity of anterior cingulate and posterior parietal regions.

Central Representation of Somatic Sensations in the Parietal Operculum (SII) and Insula

Four subjects with small restricted cerebral cortical infarcts have been examined. One had a lesion confined to the parietal operculum (SII), while in the second the SII lesion also encroached on the posterior insula; in the third subject, both banks of the sylvian fissure and the dorsal insula were involved, while in the fourth the lesion involved the upper bank of the sylvian fissure. In all cases, the postcentral gyrus (SI) was intact. Subjects 1 and 2 had mild spontaneous pain, but subjects 3 and 4 had never had spontaneous pain. In the affected areas, none could feel mechanical (skinfold pinch) pain. The 2 subjects with spontaneous pain could not discriminate sharpness (pinprick), but this was unimpaired in the third and fourth subjects. Warmth, cold, and heat pain were impaired in the 2 subjects with spontaneous pain, but not in those without; however warm-cold difference was greater in the affected regions of all subjects. The possibility must nevertheless be considered that the presence of central pain in some way alters the cortical mechanisms for the perception of thermal stimuli. Certainly, as we had earlier observed, spontaneous pain only occurs when there is interference with thermal sensation. Functional MRI (fMRI) studies following thermal stimulation in subjects 1 and 2 showed these areas, particularly SII, to be concerned with the reception of innocuous and noxious thermal stimuli, mechanical (skinfold pinch) pain and sharpness (pinprick), implying that SI is principally concerned with the reception of low-intensity mechanical stimuli, although it was activated in 1 of our fMRI-studied subjects by innocuous cooling.

Central pain in multiple sclerosis--prevalence and clinical characteristics

Pain is more common in multiple sclerosis (MS) than has previously been recognised. In the present study we have investigated the occurrence of central pain (CP) in MS and defined its characteristics. Questionnaires were sent to all 429 patients with definite MS in the patient register at our neurology department. All admitting to pain were interviewed and offered an extended interview and examination. Three hundred and sixty four patients responded (86%), of whom 57.5% reported pain during the course of their disease (21% nociceptive, 2% peripheral neuropathic and 1% related to spasticity). One hundred patients (27.5%) had CP, including 18 patients (4.9%) with trigeminal neuralgia. The non-trigeminal CP was, in 87%, located in the lower and in 31% in the upper extremities. It was mostly bilateral (76%) and constant, with 88% experiencing daily pain. Only 2% had paroxysmal attacks. Aching, burning, pricking were the commonest qualities. The pain was intense with small to moderate spontaneous variation. In 5.5% of all patients (20% of the patients with CP), pain was a presenting symptom, alone or in combination with other symptoms. The most common neurological symptoms/signs besides CP were sensory abnormalities (98%, dominated by abnormal sensibility to painful stimulus and temperature). Trigeminal neuralgia in MS started later in life and after longer disease duration than non-trigeminal pain. Both types of CP existed either chronically or as a feature of relapse. Central pain is thus an important symptom in MS (around 30%) and causes much suffering.

Distraction modulates connectivity of the cingulo-frontal cortex and the midbrain during pain—an fMRI analysis

Neuroimaging studies with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have delineated a human pain network in vivo. Despite the recognition of cerebral structures engaged in pain transmission, the cerebral mechanisms involved in pain modulation are still not well understood. Here, we investigated healthy volunteers using fMRI during experimental heat pain and distraction induced by a visual incongruent color-word Stroop task. A factorial design permitted categorical and covariation analysis of four conditions, namely innocuous and noxious heat; with and without distraction. Pain without distraction evoked an activation pattern similar to that observed in previous neuroimaging pain studies. Distraction was associated with a significant reduction of the visual analogue scale (VAS) ratings for pain intensity and unpleasantness and a reduction of pain-related activation in multiple brain areas, particularly in the so-called 'medial pain system'. Distraction significantly increased the activation of the cingulo-frontal cortex including the orbitofrontal and perigenual anterior cingulate cortex (ACC), as well as the periaquaeductal gray (PAG) and the posterior thalamus. Covariation analysis revealed functional interaction between these structures during pain stimulation and distraction, but not during pain stimulation per se. According to our results, the cingulo-frontal cortex may exert top-down influences on the PAG and posterior thalamus to gate pain modulation during distraction.

Central post-stroke pain or paresthesia in lenticulocapsular hemorrhages

Twenty patients were studied who developed central poststroke pain or paresthesia after lenticulocapsular hemorrhage. Pain or paresthesia occurred 0 to 24 months after the onset, more prominently in the leg than other body parts. The symptoms were described as numb, cold, burning, aching, swollen, and squeezing in various combinations. The mean score of the visual numerical scale was 5.6. The lesions involved the dorsal part of the posterior limb of the internal capsule, probably damaging the thalamocortical sensory pathway.

Author's experience of lateral medullary infarction--thermal perception and muscle allodynia

The patient, the author (S.K.), is a 67-year-old male. He has the typical dissociated pain, altered temperature sensation and ataxia often encountered by patients with lateral medullary infarction. This started at the time of his admission to hospital. Several weeks after discharge, he experienced the withdrawal reaction to high temperature, first mentioned by Rousseaux (Stroke 30 (1999) 2223), and movement allodynia as described by Bowsher (J Neurol Neurosurg Psychiatry 61 (1996) 62). The article describes his personal experience of the symptoms of central post-stroke pain and allodynia. A difference between muscle allodynia and conventional allodynia caused by dermal stimulation is proposed. Follow-up evaluation of sensory symptoms that appear several weeks after discharge is essential to ensure measures to alleviate them are provided.

Sensitivity of laser-evoked potentials versus somatosensory evoked potentials in patients with multiple sclerosis

OBJECTIVE

Somatosensory evoked potentials (SEPs) play a less important role in the diagnosis of multiple sclerosis (MS) than visually evoked potentials. Since standard SEPs only reflect the dorsal column function, we now investigated spinothalamic tract function in patients with MS using laser-evoked potentials (LEPs).

METHODS

LEPs to thulium laser stimuli (3ms, 540 mJ, 5mm diameter) were recorded from 3 midline positions (Fz, Cz, Pz) in 20 patients with MS, and 6 patients with possible but unconfirmed MS. Peak latencies and peak-to-peak amplitude of the vertex potential negativity (N2) and positivity (P2) were evaluated and compared with normative values from 22 healthy control subjects. Median and tibial nerve SEPs were recorded with standard methods. Depending on the results of sensory testing, two skin areas (both hands, both feet, or one hand and foot of the same body side) were assessed in each patient.

RESULTS

In group comparisons, LEPs in patients with MS were significantly delayed and reduced in amplitude compared with healthy subjects (P<0.001) or patients with suspected but unconfirmed MS (P<0.05). In intraindividual comparisons within the patients with MS, LEP amplitude was significantly lower (P<0.01) and latencies were significantly longer (N2: P<0.01; P2: P<0.05) for a clinically hypoalgesic skin area than an unaffected control area. On a single case basis, LEPs were abnormal in 12 (60%) and SEPs in 8 (40%) of the patients with MS; combined analysis of LEPs and SEPs raised sensitivity to 75% (15 patients). LEPs were also abnormal for 7 skin areas with clinically normal nociception and thermal sensitivity, indicating subclinical lesions. Standard SEPs detected subclinical lesions in 5 areas with normal tactile sensitivity.

CONCLUSIONS

In patients with multiple sclerosis, spinothalamic tract function and LEPs were impaired more often than dorsal column function and SEPs. LEPs also detected subclinical lesions. Combined assessment of LEPs and SEPs can help to document dissemination of demyelinating CNS lesions and thus contribute to the diagnosis of multiple sclerosis.

Sensory function in spinal cord injury patients with and without central pain

Spinal cord injury (SCI) frequently results in neuropathic pain. However, the pathophysiology underlying this pain is unclear. In this study, we compared clinical examination, quantitative sensory testing (QST) and somatosensory evoked potentials (SEPs) in SCI patients with and without pain below spinal lesion level, with a control group of 20 subjects without injury. All patients had a traumatic SCI with a lesion above T10; 20 patients presented with spontaneous central neuropathic pain below lesion level, and 20 patients had no neuropathic pain or dysaesthesia. Patients with and without pain had a similar reduction of mechanical and thermal detection and pain thresholds, and SEPs. SCI patients with central pain more frequently had sensory hypersensitivity (brush- or cold-evoked pain, dysaesthesia or pinprick hyperalgesia) in dermatomes corresponding to lesion level than SCI patients without pain. There was no difference in intensity of pain evoked by repetitive pinprick at lesion level between patient groups. There was a significant correlation between intensity of brush-evoked dysaesthesia at lesion level and spontaneous pain below lesion level of SCI. These data suggest that lesion of the spinothalamic pathway alone cannot account for central pain in SCI patients, and that neuronal hyperexcitability at injury or higher level may be an important mechanism for pain below injury level.

Venous malformations associated with central pain: report of a case

IMPLICATIONS

The authors describe an unusual case of central pain (CP) that resulted from giant venous hemangiomas. The patient was treated with a variety of medications, including the N-methyl-D-aspartate antagonist dextromethorphan. We report the first known association between venous malformations and CP and briefly describe why the use of dextromethorphan in this disorder requires further evaluation.

Spatial summation of pain processing in the human brain as assessed by cerebral event related potentials

To understand spatial summation of pain processing in the brain, we investigated the cerebral evoked responses to non-painful and painful contact heat stimulation (70 degrees C/s fast onset; intensity 2,4,6, corresponding to the individual's non-, slight and moderate pain) comparing one (1s) vs. two spots (2s) in 11 subjects while electroencephalographic signals were recorded. Significant spatial summation effects were shown only for the pain levels. For moderate pain, global field power examination isolated two peak activations for the vertex (Cz) N550 and P750 components. The single dipole modelling identified as likely the supplementary motor area, SMA area-6 source for N550, and posterior cingulate area-23 for P750. These source components showed a significantly faster (41.2 ms) latency and a shift in location from dorsal to ventral SMA of N550 toward cingulate area-31 between the 1s and 2s conditions. The temporal and spatial shift during spatial summation may reflect speeding up of the limbic affective reaction and prefrontal cognitive preparation in impending aversion and is deemed essential for integration of bodily sensations, such as pain.

Disappearance of central thalamic pain syndrome after contralateral parietal lobe lesion: implications for therapeutic brain stimulation

At present there is hardly any appropriate therapy for central pain syndromes available. We report on a unique case of a central thalamic pain syndrome that did not respond to any therapy but disappeared after an additional contralateral parietal lobe lesion. This example indicates that lesions affecting the bilateral balance of thalamo-parietal circuits may lead to pain relief in patients with central pain syndrome, which probably constitutes a bilateral disorder of functional plasticity. This should be taken into account in chronic brain stimulation for persistent pain states.

Disruption of thermal perception in a multiple sclerosis patient with central pain

OBJECTIVE

To investigate integrative thermal perception in a patient with multiple sclerosis.

DESIGN

Quantitative thermosensory testing was used to evaluate pain and other sensations produced by heat, cold, and the thermal grill pain illusion.

PATIENT

The authors report on a 43-year-old patient with central pain manifest most strongly in her left arm and hand, contralateral to an upper cervical spinothalamic lesion due to multiple sclerosis.

OUTCOME MEASURES AND RESULTS

Quantitative thermosensory testing showed that the patient had heat hypalgesia (no pain with stimuli of 45-50 degrees C) and cold allodynia (pain with innocuous cool temperatures, 25-10 degrees C). Whereas healthy subjects rated 20 degrees and 40 degrees C as nonpainful, but the thermal grill (intermixed 20 and 40 degrees C stimuli) as painful, the patient rated the thermal grill as less painful than 20 degrees C.

CONCLUSIONS

The absence of thermal grill-evoked pain is consistent with the hypothesis that in some cases of central pain the loss of the thermosensory pathway results in disruption of the normal cold inhibition of burning pain.

Neurogenic hyperalgesia versus painful hypoalgesia: two distinct mechanisms of neuropathic pain

Patients with sensory disturbances of painful and non-painful character show distinct changes in touch and/or pain sensitivity. The patterns of sensory changes were compared to those of human surrogate models of neuropathic pain to assess the underlying mechanisms. We investigated 30 consecutive in-patients with dysaesthesia of various origins (peripheral, spinal, and brainstem lesions) and 15 healthy subjects. Tactile thresholds were determined with calibrated von Frey hairs (1.1mm). Thresholds and stimulus-response functions for pricking pain were determined with a series of calibrated punctate mechanical stimulators (0.2mm). Allodynia was tested by light stroking with a brush, Q-tip, and cotton wisp. Perceptual wind-up was tested by trains of punctate stimuli at 0.2 or 1Hz. Intradermal injection of capsaicin (n=7) and A-fiber conduction blockade (n=8) served as human surrogate models for neurogenic hyperalgesia and partial nociceptive deafferentation, respectively. Patients without pain (18/30) showed a continuous distribution of threshold shifts in the dysaesthetic skin area with a low to moderate increase in pain threshold (by 1.52+/-0.45 log2 units). Patients with painful dysaesthesia presented as two separate groups (six patients each): one showing lowered pain thresholds (by -1.94+/-0.46 log2 units, hyperalgesia) and the other elevated pain thresholds (by 3.02+/-0.48 log2 units, hypoalgesia). The human surrogate model of neurogenic hyperalgesia revealed nearly identical leftward shifts in stimulus-response function for pricking pain as patients with spontaneous pain and hyperalgesia (by a factor of about 5 each). The sensory changes in the human surrogate model of deafferentation were similar to patients with hypoalgesia and spontaneous pain (rightward shift of the stimulus-response function with a decrease in slope). Perceptual wind-up did not differ between symptomatic and control areas. There was no exclusive association of any parameter obtained by quantitative sensory testing with a particular disease (of either peripheral or central origin). Our findings suggest that neuropathic pain is based on two distinct mechanisms: (I) central sensitization (neurogenic hyperalgesia; in patients with minor sensory impairment) and (II) partial nociceptive deafferentation (painful hypoalgesia; in patients with major sensory deficit). This distinction as previously postulated for postherpetic neuralgia, is obviously valid also for other conditions. Our findings emphasize the significance of a mechanism-based classification of neuropathic pain.

The density of remaining nerve endings in human skin with and without postherpetic neuralgia after shingles

The mechanisms of chronic neuropathic pain are not well understood. Postherpetic neuralgia (PHN), which occurs in some patients after shingles (herpes zoster), was used to investigate the neural determinants of chronic pain. Skin biopsies were obtained from 38 adults with or without PHN at least 3 months after healing of shingles on the torso. Vertical sections were immunolabeled against PGP9.5, a pan-axonal marker, to measure the density of remaining nerve endings in skin previously affected by shingles. All axons that end in the epidermis are nociceptors, neurons that transmit pain messages. The densities ranged between 2 and 3976 neurites/mm2 skin surface, but the overlap between subjects and without PHN was small. Of 19 subjects without PHN, 17 had more than 670 neurites/mm2 skin surface area (mean +/- SEM = 1569 +/- 230), and 18 of 19 subjects with PHN had 640 or fewer neurites/mm2 (mean +/- SEM = 367 +/- 92). PHN may be a 'phantom-skin' pain associated with loss of nociceptors. This threshold of approximately 650 neurites/mm2 skin surface was not detected in previous studies that used summary statistics. It implies that the absence of pain after shingles may require the preservation of a minimum density of primary nociceptive neurons, and that the density of epidermal innervation may provide an objective correlate for the presence or absence of PHN pain.

Sensory sequelae of medullary infarction: differences between lateral and medial medullary syndrome

BACKGROUND AND PURPOSE

A comparison between long-term sensory sequelae of lateral medullary infarction (LMI) and medial medullary infarction (MMI) has never been made.

METHODS

We studied 55 patients with medullary infarction (41 with LMI and 14 with MMI) who were followed up for >6 months. We examined and interviewed the patients with the use of a structured format regarding the most important complaints, functional disabilities, and the presence of sensory symptoms. The nature and the intensity of sensory symptoms were assessed with the modified McGill-Melzack Pain Questionnaire and the visual analog scale, respectively.

RESULTS

There were 43 men and 12 women, with an average age of 59 years. Mean follow-up period was 21 months. The sensory symptoms were the most important residual sequelae in LMI patients and the second most important in MMI patients. In LMI patients, the severity of residual sensory symptoms was significantly related to the initial severity of objective sensory deficits (P<0.05). Sensory symptoms were most often described by LMI patients as numbness (39%), burning (35%), and cold (22%) in the face, and cold (38%), numbness (29%), and burning (27%) in the body/limbs, whereas they were described as numbness (60%), squeezing (30%) and cold (10%), but never as burning, in their body/limbs by MMI patients. LMI patients significantly (P<0.05) more often cited a cold environment as an aggravating factor for the sensory symptoms than did the MMI patients without spinothalamic sensory impairment. The subjective sensory symptoms were frequently of a delayed onset (up to 6 months) in LMI patients, whereas they usually started immediately after the onset in MMI patients.

CONCLUSIONS

Our study shows that sensory symptoms are major sequelae in both LMI and MMI patients. However, the nature, the mode of onset, and aggravating factors are different between the 2 groups, which probably is related to a selective involvement of the spinothalamic tract by the former and the medial lemniscus by the latter. We suggest that the mechanisms for the central poststroke pain or paresthesia may differ according to the site of damages on the sensory tracts (spinothalamic tract versus medial lemniscal tract).