Okajimas Folia Anat. Jpn., 84(2): 49–60, August, 2007 Histology of the Human Carotid Sheath Revisited By Shogo HAYASHI Department of Anatomy, Tokyo Medical University, 6-1-1 Shinjuku, Shinjuku-ku, Tokyo 160-8402, Japan – Received for Publication, March 26, 2007 – Key Words: Carotid sheath, Cervical fasciae, Prevertebral lamina, Alar fascia Summary: Using semiserial sections, we histologically observed the carotid sheath and adjacent structures in 8 sides of 5 cadavers. For description, we classified the carotid sheath into 2 parts or laminae: 1) a laminar ‘‘adventitia’’ enclosing each of the cervical great vessels; and 2) a ‘‘common sheath’’ outside the adventitia. Arterial and venous adventitial structures sometimes fused and provided a definite septum between the artery and vein. Contrasting with previous de- scriptions, the common sheath did not fuse with superficial or prethracheal lamina of the cervical fasciae, but often fused with visceral fascia to provide a thick plate. The common sheath as well as the prevertebral lamina of the cervical fasciae sometimes became interrupted or unclear, but the adventitia was consistently complete circular. The alar fascia was usually considered as one layer of the multilaminar structure behind the cervical viscera, but it was difficult to identify as a single proper lamina. The carotid sheath was thus not a dissection artifact, but a definite histological structure. However, interindividual and/or site-dependent variations were evident in thicknesses of the adventitia and common sheath. Consequently, the author proposed a model of the fascial arrangement around cervical great vessels that unexpectedly differs from most descriptions in textbooks. According to textbooks of human anatomy cal or anatomical dissection. Range and Wood- (Spitzka, 1913; Schaffer, 1953; Hollinshead, 1982; burne (1964) also reported a fascial structure as a Ouchi, 1982; Clemente, 1985; Williams et al., 1995; dissection artifact. In this context, we believe that Standring, 2005), the carotid sheath, as a distinct histological observations are required to evaluate condensation of the deep cervical fasciae, com- the continuation of fascial structures, as gross dis- pletely encloses the common carotid artery, inter- sections easily reconstruct or produce a sheath-like nal jugular vein and vagus nerve. Parts of the structure along vessels and nerves (Kato et al., 2002; sheath are composed of the superficial and pre- Tamakawa et al., 2003; Kinugasa et al., 2006). Nev- tracheal laminae of the cervical fasciae, whereas the ertheless, little histological information is available sheath is believed to be separated from the pre- on the carotid sheath (Parsons, 1910). vertebral lamina of the cervical fasciae according to Furthermore, questions have been raised re- most authors. garding suggested site-dependent differences in However, Grodinsky and Holyoke (1938) noted carotid sheath arrangement (Hollinshead, 1982; that some groups have reported an incomplete Clemente, 1985). Do the superficial and pretracheal medial part and that, surprisingly, 2 or 3 research laminae of the cervical fasciae contribute to forma- groups have even denied the existence of a real tion of the carotid sheath depending on site or sheath. As a result, ‘‘strong or weak’’, ‘‘thick or supero-inferior level? However, few descriptions thin’’ and ‘‘tight or loose’’ cases of the carotid detail differences along the supero-inferior axis sheath may exist. In other words, whether the with the exception of a report by Grodinsky and sheath appears complete or incomplete may de- Holyoke (1938). Moreover, they described upper pend on interindividual variations. Moreover, a and mediastinal continuations of the carotid sheath histological study by Parsons (1910) concluded that on macroscopic observations, such as the carotid the carotid sheath is an artifact produced by surgi- sheath passing deep to the stylohyoideus and pos- Correspondence to: Shogo Hayashi, M.D., Department of Anatomy, Tokyo Medical University, 6-1-1 Shinjuku, Shinjuku-ku, Tokyo 160-8402, Japan. E-mail: [email protected] 49 50 S. Hayashi Histology of the Human Carotid Sheath Revisited 51 terior belly of the digastricus in the upper part of medial and lateral margins were trimmed. After the neck, fusing with the anteroinferior surface of a routine procedures for paraffin embedded histol- covering fascia of these muscles. However, inter- ogy, transverse or horizontal semiserial sections individual variations seemed to present in a site- (interval, 5 mm) were prepared (thickness, 10– dependent manner. 20 mm). Hematoxylin and eosin staining was The aim of this study was thus to histologically performed. examine: 1) interindividual differences, i.e., com- Protocols for the present research project did not plete or incomplete, thick or thin, tight or loose, include any specific issues that needed approval and which structures the sheath fuses with or at- from the Ethics Committee of the institutions. The taches to; and 2) site-dependent differences among present work conformed to the provisions of the 5 levels: a) level of the body of the hyoid bone; Declaration of Helsinki in 1995 (as revised in Ed- b) level of the oropharynx; c) level of the lar- inburgh in 2000). yngopharynx and thyroid gland (or, more strictly, level of the glotttis); d) level of the cricoid cartilage and uppermost trachea; and e) level of the sub- Results clavian artery. The carotid sheath, which encloses the cervical great vessels, was divided into 2 laminar structures: Materials and Methods 1) fibrous connective tissue around each of the common carotid artery and internal jugular vein; Cervical great vessels with adjacent structures and 2) a common sheath enclosing both the artery intact were obtained from 7 sides of 5 cadavers (2 and vein. The present study will apply the familiar male, 3 female) without macroscopic tumors (mean term ‘‘adventitia’’ to the former and the term age, 85 years; range, 76–95 years). These cadavers ‘‘common sheath’’ to the latter (Fig. 1). In the wider had been donated to Tokyo Medical University for meanings, this adventitia seems to be included in a education and research and had been treated post- concept of the carotid sheath because, in gross ob- mortem. The upper and lower ends of the materials servations, separation from the common sheath correspond to the base of the mandible and origin is difficult. The adventitia was consistently multi- of the subclavian artery, respectively. To preserve laminar and often >1 mm thick, but the common the prevertebral lamina of the cervical fasciae in sheath was thin and often identified as a single histology, close attention was paid to including the lamina despite being composed of bundles of fi- scalenus anterior and longus colli muscles into the brous tissues at higher magnification (Fig. 1B). The materials. After decalcification of the laryngeal vagus nerve was consistently located outside of the cartilages and hyoid bone using 5% nitric acid so- adventitia (in detail, see below), but the nerve was lution, materials were horizontally subdivided into usually enclosed in a common sheath. 5–6 pieces along the supero-inferior axis and the Most of the present figures show 2–3 levels of Common abbreviation for figures C3–C5, 3rd–5th cervical nerve roots; AC, ansa cervicalis; CC, common carotid artery; CT, cricothyroideus; DG, digastricus (an- terior or posterior belly); EC, external carotid artery; ES, esophagus; FA, facial artery; HB, hyoid bone; IC, internal carotid artery; IJ, internal jugular vein: LC, longus colli; LX, larynx; LN, cervical lymph nodes; MH, mylohyoideus; OC, oral cavity; OH, omohyoideus; PC, pharyngeal constrictor; PX, pharynx; SA, scalenus anterior; SC, subclavian artery; SH, sternohyoideus; SG, submandibular gland; SM, sternocleidomastoideus; ST, sternothyroideus; SY, sympathetic trunk; TC, thyroid cartilage; TH, thyroid gland; TR, trachea; VN, vagus nerve. Fig. 1. Well-developed carotid sheath and superior continuation. Specimen 57 right. Panel A displays the upper level crossing the hyoid bone (HB), while panel B exhibits the middle level in- cluding the thyroid cartilage (TC) and thyroid gland (TG). Magnifications of these panels are of the same. Panel C (D) is a higher magnification view of the square including C (D) in panel A (C and D were taken at the same magnification). Arrows indicate the prevertebral lamina of the cervical fasciae. In panel B, the visceral fascia is thick (white triangles) and fuses with the common sheath of the great vessels. Covering fasciae of the sternocleidomastoideus (SM) and omohyoideus (OM) are located close to the common sheath of the great vessels, but these fasciae are separated from the common sheath. Panel C displays a retrovisceral multilaminar structure. The lingual branch of the glossopharyngeal nerve (GN) passes between the superior and middle pharyngeal constrictors (PC). The ansa cervicalis (AC) is located outside of a common sheath (triangles) of the carotid sheath in this panel. Panel D demonstrates that the adventitia (double-headed arrows with AD) of the artery and vein is clearly discriminated from a common sheath (triangles) of the carotid sheath. The vagus nerve (VN) is inside the common sheath. PH, phrenic nerve. 52 S. Hayashi abundant semiserial slices of the long cervical re- made a thick part (specimen 57 right in Fig. 1B; gion, i.e., level of the hyoid bone body or the upper specimen 56 right in Fig. 5B). level; level of the larynx and thyroid gland or the A multilaminar fascial structure