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Morphological and morphometric study of insular cortex in human cadaveric brains

Introduction and Aim: The insular cortex is hidden 5 lobe of brain with variable gross anatomical patterns. Insula is situated in operculoinsular compartment of sylvian cistern. Materials and Methods: 58 human formalin fixed cerebral hemispheres were studied for the gyri and sulci variations of insular cortex and their lengths were recorded using digital vernier caliper. This study was done in department of Anatomy, Gandhi medical college Bhopal. Result: The morphological normal and variable pattern, discontinuous sulci and joined gyri were observed. Out of the (85/406) 21% variable pattern we observed (44/406) 10.3% Bifid, (31/406) 7.63%, Hypoplastic Gyri and (10/406) 2.46%Trifid. The Bifid pattern was most commonly seen in Anterior Short Gyrus18 /44 (41%). The Hypoplastic pattern was most commonly seen in Middle Short Gyri 18/31 (58%). There was presence of Transverse Gyri in 13/58 (22.41%) and Accessory gyri in 39/58 (65.51%) cerebral hemispheres. Transverse gyrus & Accessory gyrus both were absent in 19/58 (32.76%) cerebral hemispheres in insular cortex. Total number of gyri in every insula and Mean lengths of gyri were Anterior short=24.54mm, Middle short=25.04, Posterior short=34.08mm, Anterior long=36.87mm, Posterior long=37.1mm measured. The Mean lengths of all insular sulci were Central Insular=35.3mm, Superior Periinsular=52.48mm, Anterior Periinsular=24.5mm, Inferior Periinsular=26.4mm, Posterior Periinsular=12.81mm measured. Conclusion: Variation in morphological pattern of gyri are Bifid, Trifid and Hypoplastic. The Transverse and Accessory gyri were present in anterior lobule, but not observed in each insula. There was variation in number of gyri because of presence of transverse and accessory gyrus. Total number of gyri are more in anterior lobule so it is larger than Posterior lobule. The gyri of anterior lobule are shorter than gyri of Posterior lobule. Such morphological features and variations are important for surgeries.


Introduction
Insular cortex is called hidden 5 th lobe of brain with paralimbic and somatosensory, viscerosensory functions. 1 Pathology in this integrated cortical area results in many disorders in body. 2 So anatomical basis of these disorders are very important in early diagnosis and treatment. Trans-sylvian and trans-insular approach are very common in neurosugeries, it demands detailed knowledge of normal gross anatomy and variations to reduce post-operative complications. 3 Insular cortex was considered to be asymmetric pyramid or triangle with three peripheral sulci or periinsular sulci or limiting sulci (anterior, superior and inferior). 4 However, later the radiological and cadaveric studies conducted on insula described it to have a trapezoid structure. The posterior periinsular sulcus was differentiated from inferior periinsular sulcus and these two sulci have two different axis separated by an angle. 5 The Insula is a trapezoid shaped cortical structure with its apex directed anteriorly and inferiorly toward the limen insulae. The limen is located at the junction of sphenoidal and operculoinsular compartments of the sylvian fissure. 5,6 There are four periinsular sulci, cited superiorly and posteriorly in a radial manner from the apex at the limen insulae. 5,6 The central insular sulcus, the main and the deepest sulcus of the insula, courses obliquely across the insula, and extends uninterrupted from the limen insula to the superior peripheral sulcus. It divides the insula into two unequal zones: the larger anterior insula, and the smaller posterior insula. 7 The larger anterior lobule of insula is composed of three principal short insular gyri (anterior, middle and posterior). Sometimes accessory and transverse gyri are also present. The anterior, middle and posterior short insular gyri are separated by the short insular sulcus and the pre-central insular sulcus respectively. 8 Precentral gyrus of insula is posterior short gyrus term used by Nagao M. 9 The Anterior Long Gyrus and Posterior Long Gyrus form the smaller posterior lobule and are separated by the post-central insular sulcus. 8

Materials and Methods
This study was done in 29 formalin fixed brains (58 right and left cerebral hemispheres) with intact arachnoid matter, all segments of sylvian fissure, gyri and sulci of insular cortex. The study was approved by Scientific Review and Institutional Ethics Committee, Gandhi Medical College, Bhopal (M.P.).
The formalin fixed brains satisfying inclusion criteria were dissected and studied. In order to expose and visualize the insular region, the arachnoid and piamater over the sylvian fissure and middle cerebral artery branches in specially operculoinsular compartment were removed. Following standard dissecting procedure, structures under study were observed and recorded. Morphometry was done with the help of Digital Vernier caliper (sensitivity 0.1mm). Statistical analysis of results and comparisons among right and left hemispheres was done and Mean, SD, p value, Unpaired T test, Degree of freedom, SED, and r value Pearson correlation coefficient were analyzed and recorded. Inclusion criteria: Undamaged specimens of both right and left sided cerebral hemispheres cadaveric brains with intact Insula and sylvian fissure, middle cerebral artery branches of this particular area of concern. Exclusion criteria: Damaged brain specimens. Infant and Pediatric age group cadaveric brains. Morphology: The morphological normal and variable patterns as Hypoplastic, Bifid, Trifid gyri, discontinuous sulcus and some joined gyri were observed. Total number of gyri in every insula were also recorded. There was variation in number of gyri because of presence of transverse and accessory gyrus.

Transverse Gyri and Accessory Gyri
In our study total 58 cerebral hemispheres, were observed, out of which there was presence of Transverse Gyri in 13/58 ( 22.41%) cerebral hemispheres and Accessory gyri in 39/58(65.51%) cerebral hemispheres. The transverse and accessory gyri are present in anterior lobule of insular cortex. Transverse gyrus & Accessory gyrus both were absent in 19/58 (32.76%) cerebral hemispheres in insular cortex & therefore a total no of 5 gyri were found in them. Presence of Accessory gyri but absence of Transverse gyrus was observed in 26/58 (44.82 %) cerebral hemispheres in insular cortex & therefore total no of 6 gyri were found in these hemispheres. Our observation is similar to Thomas et al.

Peripheral sulci -Trapezoid Shape Insula
Radiological and cadaveric study conducted by Kaneko et al (2012) and cadaveric study conducted by Afif et al (2011) and Jeremy jones (2016) described insula as a trapezoid structure. They differentiated posterior periinsular sulcus from inferior periinsular sulcus and stated that the 2 sulci have two different axis separated by an angle. 5,6,16 Our study finding concides with presence of 4 periinsular sulci (anterior, superior, inferior, posterior) clarify trapezoid shape of insular cortex.