An Anatomical Study of the Suboccipital musculature and its Clinical Implications

 

Mamatha Hosapatna¹, Aamna Kausar², Girish Menon R³,  Rohini Punja¹*

¹Department of Anatomy, Kasturba Medical College, Manipal, Manipal Academy of Higher Education,

Manipal, Karnataka, India – 576104.

²Postgraduate Student, Department of Anatomy, Kasturba Medical College, Manipal,

Manipal Academy of Higher Education, Manipal, Karnataka, India – 576104.

³Department of Neurosurgery, Kasturba Medical College, Manipal, Manipal Academy of Higher Education, Manipal, Karnataka, India – 576104.

 

ABSTRACT:

Background: Muscles of suboccipital region play an important role in the movements of atlanto occipital and atlanto axial joints. Tension type of headache could be an indication of referred pain from the muscles of the suboccipital region. There are no cadaveric studies which have been done to determine the morphometry of the muscles of this region. This study aims to highlight these values along- with its clinical implications Methods: The study was conducted on 10 cadavers. The morphological variations and morphometry of the suboccipital muscles was determined.  Total length, thickness and width of the muscles at its attachments were measured. Results: The thickness of rectus capitis posterior major and minor was more on the left side. Obliquus capitis superior was longer on right side and obliquus capitis inferior width was more on the left side. The area of the suboccipital triangle was 1580.92 mm² and 1504.05mm² on right and left side respectively. Conclusion: Unlike the previous studies which were done mostly on the morphological variations of the muscles of SOR this study puts emphasis on the morphometric parameters. It provides reference measurements for surgeons during craniovertebral surgeries and researchers for future studies.

 

 

 

INTRODUCTION: 

The suboccipital region extends between inferior nuchal line of occipital bone and the laminae of C2. It is limited on either side by mastoid process and traverse processes of atlas and axis. The muscles of the suboccipital region (SOR) are obliquus capitis superior (OCS), obliquus capitis inferior (OCI), rectus capitis major (RCMa) and rectus capitis minor (RCMi). These muscles play a significant role in the movements of atlanto occipital and atlanto axial joints¹.

 

It has been put forth by authors that tension type of headache could be an indication of referred pain from the muscles of the SOR^2,3.

 

 

Grgic⁴ has postulated that chronic headaches are commonly of cervical origin in considerable number of patients and this has been linked to the cross sectional areas of RCMa and RCMi^4,5. An increase in duration, severity and frequency of such headaches have been linked to a reduction in cross-sectional area due to atrophy of  muscles of the SOR³. Hypertrophied RCPminor muscles can simulate this tension and therefore manifest similar symptoms⁶.

 

Surgical approach to the SOR is challenging during postero-lateral and posterior cranial fossa skull base surgeries as the 3rd part of vertebral artery is an important content and it demonstrates multiple loops in this region^7,8. Thus the morphology and morphometry of the SOR muscles have a significant role in identifying and defining the boundaries of the suboccipital triangle and avoiding damage to the vertebral artery⁹. With the advancements in microsurgery surgeons operating in the SOR need to have a thorough knowledge of the anatomy of this region to preserve the vertebral artery¹⁰.

 

There are no cadaveric studies which have been done to determine the morphometry of the muscles of the SOR, this study aims to highlight these values along- with its clinical implications

 

MATERIAL AND METHODS:

The study was conducted on 10 embalmed male cadavers (20 sides) in the Department of Anatomy after obtaining Institutional ethical clearance (IEC674-2021)

 

The cadavers were placed in the prone position and the dissection for exposing the suboccipital muscles were carried on. A vertical median incision through the skin from external occipital protuberance to the 7th cervical spine and a horizontal incision laterally extending to the acromial process. The skin flap was reflected along with the trapezius separating it from the superior nuchal line. Detatch semispinalis capitis from the occipital bone then expose all the suboccipital muscles namely the RCPMa, RCPMi, OCS and OCI. The morphological variations and morphomety of the suboccipital muscles forming the SOR was determined. The following parameters were measured (Figure 1 and 2) using a digital Vernier caliper and thread:

1)   Total length of the SOR muscles along the angle bisector (L1, L2, L3, L4)

2)   Width of the SOR muscles at its mid point (W1, W2, W3,W4)

3)   Width of the SOR muscles at its attachments (Wa1, Wb1; Wa2, Wb2; Wa3, Wb3; Wa4, Wb4)

4)   Thickness of the SOR muscles at its mid point

5)   Area of the SOT; A= 1/2* B*H

6)   Angle at the vertices (a1, a2, a3)

 

The mean and standard deviation were tabulated for the above morphometric parameters.

 

 

Figure 1: Illustration demonstrating the parameters of the SOR muscles measured. [L1- length of RCPMa; L2- length of RCPMi; L3- length of OCS; L4- length of OCI; W1- width of RCPMa at its mid point, W2- width of RCPMi at its mid point , W3- width of OCS at its mid point , W4- width of OCI at its mid point; Wa2- width of RCPMi at its origin, Wb2- width of RCPMa at its insertion]

 

 

Figure 2: Illustration demonstrating the parameters measured for calculating the area of the SOT and the angles formed between these muscles.  

 

[H- height of the SOT, B- base of the SOT; a1- angle formed between RCPMa and OCI; a2- angle formed between OCS and OCI; a3- angle formed between RCPMa and OCS; Wa1- width of RCPMa at its origin, Wb1- width of RCPMa at its insertion; Wa3- width of OCS at its origin, Wb3- width of OCS at its insertion; Wa4- width of OCI at its origin, Wb4- width of OCI at its insertion]

 

RESULTS:

The various measurements of the muscles of the SOR have been recorded and the mean and standard deviation of these values have been tabulated in Table 1. It was observed that the thickness of rectus capitis posterior major and minor was more on the left side.  While obliquus capitis superior was longer on right side and obliquus capitis inferior width was more on the left side The area of the SOT on the right side is 1580.92 mm2 and on the left side is 1504.05 mm2. The angle formed between RCPMa and OCI (a1) is 32.9o on the right and 27o on the left side. The angle formed between OCS and OCI (a2) is 64.1o on right side and 56.6o on left side. 81.9o and 90.44o are the angles formed between RCPMa and OCS on the right and left side respectively. (Fig 2). There were no morphological variations observed in the muscles of the SOR.

 

Table 1: Morphometric parameters of the SOR muscles

SOR Muscles		Length (L) mm	Width at Mid-length (W) mm	Width at Origin (Wa) mm	Width at Insertion (Wb) mm	Thickness at midpoint (T) mm
RCPMa	R	53.7±5.98	13.8±4.20	5.69±2.04	23.7±4.82	6.5±4.23
	L	55.5±5.11	13.39±4.48	5.89±2.07	23.72±5.02	9.8±6.68
RCPMi	R	34.75±3.95	12.5±2.38	5.6±1.72	21.4±3.71	2.85±0.85
	L	36.44±3.78	13.11±2.01	6.2±1.68	24.2±3.47	3.22±0.71
OCS	R	62.1±7.54	13.2±4.60	8.2±2.54	21.1±1.82	10.8±3.27
	L	58.89±8.42	12.89±5.57	8.7±3.22	23.05±3.84	9.72±3.67
OCI	R	58.2±5.08	14.55±4.76	9.4±3.65	13.5±4.82	12.3±4.32
	L	57.67±5.16	15.33±4.51	9.22±5.05	13.61±6.00	13.55±4.79

 

DISCUSSION:

The present study was conducted to evaluate the morphometry and morphology of the muscles of the SOR on embalmed cadavers. The morphometric parameters of these muscles have not been studied previously however morphological variations have been listed out in various articles.

 

Qiang Xu et al in his cadaveric study of 10 specimens measured the RCPMi and obtained the following pararmeters. Length of RCPMi along the medial and lateral margins was 27.0±5.0mm long and 24.8± 6.8mm long respectively¹¹. Length of the RCPMi along the angle bisector was 27.8±5.0mm¹¹. In our study, length of RCPMi along the angle bisector was 34.75±3.95 on the right side and 36.44±3.78 on the left side. He also noted the width of the RCPMi at its origin (Wa) and insertion (Wb) of the RCPMi were 7.9±1.4 and 27.2±4.6 respectively¹¹. These values were close to the values measured in this study however we have documented these values separately for the right and left sides of the same cadaver and it has been observed that the width at the origin and insertion of this muscle is higher on the left side compared to the right side. MR imaging studies reported by Qiang Xu et al had the following values as length of the RCPMi,  26.2±3.3mm- in males and 23.1±2.6mm in females¹¹.

 

It has been reported that atrophy and hypertrophy of the muscles of the SOR may be closely related to chronic cervical headaches. Clinical studies found that chronic cervical headaches could be a result of suboccipital muscle pathology, such as atrophy or swelling of the RCPMi^3,12,13. Hypertrophy of the nuchal muscles, with accompanying headache, has been reported in the literature¹⁴.  Outcome of Whiplash injuries may be worsened due to atrophy of the muscles of the SOR and have been associated with higher inflammatory biomarkers and hyperalgesia^15,16,17. Treatment of tension-type headaches after craniocerebral trauma could be corelated with the atrophy of RCPMi with post-traumatic complaints¹⁵.

 

The present study is unique in capturing the metric parameters of the RCPMa, OCS and OCI in addition to RCPMi which has not been done previously.

 

Suboccipital triangle (SOT) forms an anatomical landmark for the identification of third part of vertebral artery. Thus, we have calculated the area of the SOT and the values obtained were higher on the right side compared to the left side which we could justify by the atrophy of the muscles of right side. The SOT contains the vertebral artery (third part), the dorsal ramus of first cervical nerve and suboccipital venous plexus¹⁷. The angles formed between the muscles forming the boundaries of the SOT were also measured in this study.

 

The muscles forming the boundaries of SOT play a crucial role during skull base surgical procedures to approach the posterior cranial fossa¹⁸.

 

During the course of this study we did not find any morphological variations, but there are various articles reporting variations in the SOR muscles. S R Nayak’s case report on bilateral double RCPMa muscle and bilateral absence of RCPMi muscle hypothetised that additional RCPMa may put more strain to the spine of the axis and bilateral absence of RCPMi muscles would result in muscular incoordination in the SOR while balancing the head¹⁹.

 

Few authors have established a soft- tissue bridge between the RCPMi and the dura mater^20,21. Similar observations have been made in the present study and has a significant clinical relaevance. This soft-tissue bridge has been suggested to preserve and sustain the integrity of the subarachnoid space during movements of the neck. Any changes in the dimensions of the RCPMi may alter the dural tension via the soft-tissue bridge and could be a cause of tension headache due to this connection^16,22,23.

 

In a cadaveric study by Scali et al broad fascial connections between the RCPMa and the dura was observed and a physical pull of the RCPMa brought about movement of the dura from the spinal root level of the axis to the spinal root level of the first thoracic vertebra²⁴. A study conducted on the posterior intervertebral spaces of the craniovertebral joint by Kahn et al noted fascial communications between dura and OCI, RCPMi and RCPMa²⁵.

 

This cadaveric study is the first to provide comprehensive data regarding the morphometric parameters of the muscles of the SOR. The knowledge would assist in safe course during surgical procedures of this region as well as during therapeutic considerations for various dysfunctions. A few limitations of this study would be an inadequate sample size which we however justified by calculating the standard deviation and probably an absence of three-dimensional measurements which could probably explore these muscles thoroughly.

 

CONCLUSION:

Our study explored the muscles of the SOR which could significantly aid in identifying the SOT for exploring the vertebral artery as well as posterior cranial fossa surgeries via the skull base. Unlike the previous studies which were done mostly on the morphological variations of the muscles of SOR this study puts emphasis on the morphometric parameters. It provides reference measurements for surgeons during craniovertebral surgeries and researchers for future studies.

 

SOURCE OF FUNDING:

This research did not receive any specific grant from funding agencies in the public, commercial, or not for- profit sectors.

 

CONFLICT OF INTEREST:

All the authors declare that they have no conflict of interest.

 

ETHICAL APPROVAL:

This study was approved by the Institutional Ethics committee with reference number IEC: 674/2021, dated 15th September 2021.

 

AUTHORS CONTRIBUTIONS:

RP: Conceptualization, Methodology, Reviewing and Editing. AK: Data curation, Investigation, Writing- Original draft preparation. MH and GM: Supervision, Validation, Reviewing and Editing. All authors have critically reviewed and approved the final draft and are responsible for the content and similarity index of the manuscript.

 

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Accepted on 13.12.2022           © RJPT All right reserved

Research J. Pharm. and Tech 2023; 16(6):2975-2978.