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

A hidden mesencephalic variant of central pain

BACKGROUND

Central post-stroke pain (CPSP) can arise after lesions anywhere in the central somatosensory pathways, essentially within the spinothalamic system (STS). Although the STS can be selectively injured in the mesencephalon, CPSP has not been described in pure midbrain infarcts.

METHODS

Of more than 300 CPSP consecutive cases, we describe five patients who developed definite neuropathic pain following lesions circumscribed to the postero-lateral mesencephalon.

RESULTS

The mesencephalic lesion responsible for pain was always haemorrhagic and always involved the spinothalamic tract (STT), as demonstrated by suppressed laser-evoked potentials in every case, with or without preserved lemniscal function. In three cases the midbrain injury could be ascribed to trauma, presumably from the cerebellar tentorium. As a result of the paucity of sensory symptoms, the pain was considered as 'psychogenic' in two of the patients until electrophysiological testing confirmed STT involvement.

CONCLUSION

Postero-lateral midbrain lesions should be added to potential causes of CPSP. Because pain and spinothalamic deficits may be the only clinical sign, and because small lateral midbrain lesions may be difficult to trail with MRI, mesencephalic CPSP can be misdiagnosed as malingering or psychogenic pain for years.

SIGNIFICANCE

Selective spinothalamic injury caused by small lateral midbrain lesions is a very rare cause of central post-stroke pain that can remain undiagnosed for years. It appears to obey to haemorrhagic, sometimes post-traumatic lesions. Sudden development of contralateral burning pain with isolated spinothalamic deficits may be the only localizing sign, which can be easily objectively detected with electrophysiological testing.

Pain syndromes and the parietal lobe

Pain was considered to be integrated subcortically during most of the 20^th century, and it was not until 1956 that focal injury to the parietal opercular-insular cortex was shown to produce selective loss of pain senses. The parietal operculum and adjacent posterior insula are the main recipients of spinothalamic afferents in primates. The innermost operculum appears functionally associated with the posterior insula and can be segregated histologically, somatotopically and neurochemically from the more lateral S2 areas. The Posterior Insula and Medial Operculum (PIMO) encompass functional networks essential to initiate cortical nociceptive processing. Destruction of this region selectively abates pain sensations; direct stimulation generates acute pain, and epileptic foci trigger painful seizures. Lesions of the PIMO have also high potential to develop central pain with dissociated loss of pain and temperature. The PIMO region behaves as a somatosensory area on its own, which handles phylogenetically old somesthetic capabilities based on thinly myelinated or unmyelinated inputs. It integrates spinothalamic-driven information - not only nociceptive but also innocuous heat and cold, crude touch, itch, and possibly viscero-somatic interoception. Conversely, proprioception, graphesthesia or stereognosis are not processed in this area but in S1 cortices. Given its anatomo-functional properties, thalamic connections, and tight relations with limbic and multisensory cortices, the region comprising the inner parietal operculum and posterior insula appears to contain a third somatosensory cortex contributing to the spinothalamic attributes of the final perceptual experience.

Central neural substrates involved in temperature discrimination, thermal pain, thermal comfort, and thermoregulatory behavior

A phylogenetically novel pathway that emerged with primate encephalization is described, which conveys high-fidelity cutaneous thermosensory activity in "labeled lines" to a somatotopic map in the dorsal posterior insular cortex. It originates in lamina I of the superficial dorsal horn and ascends by way of the lateral spinothalamic tract and a distinct region in posterolateral thalamus. It evolved from the homeostatic sensory activity that represents the physiologic (interoceptive) condition of the body and drives the central autonomic network, which underlies all affective feelings from the body. Accordingly, human discriminative thermal sensations are accompanied by thermally motivated behaviors and thermal feelings of comfort or discomfort (unless neutral), which evidence suggests are associated with activity in the insular, cingulate, and orbitofrontal cortices, respectively. Yet, the substrates for thermoregulatory behavior have not been established, and several strong candidates (including the hypothalamus and the bed nucleus of the stria terminalis) are discussed. Finally, the neural underpinnings for relationships between thermal affect and social feelings (warm-positive/cold-negative) are addressed, including the association of hyperthermia with clinical depression.

Calcium-stimulated adenylyl cyclase subtype 1 (AC1) contributes to LTP in the insular cortex of adult mice

Long-term potentiation (LTP) of synaptic transmission in the central nervous system is a key form of cortical plasticity. The insular cortex (IC) is known to play important roles in pain perception, aversive memory and mood disorders. LTP has been recently reported in the IC, however, the signaling pathway for IC LTP remains unknown. Here, we investigated the synaptic mechanism of IC LTP. We found that IC LTP induced by the pairing protocol was N-methyl-D-aspartate receptors (NMDARs) dependent, and expressed postsynaptically, since paired-pulse ratio (PPR) was not affected. Postsynaptic calcium is important for the induction of post-LTP, since the postsynaptic application of BAPTA completely blocked the induction of LTP. Calcium-activated adenylyl cyclase subtype 1 (AC1) is required for potentiation. By contrast, AC8 is not required. Inhibition of Ca²⁺ permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (CP-AMPARs) or protein kinase M zeta (PKMζ) reduced the expression of LTP. Our results suggest that calcium-stimulated AC1, but not AC8, can be a trigger of the induction and maintenance of LTP in the IC.

Contribution of synaptic plasticity in the insular cortex to chronic pain

Animal and human studies have consistently demonstrated that cortical regions are important for pain perception and pain-related emotional changes. Studies of the anterior cingulate cortex (ACC) have shown that adult cortical synapses can be modified after peripheral injuries, and long-term changes at synaptic level may contribute to long-lasting suffering in patients. It also explains why chronic pain is resistant to conventional analgesics that act by inhibiting synaptic transmission. Insular cortex (IC), another critical cortical area, is found to be highly plastic and can undergo long-term potentiation (LTP) after injury. Inhibiting IC LTP reduces behavioral sensitization caused by injury. LTP of glutamatergic transmission in pain related cortical areas serves as a key mechanism for chronic pain.

Somatosensory deficits after stroke: a scoping review

In the past years, there have been increasing research activities focusing on somatosensory symptoms following stroke. However, as compared to the large number of clinical and neuroimaging studies on motor symptoms, the number of studies tracing somatosensory symptoms after stroke and their recovery is rather small. It is an ongoing discussion, to which extent somatosensory deficits after stroke influence patient's long-term outcome in motor and sensory performance and functional independence in activities of daily living. Modern brain imaging techniques allow for studying the impact of stroke lesion localization and size on acute and persisting clinical impairment. Here, we review the literature on somatosensory symptoms after stroke. We summarize epidemiological information on frequency and characteristics of somatosensory symptoms affecting all parts of the body in the acute and chronic stage of stroke. We further give an overview of brain imaging studies of stroke affecting the somatosensory system. Finally, we identify open questions which need to be addressed in future research and summarize the implications for clinical practice.

Effects of insular stimulation on thermal nociception

BACKGROUND

Electrical stimulation used for brain mapping in the postero-superior insula can evoke pain. The effects of prolonged high frequency insular stimulation on pain thresholds are unknown.

OBJECTIVE/HYPOTHESIS

Prolonged high frequency insular stimulation, by virtue of its inhibitory properties on networks, could decrease thermal nociception.

METHODS

Epileptic subjects had electrodes implanted in the insular cortex for the purpose of epileptic focus resection. Thermal and pressure nociceptive thresholds were tested bilaterally on the forearm on two consecutive days. Randomly assigned double-blind high frequency (150 Hz) insular stimulation took place for 10 min before pain testing either on the first day or on the second day.

RESULTS

Six subjects (three females; mean age of 35 years) were included. Insular stimulation increased heat pain threshold on the ipsilateral (p = 0.003; n = 6) and contralateral sides (p = 0.047; n = 6). Differences in cold pain threshold did not reach statistical significance (ipsilateral: p = 0.341, contralateral: p = 0.143; n = 6), but one subject had a profound decrease in both heat and cold pain responses. Pressure pain threshold was not modified by insular stimulation (ipsilateral: p = 0.1123; contralateral: p = 0.1192; n = 6). Two of the three subjects who had a postero-superior operculo-insulectomy developed central pain with contralateral thermal nociceptive deficit.

CONCLUSIONS

High frequency inhibitory postero-superior insular stimulation may have the potential to decrease thermal nociception. Together with previous studies, our data support the notion that the integrity of this brain region is necessary for thermal but not pressure nociceptive processing.

Preserved emotional awareness of pain in a patient with extensive bilateral damage to the insula, anterior cingulate, and amygdala

Functional neuroimaging investigations of pain have discovered a reliable pattern of activation within limbic regions of a putative "pain matrix" that has been theorized to reflect the affective dimension of pain. To test this theory, we evaluated the experience of pain in a rare neurological patient with extensive bilateral lesions encompassing core limbic structures of the pain matrix, including the insula, anterior cingulate, and amygdala. Despite widespread damage to these regions, the patient's expression and experience of pain was intact, and at times excessive in nature. This finding was consistent across multiple pain measures including self-report, facial expression, vocalization, withdrawal reaction, and autonomic response. These results challenge the notion of a "pain matrix" and provide direct evidence that the insula, anterior cingulate, and amygdala are not necessary for feeling the suffering inherent to pain. The patient's heightened degree of pain affect further suggests that these regions may be more important for the regulation of pain rather than providing the decisive substrate for pain's conscious experience.

Persistence of feelings and sentience after bilateral damage of the insula

It has been convincingly established, over the past decade, that the human insular cortices are involved in processing both body feelings (such as pain) and feelings of emotion. Recently, however, an interpretation of this finding has emerged suggesting that the insular cortices are the necessary and sufficient platform for human feelings, in effect, the sole neural source of feeling experiences. In this study, we investigate this proposal in a patient whose insular cortices were destroyed bilaterally as a result of Herpes simplex encephalitis. The fact that all aspects of feeling were intact indicates that the proposal is problematic. The signals used to assemble the neural substrates of feelings hail from different sectors of the body and are conveyed by neural and humoral pathways to complex and topographically organized nuclei of the brain stem, prior to being conveyed again to cerebral cortices in the somatosensory, insular, and cingulate regions. We suggest that the neural substrate of feeling states is to be found first subcortically and then secondarily repeated at cortical level. The subcortical level would ensure basic feeling states while the cortical level would largely relate feeling states to cognitive processes such as decision-making and imagination.

Reduced regional gray matter volume in patients with chronic obstructive pulmonary disease: a voxel-based morphometry study

BACKGROUND AND PURPOSE

Decreased oxygen supply may cause neuronal damage in the brains of patients with COPD, which is manifested by clinical symptoms such as neuropsychological deficits and mood disorders. The aim of the present study was to investigate brain gray matter change in COPD.

MATERIALS AND METHODS

Using voxel-based morphometry based on the high-resolution 3D T1-weighted MR images of GM volume, we investigated 25 stable patients with COPD and 25 matching healthy volunteers. A battery of neuropsychological tests was also performed.

RESULTS

Patients with COPD (versus controls) showed reduced GM volume in the frontal cortex (bilateral gyrus rectus, bilateral orbital and inferior triangular gyri, and left medial superior gyrus), right anterior insula, cingulate cortex (left anterior and middle gyri, right middle gyrus), right thalamus/pulvinar, right caudate, right putamen, right parahippocampus, and left amygdala. In COPD, in some of these regions, regional GM volume had positive correlations with arterial blood po(2), while in some regions, regional GM volume had negative correlations with disease duration. Patients with COPD (versus controls) had poorer performance in the Mini-Mental State Examination, Visual Reproduction, and Figure Memory tests. Moreover, the GM volume in the inferior triangular frontal cortex in patients with COPD was significantly correlated with the Picture Memory score.

CONCLUSIONS

Our findings suggest GM reductions in a number of brain regions in COPD, which were associated with disease severity and may underlie the pathophysiologic and psychological changes in patients with COPD.

A case of superficial hemisensory dysfunction due to operculo-insular infarction: radiological depiction of thalamocortical projections to the secondary somatosensory cortex

A 64-year-old obese man developed hypesthesia in the left arm and leg. Neurological examination revealed decreased senses of pain, touch, and temperature in the left face, arm, trunk, and leg. Remaining functions were normal. Electrocardiogram showed atrial fibrillation. Somatosensory-evoked potentials using the stimulation in the median nerve were normal on both sides. Brain magnetic resonance imaging revealed acute infarction in the right parietal operculum and insula. There were no pathognomonic lesions in the postcentral gyrus, the thalamus, or the brain stem. Cardioembolic operculo-insular infarction was diagnosed. Diffusion tensor tractography map displayed the thalamocortical projections to the primary and the secondary somatosensory cortex (S2). These radiological findings supported that the operculo-insular lesion could disrupt the thalamo-S2 pathway. Thus, the thalamocortical disconnection between the thalamus to the S2 could cause superficial hemisensory dysfunction in the present patient.

Physical temperature effects on trust behavior: the role of insula

Trust lies at the heart of person perception and interpersonal decision making. In two studies, we investigated physical temperature as one factor that can influence human trust behavior, and the insula as a possible neural substrate. Participants briefly touched either a cold or warm pack, and then played an economic trust game. Those primed with cold invested less with an anonymous partner, revealing lesser interpersonal trust, as compared to those who touched a warm pack. In Study 2, we examined neural activity during trust-related processes after a temperature manipulation using functional magnetic resonance imaging. The left-anterior insular region activated more strongly than baseline only when the trust decision was preceded by touching a cold pack, and not a warm pack. In addition, greater activation within bilateral insula was identified during the decision phase followed by a cold manipulation, contrasted to warm. These results suggest that the insula may be a key shared neural substrate that mediates the influence of temperature on trust processes.

Operculo-insular pain (parasylvian pain): a distinct central pain syndrome

Central pain with dissociated thermoalgesic sensory loss is common in spinal and brainstem syndromes but not in cortical lesions. Out of a series of 270 patients investigated because of somatosensory abnormalities, we identified five subjects presenting with central pain and pure thermoalgesic sensory loss contralateral to cortical stroke. All of the patients had involvement of the posterior insula and inner parietal operculum. Lemniscal sensory modalities (position sense, graphaestesia, stereognosis) and somatosensory evoked potentials to non-noxious inputs were always preserved, while thermal and pain sensations were profoundly altered, and laser-evoked potentials to thermo-nocoiceptive stimuli were always abnormal. Central pain resulting from posterior parasylvian lesions appears to be a distinct entity that can be identified unambiguously on the basis of clinical, radiological and electrophysiological data. It presents with predominant or isolated deficits for pain and temperature sensations, and is paradoxically closer to pain syndromes from brainstem lesions affecting selectively the spinothalamic pathways than to those caused by focal lesions of the posterior thalamus. The term 'pseudo-thalamic' is therefore inappropriate to describe it, and we propose parasylvian or operculo-insular pain as appropriate labels. Parasylvian pain may be extremely difficult to treat; the magnitude of pain-temperature sensory disturbances may be prognostic for its development, hence the importance of early sensory assessment with quantitative methods.