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Somatosensory functions are subdivided into two large groups: the elementary somatosensory functions, which consist of light touch, pain, thermal sensation, joint position sense and vibration sense, and the intermediate somatosensory functions, which include two-point discrimination, tactile localisation, weight, texture and shape perception. It is generally accepted that the functional localisation of somatosensory function is the postcentral gyrus, but some researchers suggested that the insula is also involved in somatosensory function.1
Insula is a multimodal area and has a major role as a convergence zone implicated in the coordination between internal and external information through emotional subjective awareness. However, the subregional specificity of somatosensory functions in the insula has not been fully elucidated.2 We experienced three cases of stroke restricted exactly to the subregions of the insula which showed heterogeneous somatosensory dysfunction. For these patients, we performed a close examination of the intermediate somatosensory function, and discussed the relation between the insula and intermediate somatosensory function.
Patients and methods
An 81-year-old, right-handed woman who had difficulty speaking in February 2017 was transferred to our hospital. MRI revealed a lesion in the right middle insula (figure 1A). The symptom improved at the time of hospital arrival and she underwent the best medical treatment. The symptom disappeared on the next day. Intelligence and elementary somatosensory functions were normal. The results of the intermediate somatosensory examinations showed that two-point discrimination, tactile localisation and graphesthesia were significantly impaired.
An 80-year-old, right-handed man revealed weakness of the left upper and lower extremities in January 2014. MRI on the onset revealed a lesion in the right middle-to-posterior insular cortex (figure 1B). The symptom disappeared in the ambulance and he underwent the best medical treatment. Intelligence was normal. As for the elementary somatosensory functions, mild impairment was observed in pain sensation. The results of the intermediate somatosensory examinations showed that two-point discrimination, texture perception and graphesthesia were significantly impaired.
A 75-year-old, right-handed woman developed weakness of the left upper and lower extremities in September 2012. MRI revealed a lesion in the right posterior insular cortex (figure 1C). The symptom disappeared in the ambulance and she underwent the best medical treatment. Intelligence was normal and pain sensation was mildly impaired. The results of the intermediate somatosensory examinations showed that two-point discrimination, texture perception and tactile localisation were significantly impaired.
For intellectual ability, the Japanese Raven’s Colored Progressive Matrices was performed. Intermediate somatosensory functions were assessed according to the reported literature.3 For 2-point discrimination, the patients report the number of touched needles, ‘one’ or ‘two’. Forty-two trials were performed for each hand. For tactile localisation, the patients indicate the location of the point using the first finger of the same hand. Twenty-four trials were performed for each hand. For texture perception, the patients palpated two textures serially with their eyes closed. Then they were asked to do the ‘same-different’ discrimination. Twenty-five trials were performed for each hand. For tactile object discrimination, the patients palpated two daily necessaries of them serially with their eyes closed, and then they were asked whether these objects were the same. Twenty trials were performed for each hand. For tactile object naming, the patients were asked to name a single manipulated object. Twelve trials were performed for each hand. For graphesthesia, the patients report the letter that is written. Sixteen trials were performed for each hand. For statistical analysis, t-test was used, and the level of significance was set at p<0.05.
All patients did not have aphasia and their intelligence was normal. As for the elementary somatosensory functions, two patients had mild impairment in pain sensation, but other elementary somatosensory functions were normal. Our patients showed lesions restricted to the middle or posterior portion of the right insula, as well as impairment in intermediate somatosensory function. Impairment in graphesthesia was observed in a patient with a middle insula lesion, but not with a posterior insular lesion. Texture perception was impaired by a posterior insular lesion, but not by a middle insular one. Two-point discrimination was impaired through the lesion at the middle-to-posterior insula.
The present patients exhibited no aphasia and only limited abnormality in elementary somatosensory functions (mild impairment in pain sensation in two of three patients), and therefore enabled us to estimate meticulous localisation of neural function. Several previous studies have identified that the posterior insula had a relationship with somatosensory functions.4 However, few reports are available on the relationship with intermediate somatosensory function.5 One paper reported that an infarct in the right posterior insula showed the deficit of graphesthesia and stereognosis, but the patient also showed hypesthesia. They further reported that an infarct in the left posterior insula showed the deficit of two-point discrimination, graphesthesia and stereognosis, but the patient showed non-fluent aphasia. Another patient who had an ischaemic lesion in the left posterior insula showed mild deficit of graphesthesia and stereognosis, but the patient also showed impairment in elementary somatosensory function and transient fluent aphasia. In this way, the other overlapping symptoms might have some effect on the estimation of intermediate somatosensory function. Our patients had no aphasia and their intelligence was normal, and all patients had a lesion in the right middle-to-posterior insular cortex on diffusion-weighted image (DWI) in the acute phase, and follow-up MRI of the fluid attenuated inversion recovery images showed lesional distribution similar to the DWI lesions, thereby indicating structural damages. We suggest, through close evaluation of sensory function, that middle-to-posterior insula has a relationship with intermediate somatosensory function.
Our patients had a localised lesion which can be correlated with the types of intermediate somatosensory function. The subregional distribution of intermediate somatosensory function in the insula might be as follows (figure 1D): two-point discrimination is recognised in the middle and posterior insula. Graphesthesia is recognised in the middle insula, whereas texture perception is in the posterior insula. Therefore, we may say that the insula functions as the convergence zone of the intermediate somatosensory information.
The limitations of this study include its small sample size and the lack of patients with left insular lesion, so we do not know whether the same symptom occurs even by the left side lesion or it is specific to the right side. Further studies are needed to clarify the relationship between the insula and somatosensory function, especially in terms of the relationship between the subregional distribution of insular lesions and another intermediate somatosensory function.
Contributors MS: planning the study. KaM: acquisition of data, conducting and reporting the work. MS, KaM, KT, YU: interpretation of data and discussion of the results. MS and HT: revising the manuscript critically for important intellectual content. AI, HI, KoM, AS, MA, AN, KeM: the attending physicians of each patient, and indication the relationship with the patients. HT: approval of the version of the manuscript to be published. All authors approved the final version of the manuscript, and agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent Obtained.
Ethics approval Basic study of the diagnosis of dementia by visual, auditory, tactile stimuli: Study of brain mechanisms by fMRI.
Provenance and peer review Not commissioned; externally peer reviewed.
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