Clinical Anatomy 9:291-295 (1996)

ORIGINAL COMMUNICATIONS

The Dimensions and Vascular Anatomy of the Cricothyroid Membrane:

Relevance to Emergent Surgical Airway Access KEVIN DOVER, THOMAS R. HOWIESHELL, mi) GENE L. COLBORN

Deportmentr of Anatomy und Surgery, hledirii/ CoDegf of GeorgiSln, Augusta

Following traumatic injury, rapid surgical access to the airway may be required, with surgical cricothyroidostomy the procedure of choice. Immediate complications of cricothyroidostomy include unsuccessful or incorrect site of tube placement and hemorrhage. Subglottic stenosis is the most common late complication. This project was undertaken to better define the di- mensions and vasculature of the cricothyroid region. In 15 cadaveric specimens, cervical dis- section revealed the average width of the cricothyroid membrane visible between cricothyroid muscles to be 8.2 mm, and the average height 10.4 mm. Latex injection of the carotid artery demonstrated a transverse cricothyroid artery arising from the superior thyroid artery in 93% of cases. The cricothyroid artery crossed the upper one-half of the cricothyroid membrane in all but one specimen. Branches of the cricothyroid artery penetrated the membrane and as- cended along the undersurface of the thyroid cartilage. Unilateral superior thyroid artery in- jection demonstrated anastomoses between right and left cricothyroid arteries. In 54% of spec- imens, the superior thyroid artery coursed anterior to the sternothyroid muscle and then the lateral edge of the cricothyroid membrane. The membrane was also crossed by venous tribu- taries to the superior and inferior thyroid veins. To lessen the possibility of complications fol- lowing cricothyroidostomy, a knowledge of the dimensions, relations, and vasculature of the cricothyroid membrane is of inestimable value.

Key words: cricothyroidostomy, airway, cricothyroid membrane, superior thy-

o 1996 Wiley-Liss, h c .

roid artery

INTRODUCTION Orotracheal intubation via direct laryngoscopy is the

preferred method of initiating general anesthesia for surgical procedures and to establish an airway in non- traumatic cardiopulmonary resuscitation (Ampel et al., 1988). T h e advantages of oral intubation are that it can be performed rapidly under direct vision and accom- plished both in awake and apneic patients. In per- forming airway maneuvers after acute trauma, how- ever, one must always consider the possibility of injury to the cervical spine (Bucholz et al., 1979). This can be an occult dislocation, or an unstable fracture that has yet to produce injury to the spinal cord, or an incom- plete spinal cord lesion that can be aggravated by un- controlled manipulation of the neck. Oral intubation requires some degree of cervical hyperextension for glottic visualization and is to be avoided in victims of blunt trauma until the cervical spine has been evalu- ated. This route can also be difficult or contraindicated in patients with maxillofacial and mandibular fractures,

laryngeal fracture, or thermal burn with inhalation in- jury and associated laryngeal edema.

Nasotracheal intubation, popularized by Magill (1930), is a blind technique directed primarily by the sound of the air moving through the endotracheal tube. I t has several advantages in trauma, requiring no cervi- cal manipulation or direct laryngoscopy, and it can be performed in the awake patient. Disadvantages in- clude limited success in the presence of apnea, retropharyngeal perforation, and vocal cord disruption. I t is contraindicated in maxillofacial trauma with insta- bility of the midface or suspected fracture of the crib- riform plate, where a misdirected endotracheal tubc

Rcceivcd for publication June 27, 1995; revised August 8, 1995.

Address reprint q u e s t s to 'Thomas R. Howdieshell, hlI), FACS, l'rauma/Surgical Critical Carc, Medical College of Georgia, RA- 4411, Augusta, GA 30912.

Presented a t thc American Association of Clinical Anatomists An- nual Meeting, liniversity of 'I'exas Medical 13rdnch, (;atveston, Texas, June 1994.

0 1996 Wiley-Liss, Inc.

292 Dover et al.

may enter the frontal cranial fossa (’Tintinalli and Claf- fey, 1981).

Inability to intubate the trachea is a clear indication for creating a surgical airway. With few exceptions, tra- cheostomy is a poor choice for emergent airway access in the trauma victim. T h e trachea resides deeper in the neck than is commonly appreciated; it is surrounded by numerous veins capable of alarming hemorrhage, and the location for recommended tracheal incision is fre- quently obscured by the isthmus of the thyroid gland (Skaggs and Cogbill, 1969). Also, the necessary light, instruments, and assistance for tracheostomy are rarely available a t the emergency site. For these reasons, technical misadventures may result in complications, including hemorrhage, hypoxemia, injury to the recur- rent laryngeal nerve, pneumothorax, or esophageal perforation (McClelland, 1965).

Surgical cricothyroidostomy is the procedure of choice in circumstances in which emergency surgical airway access is indicated (Walls, 1988). T h e procedure requires elementary technical skills, a few basic instru- ments, and can be performed rapidly. T h e subcuta- neous cricothyroid membrane is identified by pal- pation, the larynx stabilized, and a short transverse incision made through skin, subcutaneous tissue, and membrane. ’The incision is dilated with an instrument or the tip of a gloved finger, and a tracheostomy tube or endotracheal tube is inserted into the tracheal lumen (Phillips, 1991). Reported complication rates vary from 6% to 40%, with complications including incorrect site of tube insertion, damage to cricoid and thyroid cartilages, stoma1 or endobronchial hemor- rhage, and subglottic stenosis.

T h e cricothyroid membrane, o r ligament, is seen initially as a trapezoidal, tough band of tissue extend- ing in the midline from the cricoid cartilage below to the thyroid cartilage above. It should be referred to more specifically as the median cricothyroid ligament, for it is the superficial, thickened, and antcromedial part of the conus elasticus which lies beneath the laryngeal mucosa. ’This membrane arises from the arch of the cricoid cartilage and attaches superiorly to the thyroid and arytenoid cartilages. Its free superior margin forms the focal ligament or cord.

7’he cricothyroid artery is mentioned briefly in most classic anatomic reference texts. ‘I‘he British edition of Gray’s Anatomy (Williams et a]., 1989) states that the artery crosses the superior portion of the median cricothyroid ligament. ‘The cricothyroid artery usually arises from the superior laryngeal branch of the supe- rior thyroid artery and commonly has rich anastomoses with the superior laryngeal artery deep to the thyroid lamina (Lippert c t al., 1985; Bergmann et al., 1988). In approximately 5% of cases, the cricothyroid artery may

totally replace the superior laryngeal artery. Occasion- ally, two enlarged cricothyroid arteries anastomose in the midline, giving origin to a median descending artery supplying a pyramidal thyroid lobe.

T h e purpose of this paper is to provide data con- cerning the dimensions and vascular relations of the cricothyroid membrane. This information may help the clinician avoid, or at least manage, the complica- tions following cricothyroidostomy.

MATERIAL AND METHODS

In this study, 15 cadaveric specimens were exam- ined. Nine of the specimens were male (60%) and six were female (40%), with ages ranging from 70 to 92 years. Red arterial latex was injected in nine cadav- ers via the right common carotid artery a t the time of embalming. Six embalmed cadavers were dissected without latex injection. Selective latex injection of the superior thyroid artery was performed in three of the six cadavers not previously injected to evaluate the collateral circulation across the cricothyroid region.

Neck dissection was performed via a collar incision beginning at the level of the clavicles and suprasternal notch. T h e skin, subcutaneous tissue, and platysma were elevated as a flap to the level of the mandible. The sternohyoid and omohyoid muscles were detached from their origins and elevated cephalad to expose the cricothyroid membrane region for dimensional mea- surements and identification of vasculature. Only tha t part of the membrane seen clinically was measured; that is, the area between the cricothyroid muscles later- ally, the thyroid cartilage superiorly, and the cricoid cartilage inferiorly. Because the exposed portion of membrane is actually trapezoidal in shape, wider above than below, measurements included the width at three levels and the height.

RESULTS T h e dimensions of the exposed portion of cricothy-

roid membrane were measured with calipers following careful dissection of the pretracheal fascia and associ- ated loose areolar tissue. T h e width (three levels) and height for males, females, and the total group are shown in Figure 1. ‘The average width and height of the cricothyroid membrane were consistently smaller in fcmales.

In 13 of the 15 cadavers (87%), the superior thyroid artery originated from the external carotid artery. In two cadavers, both male, the superior thyroid artery originated from the common carotid artery. A trans- verse cricothyroid artery was identified in 14 cadavers (93%), originating from the superior thyroid artery.

Cricothyroid Memhrane Anatomy 293

posterior to the sternothyroid muscle to supply the pos- terior aspect of the thyroid lobe. T h e other branch passed anterior to the sternothyroid muscle to supply the anterior aspect of the thyroid lobe and isthmus. In these nine cadavers, the cricothyroid artery arose from the anterior division of the superior thyroid artery. In six cadavers (40%), the superior thyroid artery failed to branch prior to reaching the upper pole of the thyroid gland. In these cadavers, the cricothyroid artery arose from the intrathyroid portion of the superior thyroid artery and coursed posterior to the sternothyroid mus- cle to reach the cricothyroid membrane.

[Jnikiteral superior thyroid artery latex injection demonstrated communications between the right and left superior thyroid artery. Collateral anastomoses were noted both at the level of the cricothyroid mem- brane via the cricothyroid artery, and at the level of the thyroid isthmus. Several branches of the cricothyroid artery supplied the strap muscles, while others pene- trated the cricothyroid membrane and ascended along the undersurface of the thyroid cartilage to supply the laryngeal mucosa and anastomose with branches of the superior laryngeal artery (Fig. 3).

Numerous veins were also identified in the cricothy- roid region. In the ~ i ibcu taneou~ tissue, paired anterior jugular veins crossed the membrane in a vertical direc- tion in the majority of specimens. Deep to the ster- nohyoid muscles, venous tributaries of superior and in-

Fig. 1. Dimensions of cricothyroid membrane. Range and (mean) values reported in millimeters.

T h e cricothyroid artery coursed across the upper one- third of the cricothyroid membrane in 13 specimens (93%), and across the lower portion of the membrane in one cadaver (Fig. 2). In nine specimens (60%), the superior thyroid artery branched prior to reaching the upper pole of the thyroid lobe. One branch coursed

Fig. 3. Branches of cricothyroid artery supplying strap muscles (large arrows) and penetrating cricothyroid membrane (small arrows). C = cricothyroid muscles, S = strap muscle.

Fig. 2. membrane.

Cricothyroid artery (arrow) travcrsing upper portion of

294 Dover et al.

ferior thyroid veins were found crossing the cricothy- mid membrane. In 12 of the specimens (80%), small veins from the region of the thyroid isthmus traversed the cricothyroid membrane, followed the cricothyroid and superior thyroid arteries, and drained into the in- ternal jugular vein.

DISCUSSION

Jackson's (1921) landmark study proposed the see- ond to fourth tracheal rings as the only acceptable loca- tions for establishment of a surgical airway. Citing unac- ceptable complication rates at higher laryngotrachcal levels, the Jacksonian dictum received no serious chal- lenge for over 50 years. 'I'horacic surgeons Brantigan and Grow (1976), wishing to maximize the distance between clean median sternotomy wounds and colonized trd- cheostomy stomal sites, reported a series of 655 elective cricothyroidostomies with lower morbidity than that re- ported in series of tracheostomies. This report stimu- lated renewed interest in cricothyroidostomy, to include its use as an emergency airway access procedure. Sub- sequent reports (Boyd et al., 1979; Cole and Aguilar, 1988) of emergency cricothyroidostomy documented complication rates of 6.8-32%. Immediate complica- tions included incorrect site of tube placement, subcu- taneous emphysema, pneumomediastinum, and hemor- rhage. Late complications included hoarseness or voice change, vocal fold paresis, laryngeal fracture with per- manent dysphonia, and subglottic stenosis.

Inappropriate placement of the skin incision can in- crease the risk of complications, including incorrect site of tube placement and stomal hemorrhage. Cervi- cal hematoma, edema, or subcutaneous emphysema may make palpation of topographic landmarks, such as the thyroid and cricoid cartilages, difficult or impossi- ble. T h e placement of a horizontal skin incision too cephalad or caudad may lead to inadvertent thyrohyoid membrane or tracheal insertion of the airway. ( h e s of stomal or surface hemorrhage requiring vessel ligation have been reported following horizontal skin incisions (McGill et al., 1982). From our investigation, it is easy to imagine injuries to the anterior jugular veins or an- terior branch of the superior thyroid artery as causes of wound bleeding following horizontal skin incision.

Fatal airway hemorrhage has also been reported fol- lowing transtracheal needle aspiration and cricothyroi- dostomy (1Jnger and Moser, 1973; Schillaci e t al., 1976; Safar and Penninckx, 1967). T h e cricothyroid artery was lacerated with its freely bleeding transected ends forced into the lumen of the airway resulting in endobronchial hemorrhage and asphyxia. As illustrated in our dissec- tions, injury to the transverse cricothyroid artery or an-

terior branches of the superior thyroid artery, with their rich collateral flow and systemic pressure, might result in serious endobronchial or wound bleeding.

I,atc complications, including voice change, vocal cord injury, laryngeal fracture, and subglottic stenosis have been associated with forceful placement of over- sized tracheostomy tubes through the cricothyroid membrane (McGill eta]., 1982). In our study, measure- ments of the exposed portion of the cricothyroid mem- brane demonstrated the average size of the space to be 6.9 mm by 9.5 mm in females and 8.8 mm by 10.9 mm in males. Tubes frequently utilized for tracheostomy, #8 and #10 Shiley tracheostomy tubes with outer diam- eters of 12 mm and 13 mm respectively, could cause la- ryngeal injury when used for cricothyroidostomy.

CONCLUSIONS

Based on our results, we suggest a vertical skin inci- sion, as it provides maximal flexibility for exposure while minimizing the chance of soft tissue hemorrhage from vertically oriented anterior jugular veins or ante- rior branches of the superior thyroid artery located su- perficial to the strap muscles. Palpation of the cricothy- roid membrane should be repeated after the skin incision is made to confirm the correct position of the membrane. A transverse stab incision of the membrane near its lower border adjacent to the cricoid cartilage is recommended to avoid injury to the cricothyroid artery. If bleeding occurs, bilateral ligation of the ante- rior branches of the superior thyroid artery may be nec- essary to control hemorrhage, due to the extensive col- lateral blood supply involving the cricothyroid region. Consideration of limiting airway tube size, particularly in females, to no greater than 9-10 mm outer diameter may decrease the incidence of laryngeal injury. When rapid surgical access to the airway is required following traumatic injury, cricothyroidostomy is the procedure of choice. '1'0 avoid or manage complications following surgical cricothyroidostomy, a knowledge of the di- mensions, relations, and vasculature of the cricothyroid membrane is imperative.

REFERENCES Ampel, L., K.A. Hott, (;.W. Sielaff and '1'.13. Sloan 1988 An ap-

proach to airway management in the acutely head-injured pa- tient. J. Emerg. hled. 61-7.

Bergmann, R.A., S.A. 'I'hompson, A.K. Afifi and Iq'.A. Saadeh 1988 Compendium of I luman Anatomic Yariation. 13alti- more: lrrban and Schwarzenberg, pp, 353-367.

Boyd, A.D., M.C. Iiomita, A.A. Conlan, S.D. I:ink and I:.(:. Spencer I979 A clinical evaluation of cricotliyroidotom).. Surg. Gynecol. Obstet . /49:365-368.

Cricothyroid Membrane Anatomy 295

Rrantigan, C.O. and J.R. Grow Sr. 1976 (:ricothyroidotomy: Elective use in respiratory problems requiring tracheotomy. J. Thorac. Cardiovasc. Surg. 7/:72-81.

Bucholz, K.W., W.Z. Burkhead, W. (haham and C. Petty 1979 Occult cervical spine injuries in fatal traffic accidents. J. Trauma 19768-771.

Cole, K.R. and E.A. Aguilar 111 1988 Cricothyroidotomy versus tra- cheostomy: An otolaryngologist’s perspective. 1,aryngoscope

Jackson, C. 1921 High tracheostomy and other errors the chief causes of chronic laryngeal stenosis. Surg. Gynecol. Obstet. .?2:392-398.

t ippert , 11. and K. Pahst 1985 Arterial Variations in Man: (XIS- sitication and Frequency. Munich: J.F. Hergman, pp. 83-84.

Magill, I.W. 1930 ‘I’echnique in endotracheal anaesthesia. Hr. hled. J. 2817-819.

blc(:lelland, K.M. 1965 (hmplications of tracheostomy. Rr. bled. J. 2567-569.

McGill, J . , J.E. Clinton and E. Kuiz 1982 Cricothyrotomy in the emergency department. Ann. Emerg. bled. //:361-364.

98: 1 3 1-1

Phillips, T.1:. 1991 Airway management. In Trauma. R.E. Moore, K.1,. blattox and I1.V. Feliciano (eds.) Xorwalk: Appleton and Lange, pp. 127-142.

Safar, 1’. and J. Penninckx 1967 Cricothyroid membrane punc- ture with special cannula. Anesthesiology 28:943-948.

Schillaci, K.F., V.E. Iacovoni and R.S. Conte 1976 Transtracheal aspiration complicated by fatal endotracheal hemorrhage. N e w Engl. J . Med. 29.5:488-490.

Skaggs, J.A. and (:.I,. Cogbill 1969 Tracheostomy: hlanage- ment, mortality, complications. Am. Surg

’I‘intinalli, J.E. and J. Claffey 1981 Complications of nasotra- cheal intubation. Ann. Emerg. bled. /0:142-144.

Ilnger, K .M. and K.1LI. Moser 1973 1;atal complication of transtracheal aspiration. Arch. Intern. hled. /.?2:437-439.

Walls, R.M. 1988 <:ricothyroidotomy. Emerg. bled. (:lin, North. Am. 6725-736.

Williams, P.L., K. Warwick, M. Dyson and L.H. Bannister (eds.) 1989 Gray’s Anatomy. N e w York: Churchill Livingstone, pp. 1253-1 254.