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Anatomical characterisation of the cricothyroid membrane in females of childbearing age using computed tomography
International Journal of Obstetric Anesthesia (2014) 23, 29–34 0959-289X/$ - see front matter c 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijoa.2013.07.007
ORIGINAL ARTICLE
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Anatomical characterisation of the cricothyroid membrane in females of childbearing age using computed tomography N. Long,a S. Ng,b G. Donnelly,b,c M. Owens,c M. McNicholas,a K. McCarthy,b C. McCaulb,c,d a
Department of Radiology, The Mater Misericordiae University Hospital, Dublin, Ireland Department of Anaesthesia, The Rotunda Hospital, Dublin, Ireland c Department of Anaesthesia, Mater Misericordiae, University Hospital, Dublin, Ireland d School of Medicine and Medical Sciences, University College Dublin, Ireland b
ABSTRACT Background: In the event of failure to secure the airway by conventional means, it may be necessary to perform invasive airway access via the cricothyroid membrane. No studies have addressed anatomy of this structure in the obstetric population. We aimed to review the anatomical variation of this structure in a population of childbearing age. Methods: We searched the radiology database for computed tomography studies of the neck performed in a 13-month period in consecutive patients aged 15–55 years. Studies on 18 females and 22 males were reviewed. Male patients were included for comparison. Data were reconstructed using a high spatial frequency algorithm to optimise spatial resolution. Five parameters were measured: distance from the skin to the membrane, maximum midline height of the membrane in the vertical plane, maximum transverse diameter of the membrane, neck diameter and cartilaginous calcification. Results: The distance (mean range) from skin to the membrane was similar in females and males (16.2 [3–33] vs. 13.9 [3–37] mm, P = 0.42). The vertical height (9.9 [7–17] vs. 11.4 [8–15] mm, P = 0.04) and maximum width of the membrane (14.5 [10–17] mm vs. 12.5 [10–15] mm, P < 0.01) were greater in males. Cartilaginous calcification was low and did not differ between genders. Conclusions: The cricothyroid membrane is not necessarily a superficial structure and consequently may be difficult to palpate. The smallest dimensions of the membrane indicate that smaller than recommended cricothyroidotomy devices may be required in some patients as the external diameter of commercial trocar devices and tracheal tubes may exceed 7 mm. c 2013 Elsevier Ltd. All rights reserved.
Keywords: Airway anatomy; Cricothyroid membrane; Cricoid cartilage; Thyroid cartilage
Introduction Failure to intubate the trachea is more common in obstetric anaesthesia than any other subspeciality and is thought to occur in 1/100–1/250 patients.1–3 Current teaching prioritises maintenance of oxygenation using a failed intubation algorithm, the ultimate component of which is surgical airway access.1,4 Although rare, cases in which emergency surgical airway access was required were reported in recent studies of obstetric patients in both the UK and USA.1,2,5 In two of the four obstetric cases reported in the 4th Audit National Audit Project (NAP4) study, emergency surgical access was attempted.5 This failed in one patient and was successfully performed by an otolaryngology (ENT) surgeon in the Accepted July 2013 Correspondence to: Dr Conan Liam McCaul, Department of Anaesthesia, The Rotunda Hospital, Dublin 1, Ireland. E-mail address: [email protected]
other.5 Consistent with that observed in experimental series, the overall incidence of failure of emergency surgical airway success performed by anaesthetists in NAP4 was 9 out of 25 (36%). We performed this study in an attempt to add to anatomical knowledge regarding the cricothyroid membrane which is the recommended site for emergency airway access when other techniques fail.4,6 The anatomy of the cricothyroid membrane (CTM) has not been studied extensively and has been derived from a small number of cadaveric dissections in patients of unspecified ages. In particular, the relationship between obesity and the subcutaneous depth of the CTM, which may have important implications for its localizability by palpation, has not been determined. We assessed the structure of the CTM through three-dimensional reconstruction of stored computed tomography images of the neck region. We sought to establish the range of measurements of the vertical and transverse dimensions of the CTM and its depth from overlying skin. We also
30
Cricothyroid membrane anatomy
sought to determine if differences existed between males and females in these measurements.
Methods Following institutional ethics committee approval, computed tomography (CT) studies of consecutive adult patients <55 years of age, scanned between Feb 2010 and Feb 2011, were reviewed. Patients with a history of laryngeal pathology, surgery to the cervical region or previous radiotherapy were excluded to yield 40 patients who met the inclusion and exclusion criteria. Twenty-two male patients and 18 female patients were included in the study. The CT studies were reviewed in three dimensions (axial, sagittal and coronal) and measurements of the CTM were taken in orthogonal planes. Four measurements were taken, and the degree of cartilage calcification was scored. 1. The anterior-posterior (AP) distance from the skin to CTM. This measurement was taken from a point directly superior to the cricoid cartilage to the surface of the skin in the midline (Fig. 1). 2. Maximum midline height of the CTM in the midline sagittal plane. This was taken from the superior edge of the cricoid cartilage to the inferior edge of the thyroid cartilage in the midline (Fig. 2). 3. The maximum transverse measurement taken between the inferior aspect of the thyroid cartilage and the superior aspect of the cricoid cartilage which approximates to the maximum transverse diameter of the CTM (Fig. 3).
Fig. 2 Sample CT image demonstrating the maximum midline height of the cricothyroid membrane from the superior edge of the cricoid cartilage to the inferior edge of thyroid cartilage in the midline sagittal plane.
Fig. 3 Sample coronal CT image demonstrating the maximum transverse measurement of the cricothyroid membrane. This is taken between the inferior aspect of thyroid cartilage and the superior aspect of cricoid cartilage.
4. The thickness of the neck from front to back in the midline at the level of the CTM (Fig. 4).
Fig. 1 Sample axial CT image demonstrating the AP distance from skin to the cricothyroid membrane.
The degree of calcification of the thryoid cartilage was classified as complete or incomplete. Cricoid cartilage calcification was scored 1–3 as follows: (1) not calcified; (2) calcified posteriorly and/or laterally but not anteriorly; and (3) calcified circumferentially. In patients where the cricoid cartilage was not calcified or incompletely calcified, the anterior aspect of the
N. Long et al.
31 Table 1
Indication for CT scan
Indication
Number
Investigation of mass Infection (head & neck) Oncology staging Trauma Vascular Dysphagia Pain
4 (10%) 5 (12.5%) 12 (48.0%) 16 (64.0%) 1 (2.5%) 1 (2.5%) 1 (2.5%)
Data are number (%).
Fig. 4 Sample axial CT image demonstrating the thickness of the neck from front to back in the midline at the level of the cricothyroid membrane.
cricoid cartilage was identified by a characteristic crescentic region of higher attenuation anteriorly. All studies were performed on a 128 slice Siemens CT scanner using the following imaging parameters: 100–120 kV tube voltage; 90–100 mA tube current; slice collimation of 0.6 mm; slice thickness of 5 mm. Thirtytwo studies had been performed with intravenous contrast and eight studies without. Data were reconstructed using a soft tissue with 1 mm reconstruction section thickness, 0.7 mm reconstruction interval, 284– 380 mm2 field of view and 512 · 512 matrix. The standard position for these CT studies is supine and anatomically neutral.
Statistical analysis Statistics were performed using Sigma Stat (Jandel Scientific, San Jose, CA, USA). Continuous data were analysed with the Student’s t test. Normally distributed data are presented as mean and standard deviation (SD) and data that were not normally distributed are presented as medians and range. Associations were determined by linear regression analysis.
Results
Fig. 5 (A) Subcutaneous depth of the cricothyroid membrane. (B) Maximum transverse diameter of the cricothyroid membrane. Box and whisker plots showing median, interquartile ranges and 95% CI.
Sixty-two consecutive stored images were evaluated of which 40 were suitable for use. Images from 22 male and 18 female patients aged 34.1 ± 9.9 and 38.1 ± 9.8 years, respectively, were studied. The indications for the CT scan are detailed in Table 1. The mean [range] subcutaneous depth of the CTM was 16.2 [3–33] in females and 13.9 [3–37] mm in males and did not differ between groups (P = 0.42) (Fig. 5A). The vertical height of the membrane was greater in males 11.4 [8–15] mm than females 9.9 [7–17] mm (P = 0.04) (Fig. 6). The maximum transverse diameter of the membrane was greater in males 14.5 [10–17] mm than females 12.5 [10–15] mm (P < 0.01)
(Fig. 5B). The AP neck diameter was greater in males than females (Table 2). The degree of cricoid calcification did not differ between genders. Cricothyroid membrane depth correlated with both body mass index (P = 0.04) and weight (P = 0.01). Cricoid calcification correlated positively with increased age (P = 0.001). Increased AP neck diameter correlated with subcutaneous depth (P < 0.0001) and width (P = 0.031) but not height (P = 0.263) of the CTM. There was no correlation between the height and width of the CTM (P = 0.102). Five of 18 (28%) females and 2 of 22 (9%) males had CTM heights of <9 mm.
32
Cricothyroid membrane anatomy
Fig. 6 Vertical height of the cricothyroid membrane. Box Whisker plots showing median, interquartile ranges and 95% CI.
Discussion The data from this study indicate that there is considerable anatomic variation in all measured aspects of the CTM. Most notably, this structure is frequently not superficial and therefore may be difficult to palpate. Furthermore, its vertical dimension may be sufficiently small in some patients that some devices used to achieve emergency oxygenation via the CTM will be too large. These findings challenge the commonly held assumptions regarding the ease of localising the CTM which has frequently been described as a subcutaneous, superficial and easily identifiable structure.7–10 While this may be true for thin patients, it does not extend to the obese population in whom total neck tissue and fat volume are increased.11,12 Previous ultrasonic airway studies in obese subjects determined pre-tracheal soft tissue depth at the level of the vocal cords as 28 ± 2.7 mm vs. 17.5 ± 1.8 mm in those who were difficult and easy to intubate, respectively, indicating that a greater impediment to accurate localisation of the CTM exists in those where intubation is difficult.13 Published data on the anatomical dimensions of the CTM are surprisingly scant and many of the published cadaveric studies provide limited data on the age or Table 2
gender of their subjects. The limited data that exist show that there is considerable variation in the AP, transverse and vertical dimensions of the thyroid and cricoid cartilages in both males and females.14,15 The vertical height of the CTM tends to be smaller in females than males. In a large cadaver-based study, Randestad et al. showed the mean vertical size of the cricothyroid ligament to be 6.4 mm in females and 7.6 mm in male specimens.16 In the current study the vertical height of the CTM in females is as high as 17 mm but it is a little as 7 mm in others. The clinical implication of this is that placement of a 6-mm internal diameter tracheal tube may be difficult as its outside diameter is 8.2 mm, not including the cuff. This size of tube is generally recommended as it is the smallest that will railroad over the common bougie (15 French gauge).17,18 Using a 6-mm tube may cause tissue injury, particularly to the vascular structures as they predominantly cross the upper third of the CTM. Placing any device in the inferior aspect of the structure, as recommended, may not be possible.19,20 However, it is possible that the aperture between the cricoid and thyroid cartilages may distend with pressure as movement can occur at the cricothyroid joint. This joint connects the inferior cornu of the thyroid cartilage on its medial aspect to the lateral aspects of the posterior portion of the cricoid cartilage and permits vertical, horizontal and rotational movement. The thyroid cartilage can move visor-like relative to the cricoid cartilage. This has been measured in a recent cadaveric dissection study by Windisch et al. as 9.2 ± 2.47 mm in males and 6.6 ± 1.77 mm in females.21 Vertical gliding of the thyroid relative to the cricoid is also described and is influenced by the mechanical structure of the joint.21 Palpating the CTM may be difficult and is known to be inaccurate in the obese population.22 This has led to the suggestion that ultrasound may be of use during emergency surgical airway management.17,19,20 This approach is complicated by the ultrasonic characteristics of the CTM which is not clearly visible on ultrasound examination. This is reflected in the low calcification scores seen in the CTM in our study. We also sought to determine the extent of calcification of the thyroid and cricoid cartilages in an attempt to predict their ultrasonographic characteristics. Even in the absence of
Antero-posterior neck diameter and cartilaginous calcification
Neck diameter (mm) Thyroid cartilage calcification complete incomplete Cricoid calcification grade 1 2 3 Data are mean ± SD [range] or percentage.
Female (n = 18)
Male (n = 22)
P Value
116.2 ± 15.0 [96–150]
130.9 ± 24.9 [100–186]
0.035
94.4% 5.6%
95.4% 4.6%
1.0 1.0
27.8% 66.7% 16.7%
9.9% 59.1% 31.8%
0.21 0.87 0.46
N. Long et al. calcification of the CTM, the presence of calcification within the adjacent cartilaginous structures of the larynx, such as the thyroid and cricoid cartilages, is potentially advantageous in locating the CTM as such calcifications are bright and echogenic on ultrasound. In our study, while increased calcification was associated with increased age, there was no consistent relationship between calcification in the thyroid and cricoid cartilages. This is consistent with data from imaging studies which suggest that thyroid cartilage calcification is chronologically independently of that in the cricoid cartilage.18 In a large study of lateral cepahalometric radiographs in patients aged 10–59 years, Mupparapu and Vuppalapati demonstrated that ossification of the laryngeal cartilages was uncommon in the first two decades of life.23 In the fourth decade, however, ossification was present in 36.5% and 24.3% of thyroid and cricoid cartilages in females and 75% and 45% in male subjects, respectively.23 These data suggest that cartilaginous calcification is not present sufficiently consistent to be a reliable sonographic landmark for use of ultrasound in locating the CTM particularly in younger age groups. Despite a relatively high rate of failed intubations, emergency surgical airway access is rarely performed in obstetric anaesthesia practice. This is probably because of the use of neuraxial anaesthesia and the frequent success of supraglottic airway devices when failure to intubate occurs. Emergency surgical access was performed in one obstetric patient in the UKOSS study,1 two in NAP42 and in one patient in Palanisamy’s series.5 In two cases the procedure was performed by an ENT surgeon, a luxury that will not always be available at short notice in an obstetric unit. Both obstetric patients in the NAP4 study were obese. In one of these, the attempted narrow-bore needle cricothyroidotomy failed twice. Of all 58 attempted emergency surgical access attempts in the NAP4 study, six patients died, three attributable to failure of the technique. In 25 cases, the primary attempt was made by an anaesthetist and only nine were successful. In most of these cases, a narrow bore cannula was used. Among the listed causes of failure was ‘‘misplacement’’ which has the capacity not only to fail to improve oxygenation but to increase the likelihood of airway injury.24 The high prevalence of obesity in both obstetric and non-obstetric series, previously described has a number of implications. The CTM may not be easily identifiable and the airway devices may not be long enough to reach the airway lumen. This study has a number of limitations. Firstly, radiologic reconstruction may not have the precision of anatomic measurement of cadaveric dissection and examination but does have the advantage of access to large numbers of patient images which increases the generalizability of the data compared to smaller cadaveric series. Secondly, the studies were performed in the anatomically neutral position and any dynamic change
33 that might be seen with head extension was not assessed. Third, these studies were performed to investigate disease which might potentially distort neck anatomy. Last, none of the studies were performed on pregnant patients who may have increased neck dimensions due to the normal physiological changes of pregnancy. In conclusion, this study demonstrates that the CTM is not invariably a subcutaneous structure and may explain at least partially the high failure rate for emergency cricothyroid oxygenation seen in the recent literature. Furthermore, the vertical height of the CTM may be quite small and clinicians should be prepared to use devices with narrower diameters than commonly recommended for emergency cricothyroid access.
Disclosure This study received departmental funding only and the authors have no conflicts of interests to declare
References 1. Quinn AC, Milne D, Columb M, Gorton H, Knight M. Failed tracheal intubation in obstetric anaesthesia: 2 year national case control study in the UK. Br J Anaesth 2013;110:74–80. 2. Palanisamy A, Mitani AA, Tsen LC. General anesthesia for caesarean delivery at a tertiary care hospital from 2000 to 2005; a retrospective analysis and 10-year update. Int J Obstet Anesth 2011;20:10–6. 3. McDonnell NJ, Paech MJ, Clavisi OM, Scott KL. Difficult and failed intubation in obstetric anaesthesia: an observational study of airway management and complications associated with general anaesthesia for caesarean section. Int J Obstet Anesth 2008;17:292–7. 4. Mhyre JM, Healy D. The unanticipated difficult intubation in obstetrics. Anesth Analg 2011;112:648–52. 5. Cook TM, Woodhall N, Frerk C. Major complications of airway management in the UK: results of the Fourth National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society. Part 1: anaesthesia. Br J Anaesth 2011;106:617–31. 6. Henderson JJ, Popat MT, Latto IP, Pearce AC. Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia 2004;59:675–94. 7. Ellis H, Feldman S, Harrop-Griffiths S. Anatomy for anaesthetists. 8th ed. Chichester: Wiley-Blackwell; 2004. 8. Dover K, Howdieshell TR, Colborn GL. The dimensions and vascular anatomy of the cricothyroid membrane: relevance to emergent surgical airway access. Clin Anat 1996;9:291–5. 9. Katos MG, Goldenberg D. Emergency cricothyrotomy. Operative Techniques in Otolaryngology 2007;18:110–4. 10. Vanner R. Emergency cricothyrotomy. Curr Anaesth Crit Care 2001;12:238–43. 11. Dob DP, McLure HA, Soni N. Failed intubation and emergency percuteneous tracheostomy. Anaesthesia 1998;53:69–78. 12. Boon JM, Abrahams PH, Meiring JH, Welch T. Cricothyroidotomy: a clinical anatomy review. Clin Anat 2004;17:478–86. 13. Ezri T, Gewu¨rtz G, Sessler DI, et al. Prediction of difficult laryngoscopy in obese patients by ultrasound quantification of anterior neck soft tissue. Anaesthesia 2003;58:1111–4. 14. Eckel HE, Sittel C. Morphometry of the larynx in horizontal sections. Am J Otololaryngol 1995;16:40–8.
34 15. Shin H, Ahn Y, MW S, KIm K, Kwon T. Measurement of crosssectional dimensions of the cricoid cartilage: a computed tomographic study. Ann Otol Rhinol Laryngol 2009;118:253–8. 16. Randestad A, Lindholm CE, Fabian P. Dimensions of the cricoid cartilage and the trachea. Laryngoscope 2000;110:1957–61. 17. Suzuki A, Iida T, Kunisawa T, Henderson JJ, Fujita S, Iwasaki H. Ultrasound-guided cannula cricothyroidotomy. Anesthesiology 2012;117:1128. 18. Sakai F, Gamsu G, Dillon WP, Lynch DA, Gilbert TJ. MR Imaging of the larynx at 1.5 T. J Comput Assist Tomogr 1990;14:60–71. 19. Dinsmore J, Heard A, Green R. The use of ultrasound to guide time-critical cannula tracheotomy when anterior neck airway anatomy is unidentifiable. Eur J Anaesthesiol 2011;28:506–10. 20. Kleine-Brueggeney M, Greif R, Ross S, et al. Ultrasound-guided percutaneous tracheal puncture: a computer-tomographic controlled study in cadavers. Br J Anaesth 2011;106:738–42.
Cricothyroid membrane anatomy 21. Windisch G, Hammer G, Prodinger P, Friedrich G, Anderhuber F. The functional anatomy of the cricothyroid joint. Surg Radiol Anat 2010;32:135–9. 22. Aslani A, Ng S-C, Hurley M, McCarthy KF, McNicholas M, McCaul CL. Accuracy of identification of the cricothyroid membrane in female subjects using palpation: an observational study. Anesth Analg 2012;114:987–92. 23. Mupparapu M, Vuppalapati A. Ossification of laryngeal cartilages on lateral cephalometric radiographs. Angle Orthod 2005;75:196–201. 24. Salah N, El Saigh I, Hayes N, McCaul C. Airway injury during emergency transcutaneous airway access: a comparison at cricothyroid and tracheal sites. Anesth Analg 2009;109:1901–7.
ORIGINAL ARTICLE
www.obstetanesthesia.com
Anatomical characterisation of the cricothyroid membrane in females of childbearing age using computed tomography N. Long,a S. Ng,b G. Donnelly,b,c M. Owens,c M. McNicholas,a K. McCarthy,b C. McCaulb,c,d a
Department of Radiology, The Mater Misericordiae University Hospital, Dublin, Ireland Department of Anaesthesia, The Rotunda Hospital, Dublin, Ireland c Department of Anaesthesia, Mater Misericordiae, University Hospital, Dublin, Ireland d School of Medicine and Medical Sciences, University College Dublin, Ireland b
ABSTRACT Background: In the event of failure to secure the airway by conventional means, it may be necessary to perform invasive airway access via the cricothyroid membrane. No studies have addressed anatomy of this structure in the obstetric population. We aimed to review the anatomical variation of this structure in a population of childbearing age. Methods: We searched the radiology database for computed tomography studies of the neck performed in a 13-month period in consecutive patients aged 15–55 years. Studies on 18 females and 22 males were reviewed. Male patients were included for comparison. Data were reconstructed using a high spatial frequency algorithm to optimise spatial resolution. Five parameters were measured: distance from the skin to the membrane, maximum midline height of the membrane in the vertical plane, maximum transverse diameter of the membrane, neck diameter and cartilaginous calcification. Results: The distance (mean range) from skin to the membrane was similar in females and males (16.2 [3–33] vs. 13.9 [3–37] mm, P = 0.42). The vertical height (9.9 [7–17] vs. 11.4 [8–15] mm, P = 0.04) and maximum width of the membrane (14.5 [10–17] mm vs. 12.5 [10–15] mm, P < 0.01) were greater in males. Cartilaginous calcification was low and did not differ between genders. Conclusions: The cricothyroid membrane is not necessarily a superficial structure and consequently may be difficult to palpate. The smallest dimensions of the membrane indicate that smaller than recommended cricothyroidotomy devices may be required in some patients as the external diameter of commercial trocar devices and tracheal tubes may exceed 7 mm. c 2013 Elsevier Ltd. All rights reserved.
Keywords: Airway anatomy; Cricothyroid membrane; Cricoid cartilage; Thyroid cartilage
Introduction Failure to intubate the trachea is more common in obstetric anaesthesia than any other subspeciality and is thought to occur in 1/100–1/250 patients.1–3 Current teaching prioritises maintenance of oxygenation using a failed intubation algorithm, the ultimate component of which is surgical airway access.1,4 Although rare, cases in which emergency surgical airway access was required were reported in recent studies of obstetric patients in both the UK and USA.1,2,5 In two of the four obstetric cases reported in the 4th Audit National Audit Project (NAP4) study, emergency surgical access was attempted.5 This failed in one patient and was successfully performed by an otolaryngology (ENT) surgeon in the Accepted July 2013 Correspondence to: Dr Conan Liam McCaul, Department of Anaesthesia, The Rotunda Hospital, Dublin 1, Ireland. E-mail address: [email protected]
other.5 Consistent with that observed in experimental series, the overall incidence of failure of emergency surgical airway success performed by anaesthetists in NAP4 was 9 out of 25 (36%). We performed this study in an attempt to add to anatomical knowledge regarding the cricothyroid membrane which is the recommended site for emergency airway access when other techniques fail.4,6 The anatomy of the cricothyroid membrane (CTM) has not been studied extensively and has been derived from a small number of cadaveric dissections in patients of unspecified ages. In particular, the relationship between obesity and the subcutaneous depth of the CTM, which may have important implications for its localizability by palpation, has not been determined. We assessed the structure of the CTM through three-dimensional reconstruction of stored computed tomography images of the neck region. We sought to establish the range of measurements of the vertical and transverse dimensions of the CTM and its depth from overlying skin. We also
30
Cricothyroid membrane anatomy
sought to determine if differences existed between males and females in these measurements.
Methods Following institutional ethics committee approval, computed tomography (CT) studies of consecutive adult patients <55 years of age, scanned between Feb 2010 and Feb 2011, were reviewed. Patients with a history of laryngeal pathology, surgery to the cervical region or previous radiotherapy were excluded to yield 40 patients who met the inclusion and exclusion criteria. Twenty-two male patients and 18 female patients were included in the study. The CT studies were reviewed in three dimensions (axial, sagittal and coronal) and measurements of the CTM were taken in orthogonal planes. Four measurements were taken, and the degree of cartilage calcification was scored. 1. The anterior-posterior (AP) distance from the skin to CTM. This measurement was taken from a point directly superior to the cricoid cartilage to the surface of the skin in the midline (Fig. 1). 2. Maximum midline height of the CTM in the midline sagittal plane. This was taken from the superior edge of the cricoid cartilage to the inferior edge of the thyroid cartilage in the midline (Fig. 2). 3. The maximum transverse measurement taken between the inferior aspect of the thyroid cartilage and the superior aspect of the cricoid cartilage which approximates to the maximum transverse diameter of the CTM (Fig. 3).
Fig. 2 Sample CT image demonstrating the maximum midline height of the cricothyroid membrane from the superior edge of the cricoid cartilage to the inferior edge of thyroid cartilage in the midline sagittal plane.
Fig. 3 Sample coronal CT image demonstrating the maximum transverse measurement of the cricothyroid membrane. This is taken between the inferior aspect of thyroid cartilage and the superior aspect of cricoid cartilage.
4. The thickness of the neck from front to back in the midline at the level of the CTM (Fig. 4).
Fig. 1 Sample axial CT image demonstrating the AP distance from skin to the cricothyroid membrane.
The degree of calcification of the thryoid cartilage was classified as complete or incomplete. Cricoid cartilage calcification was scored 1–3 as follows: (1) not calcified; (2) calcified posteriorly and/or laterally but not anteriorly; and (3) calcified circumferentially. In patients where the cricoid cartilage was not calcified or incompletely calcified, the anterior aspect of the
N. Long et al.
31 Table 1
Indication for CT scan
Indication
Number
Investigation of mass Infection (head & neck) Oncology staging Trauma Vascular Dysphagia Pain
4 (10%) 5 (12.5%) 12 (48.0%) 16 (64.0%) 1 (2.5%) 1 (2.5%) 1 (2.5%)
Data are number (%).
Fig. 4 Sample axial CT image demonstrating the thickness of the neck from front to back in the midline at the level of the cricothyroid membrane.
cricoid cartilage was identified by a characteristic crescentic region of higher attenuation anteriorly. All studies were performed on a 128 slice Siemens CT scanner using the following imaging parameters: 100–120 kV tube voltage; 90–100 mA tube current; slice collimation of 0.6 mm; slice thickness of 5 mm. Thirtytwo studies had been performed with intravenous contrast and eight studies without. Data were reconstructed using a soft tissue with 1 mm reconstruction section thickness, 0.7 mm reconstruction interval, 284– 380 mm2 field of view and 512 · 512 matrix. The standard position for these CT studies is supine and anatomically neutral.
Statistical analysis Statistics were performed using Sigma Stat (Jandel Scientific, San Jose, CA, USA). Continuous data were analysed with the Student’s t test. Normally distributed data are presented as mean and standard deviation (SD) and data that were not normally distributed are presented as medians and range. Associations were determined by linear regression analysis.
Results
Fig. 5 (A) Subcutaneous depth of the cricothyroid membrane. (B) Maximum transverse diameter of the cricothyroid membrane. Box and whisker plots showing median, interquartile ranges and 95% CI.
Sixty-two consecutive stored images were evaluated of which 40 were suitable for use. Images from 22 male and 18 female patients aged 34.1 ± 9.9 and 38.1 ± 9.8 years, respectively, were studied. The indications for the CT scan are detailed in Table 1. The mean [range] subcutaneous depth of the CTM was 16.2 [3–33] in females and 13.9 [3–37] mm in males and did not differ between groups (P = 0.42) (Fig. 5A). The vertical height of the membrane was greater in males 11.4 [8–15] mm than females 9.9 [7–17] mm (P = 0.04) (Fig. 6). The maximum transverse diameter of the membrane was greater in males 14.5 [10–17] mm than females 12.5 [10–15] mm (P < 0.01)
(Fig. 5B). The AP neck diameter was greater in males than females (Table 2). The degree of cricoid calcification did not differ between genders. Cricothyroid membrane depth correlated with both body mass index (P = 0.04) and weight (P = 0.01). Cricoid calcification correlated positively with increased age (P = 0.001). Increased AP neck diameter correlated with subcutaneous depth (P < 0.0001) and width (P = 0.031) but not height (P = 0.263) of the CTM. There was no correlation between the height and width of the CTM (P = 0.102). Five of 18 (28%) females and 2 of 22 (9%) males had CTM heights of <9 mm.
32
Cricothyroid membrane anatomy
Fig. 6 Vertical height of the cricothyroid membrane. Box Whisker plots showing median, interquartile ranges and 95% CI.
Discussion The data from this study indicate that there is considerable anatomic variation in all measured aspects of the CTM. Most notably, this structure is frequently not superficial and therefore may be difficult to palpate. Furthermore, its vertical dimension may be sufficiently small in some patients that some devices used to achieve emergency oxygenation via the CTM will be too large. These findings challenge the commonly held assumptions regarding the ease of localising the CTM which has frequently been described as a subcutaneous, superficial and easily identifiable structure.7–10 While this may be true for thin patients, it does not extend to the obese population in whom total neck tissue and fat volume are increased.11,12 Previous ultrasonic airway studies in obese subjects determined pre-tracheal soft tissue depth at the level of the vocal cords as 28 ± 2.7 mm vs. 17.5 ± 1.8 mm in those who were difficult and easy to intubate, respectively, indicating that a greater impediment to accurate localisation of the CTM exists in those where intubation is difficult.13 Published data on the anatomical dimensions of the CTM are surprisingly scant and many of the published cadaveric studies provide limited data on the age or Table 2
gender of their subjects. The limited data that exist show that there is considerable variation in the AP, transverse and vertical dimensions of the thyroid and cricoid cartilages in both males and females.14,15 The vertical height of the CTM tends to be smaller in females than males. In a large cadaver-based study, Randestad et al. showed the mean vertical size of the cricothyroid ligament to be 6.4 mm in females and 7.6 mm in male specimens.16 In the current study the vertical height of the CTM in females is as high as 17 mm but it is a little as 7 mm in others. The clinical implication of this is that placement of a 6-mm internal diameter tracheal tube may be difficult as its outside diameter is 8.2 mm, not including the cuff. This size of tube is generally recommended as it is the smallest that will railroad over the common bougie (15 French gauge).17,18 Using a 6-mm tube may cause tissue injury, particularly to the vascular structures as they predominantly cross the upper third of the CTM. Placing any device in the inferior aspect of the structure, as recommended, may not be possible.19,20 However, it is possible that the aperture between the cricoid and thyroid cartilages may distend with pressure as movement can occur at the cricothyroid joint. This joint connects the inferior cornu of the thyroid cartilage on its medial aspect to the lateral aspects of the posterior portion of the cricoid cartilage and permits vertical, horizontal and rotational movement. The thyroid cartilage can move visor-like relative to the cricoid cartilage. This has been measured in a recent cadaveric dissection study by Windisch et al. as 9.2 ± 2.47 mm in males and 6.6 ± 1.77 mm in females.21 Vertical gliding of the thyroid relative to the cricoid is also described and is influenced by the mechanical structure of the joint.21 Palpating the CTM may be difficult and is known to be inaccurate in the obese population.22 This has led to the suggestion that ultrasound may be of use during emergency surgical airway management.17,19,20 This approach is complicated by the ultrasonic characteristics of the CTM which is not clearly visible on ultrasound examination. This is reflected in the low calcification scores seen in the CTM in our study. We also sought to determine the extent of calcification of the thyroid and cricoid cartilages in an attempt to predict their ultrasonographic characteristics. Even in the absence of
Antero-posterior neck diameter and cartilaginous calcification
Neck diameter (mm) Thyroid cartilage calcification complete incomplete Cricoid calcification grade 1 2 3 Data are mean ± SD [range] or percentage.
Female (n = 18)
Male (n = 22)
P Value
116.2 ± 15.0 [96–150]
130.9 ± 24.9 [100–186]
0.035
94.4% 5.6%
95.4% 4.6%
1.0 1.0
27.8% 66.7% 16.7%
9.9% 59.1% 31.8%
0.21 0.87 0.46
N. Long et al. calcification of the CTM, the presence of calcification within the adjacent cartilaginous structures of the larynx, such as the thyroid and cricoid cartilages, is potentially advantageous in locating the CTM as such calcifications are bright and echogenic on ultrasound. In our study, while increased calcification was associated with increased age, there was no consistent relationship between calcification in the thyroid and cricoid cartilages. This is consistent with data from imaging studies which suggest that thyroid cartilage calcification is chronologically independently of that in the cricoid cartilage.18 In a large study of lateral cepahalometric radiographs in patients aged 10–59 years, Mupparapu and Vuppalapati demonstrated that ossification of the laryngeal cartilages was uncommon in the first two decades of life.23 In the fourth decade, however, ossification was present in 36.5% and 24.3% of thyroid and cricoid cartilages in females and 75% and 45% in male subjects, respectively.23 These data suggest that cartilaginous calcification is not present sufficiently consistent to be a reliable sonographic landmark for use of ultrasound in locating the CTM particularly in younger age groups. Despite a relatively high rate of failed intubations, emergency surgical airway access is rarely performed in obstetric anaesthesia practice. This is probably because of the use of neuraxial anaesthesia and the frequent success of supraglottic airway devices when failure to intubate occurs. Emergency surgical access was performed in one obstetric patient in the UKOSS study,1 two in NAP42 and in one patient in Palanisamy’s series.5 In two cases the procedure was performed by an ENT surgeon, a luxury that will not always be available at short notice in an obstetric unit. Both obstetric patients in the NAP4 study were obese. In one of these, the attempted narrow-bore needle cricothyroidotomy failed twice. Of all 58 attempted emergency surgical access attempts in the NAP4 study, six patients died, three attributable to failure of the technique. In 25 cases, the primary attempt was made by an anaesthetist and only nine were successful. In most of these cases, a narrow bore cannula was used. Among the listed causes of failure was ‘‘misplacement’’ which has the capacity not only to fail to improve oxygenation but to increase the likelihood of airway injury.24 The high prevalence of obesity in both obstetric and non-obstetric series, previously described has a number of implications. The CTM may not be easily identifiable and the airway devices may not be long enough to reach the airway lumen. This study has a number of limitations. Firstly, radiologic reconstruction may not have the precision of anatomic measurement of cadaveric dissection and examination but does have the advantage of access to large numbers of patient images which increases the generalizability of the data compared to smaller cadaveric series. Secondly, the studies were performed in the anatomically neutral position and any dynamic change
33 that might be seen with head extension was not assessed. Third, these studies were performed to investigate disease which might potentially distort neck anatomy. Last, none of the studies were performed on pregnant patients who may have increased neck dimensions due to the normal physiological changes of pregnancy. In conclusion, this study demonstrates that the CTM is not invariably a subcutaneous structure and may explain at least partially the high failure rate for emergency cricothyroid oxygenation seen in the recent literature. Furthermore, the vertical height of the CTM may be quite small and clinicians should be prepared to use devices with narrower diameters than commonly recommended for emergency cricothyroid access.
Disclosure This study received departmental funding only and the authors have no conflicts of interests to declare
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