|Year : 2021 | Volume
| Issue : 2 | Page : 81-84
Omega sign: An indicator of motor hand area on cerebral hemisphere
Shilpa Nandkishor Gosavi, Rajendra Somnath Garud
Department of Anatomy, Bharati Vidyapeeth (DTU) Medical College, Pune, Maharashtra, India
|Date of Submission||05-Aug-2020|
|Date of Acceptance||28-Apr-2021|
|Date of Web Publication||30-Jun-2021|
Dr. Shilpa Nandkishor Gosavi
Department of Anatomy, Bharati Vidyapeeth (DTU) Medical College, Pune, Maharashtra
Source of Support: None, Conflict of Interest: None
Introduction: With advances in the field of technology microneurosurgery is performed with the use of transcisternal, transfissural, and transsulcul approaches, where sulci present on brain are used as fundamental landmarks. Detailed knowledge of various cerebral sulci and gyri is also essential for neuroimaging techniques. Often neurosurgeons need to work in the central lobe to approach the cortical or subcortical lesions. The aim of the study was to identify and locate omega sign on the precentral gyrus as an indicator of hand area and to provide anatomical basis for the surgical landmark on the cortical surface. Material and Methods: Fifty-five cerebral hemispheres were studied in the Department of Anatomy. On the superolateral surface, the central sulcus, pre- and post-central gyri were identified. On the precentral gyrus, the presence of omega sign was observed. When present the height of the omega sign, width at the base, its distance from superior and inferior Rolandic point was noted. Data collected were statistically analyzed using SPSS version 25.0 software. Results: We observed the presence of omega sign in 26 hemispheres (47.27%). The average height of omega was 9.31 ± 2.94 mm, average width at base was 16.03 ± 3.34 mm. Distance from the superior Rolandic point was 27.53 ± 7.05 mm, while from inferior Rolandic point, it was 52.55 ± 7.8 mm. Discussion and Conclusions: Although technology offers modern intraoperative localization tools such as MRI and neuronavigation, anatomical knowledge is important for the surgical planning.
Keywords: Cerebrum, motor hand area, omega sign, sulci and gyri
|How to cite this article:|
Gosavi SN, Garud RS. Omega sign: An indicator of motor hand area on cerebral hemisphere. J Anat Soc India 2021;70:81-4
| Introduction|| |
With advances in the field of technology, microneurosurgery is performed with the use of transcisternal, transfissural, and transsulcul approaches. For which the sulci present on the surface of brain are used as fundamental landmarks. The detailed knowledge about the shape, position, and form of various cerebral sulci and gyri is essential for neuroimaging techniques. They also serve as a guide to a particular functional area during the surgery. Many times the neurosurgeons need to work in the central lobe to approach the cortical or subcortical lesions. Even though modern technology like magnetic resonance imaging (MRI) is available, detailed knowledge about the anatomy of sulci and gyri is important to plan a surgery.
On the superolateral surface of cerebrum, the region formed by precentral gyrus and postcentral gyrus is known as central lobule. It is bounded anteriorly by precentral sulcus and posteriorly by postcentral sulcus. Inferiorly, it is bounded by lateral sulcus and superiorly by the upper margin of the cerebrum.
Various methods have been used to identify the motor hand area on precentral gyrus by various authors to help the neurosurgeons. When observed on the surface, the motor hand area is present where the superior frontal sulcus meets the precentral sulcus (at the same sagittal plane on the precentral gyrus). In another method, the precentral knob can be localized in the form of the Greek letter inverted omega in the axial plane. Direct electrical stimulation and the functional MRI (fMRI) studies have confirmed the presence of the motor hand area in the superior part of the precentral gyrus.
Although variation in anatomy of sulci and gyri is observed commonly, it is a known fact that anatomically constant sulci are topographically related to more specialized areas. During surgery, due to the presence of arachnoid mater, it may sometimes be difficult for a surgeon to understand the anatomy of sulci and gyri. The variability of these sulci is well known. Therefore, neurosurgeons need multiple methods, such as identification of landmarks, some morphological and morphometric methods, to identify the sulci and gyri in this region. Although various techniques have been used including intrasurgical electrical stimulation for the identification of central region structures, researchers agree that additional landmarks for mapping of the motor cortex are always useful.
With newer microsurgical techniques, and due to advances in the field of neuroradiological and neuroanesthesia in addition to good monitoring techniques during and after surgeries, many of the lesions located in the central sulcus region can be safely approached surgically and cured if the removal is based on anatomical data and refined microsurgical technique. When the normal cerebral anatomy is distorted, identification of structural landmarks such as omega sign is important, with the help of which the central sulcus can be identified through MRI imaging. Many scientists have described a hook-shaped hand area present in upper part of precentral gyrus.,,
| Material and Methods|| |
After performing the craniotomy, dura mater was removed, and the brain was removed carefully. It was fixed in 10% formalin. Coverings of the brain and blood vessel were removed carefully. Fifty-five cerebral hemispheres (25 complete brain and five half brain) (right – 28, left – 27) were studied in the Department of Anatomy.
On the superolateral surface, the central sulcus, pre- and post-central gyri were identified. On the precentral gyrus, the presence of knob-like structure (omega sign) was observed in each the hemisphere [Figure 1]. If present, the height of the omega sign was measured form its base to the apex. Width at the base was measured as the maximum distance at the base of the knob [Figure 2]. Distance of the omega from the medial longitudinal fissure (Superior Rolandic point) and from the lateral sulcus (Inferior Rolandic point) was noted [Figure 3]. Data collected were statistically analyzed using SPSS version 25.0 software (C-Dot System Pvt. Ltd, Pune, India).
|Figure 3: Distance of omega sign from other landmarks. C (Yellow line) – Distance between superior rolandic point and omega sign. D (Blue line) – Distance between inferior rolandic point and omega sign|
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| Results|| |
After studying 55 cerebral hemispheres, we observed the presence of omega sign in 26 hemispheres. (47.27%) (right – 12 [42.85%], left– 14 [51.85%]). Bilateral presence was noted in seven brains out of 25 complete brains observed.
The average height of omega was 9.31 ± 2.94 mm (range: 3.3–13.93 mm), the average width at the base was 16.03 ± 3.34 mm (range: 11.1–23.44 mm). Its distance from the superior rolandic point was 27.53 ± 7.05 mm (range: 17.87–39.23 mm), while from inferior Rolandic point was 52.55 ± 7.8 mm (range: 32.63–65.84 mm) [Table 1].
| Discussion|| |
On the superolateral surface of cerebrum, central sulcus is observed as one of the most important and consistent landmarks. As it descends down, it separates motor and sensory areas. It also forms the boundary between frontal and parietal lobes. Traditionally, three different curves or genua have been described along its course giving it an “S-”shaped appearance. The superior and inferior genua show convexity forward while the middle one, the deepest curve shows concavity forward. Thus, the middle curve resembles an inverted Greek letter omega.
During 5th and 6th month of intrauterine life, the central sulcus appear in two parts, upper and lower, which usually coalesce shortly afterward, although they may remain discontinuous. Alkadhi and Kollias. suggested that, as the two sulci fuse with each other, the intervening eminence gets undermined (borne down) into the sulcus. Thus, the intervening portion is never entirely obliterated but can be discerned in the bottom of the sulcus. This is the pli de passage fronto-parie tal moyen (PPFM) described by Broca or deep annectant gyrus described by Cunnigham.
Hopkins et al. in their study of evolution of central sulcus morphology in primates have shown that the surface area, shape, and folding pattern of the central sulcus changed during Old World anthropoid primate evolution. This probably reflects the increasing importance of somatosensory and motor integration of hand functions. They also observed that, as brain size increased, folding in the central sulcus had to accommodate the increasing size of the PPFM, leading to the anatomical formation of the motor-hand area or Knob, seen on the cortical surface of the central sulcus.
In their study of fMRI, Yousry et al. defined the motor hand area in the axial plane as a knob-like, broad-based, posterolaterally directed structure of the precentral gyrus, which was inverted omega shape and sometimes a horizontal epsilon shape. They observed sites of signal intensity changes located in the precentral gyrus in 11 hemispheres out of 28 they studied (39.29%). With MRI, they detected the knob-like structure was in all 59 hemispheres examined in the axial plane. Caulo et al. in their MRI study observed omega sign present in 78.2%.
The knob on the precentral gyrus was mainly formed by two sulci perpendicular to the central sulcus. These sulci were more prominent at deeper levels of the central sulcus and become smooth or even disappear on the cortex. Due to this sulcal configuration, Rodrigues et al. observed that, on direct cortical inspection sometimes, it was difficult to identify the presence of the knob on the precentral gyrus. Thus, they also noted the difficulty in finding the knob even if it was obvious on axial imaging (showing the omega sign).
In their cadaveric study, it was found that the knob on the precentral gyrus, which represents the motor hand area, was present in 64 out of 82 hemispheres (78.05%). While in our study of cadaveric cerebral hemispheres, we observed it in 47.27% of the hemispheres studied.
After studying ten cadaveric cerebral hemispheres, Campero et al. noted that the average height of omega was 11.2 ± 3.35 mm and width at the base was 18.7 ± 2.49 mm. In the present study, the average height of omega was 9.31 ± 2.98 mm (range: 3.3–13.93 mm), the average width at the base was 16.03 ± 3.34 mm. Rodrigues et al. noted average height of omega as 6.49 mm and average width as 17.66 mm. As majority of the individuals are right handed, height and width at the base of omega on the left side were expected to be larger than the right, but in the present study, the difference was statistically not significant. It might surface with a study with larger sample size.
The point where the central sulcus meets the interhemispheric fissure (superior longitudinal fissure) is known as superior Rolandic point. It lies about 5 cm behind the bregma, which is approximately 12 cm posterior to the nasion. The meeting point of the central sulcus with the Sylvian fissure is known as inferior Rolandic point, real, or projected.
Campero et al. observed the average distance between the medial edge of the hemisphere and the medial limit of the omega as 24.5 ± 5.35 mm. Rodrigues et al. noted it as 20.617 mm and 18.633 mm on the right and left hemisphere, respectively. While in the present study, it was 28.53 mm ion the right side and 26.55 mm on the left side.
In a cadaveric study of 82 hemispheres by Rodrigues et al., the average distance of the knob on the precentral gyrus to the posterior ramus of the lateral sulcus was observed as 41.529 mm and 41.166 mm on the right and left hemisphere, respectively. In the present study, it was 52.13 mm and 52.98 mm, respectively. The difference can be attributed to the study population being different.
In recent years, different intraoperative monitoring methods have been often used. While operating on central sulcus lesions generally, direct cortical stimulation, phase reversal technique, and subcortical stimulation are used. Rodrigues et al. suggested a combination method of anatomical landmarks along with morphometric measures to improve the accuracy along with the multimodality.
Rodrigues et al. observed that it was difficult to find the knob present on the precentral gyrus even if it was obvious on axial imaging showing the omega sign. They therefore highlighted the importance of the morphometric data, various distances from the fixed points such as superior and inferior Rolandic points. Yousry et al. also suggested the use of morphometric data for localization of the hand area intraoperatively.
| Conclusion|| |
Omega sign was observed on the precentral gyrus in 49.56% of cerebral hemispheres studied in the present study. Bilateral presence was observed in 28%. The morphometric data from the cadaveric study will be useful as an additional tool for locating the knob on the precentral gyrus for neurosurgeons. Variations in the gyral pattern are of interest for anatomists too.
We are thankful to Mrs. Vandana Tendolkar for her help.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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