|Year : 2016 | Volume
| Issue : 3 | Page : 300-306
Upper airway imaging and its role in preoperative airway evaluation
Jagadish G Sutagatti1, Madhuri S Kurdi2
1 Department of Radiodiagnosis, Karnataka Institute of Medical Sciences, Hubli, Karnataka, India
2 Department of Anaesthesiology, Karnataka Institute of Medical Sciences, Hubli, Karnataka, India
|Date of Web Publication||17-May-2016|
Jagadish G Sutagatti
Department of Radiodiagnosis, Karnataka Institute of Medical Sciences, Hubli, Karnataka
Source of Support: None, Conflict of Interest: None
Ultrasonography (USG) is well-known as a fast, safe, and noninvasive technique. Its application for imaging of the airway is now gaining momentum. The upper airway has a complex anatomy, and its assessment forms a vital part of every preanesthetic evaluation. Ultrasound (US) imaging can help in upper airway assessment in the preoperative period. There are various approaches to upper airway USG. The technique has its own advantages, disadvantages, and limitations. This simple yet challenging imaging technique is all set to become an important part of routine preoperative airway evaluation. This article reviews the various approaches to upper airway US imaging, interpretation of the images, limitations, and disadvantages of the technique and its varied clinical applications in the preoperative period. The scientific material presented here was hand searched from textbooks and journals, electronically from PubMed, and Google scholar using text words.
Keywords: Airway, preoperative period, ultrasound imaging
|How to cite this article:|
Sutagatti JG, Kurdi MS. Upper airway imaging and its role in preoperative airway evaluation. Med J DY Patil Univ 2016;9:300-6
| Introduction|| |
The upper airway is an extraordinarily complex anatomical region.  Anticipating and preparing for difficulty in airway management is crucial to avoid airway catastrophes.  Routine preoperative airway examination usually includes an assessment of mouth opening and dentition, Mallampati classification, thyromental distance measurement, and evaluation of neck mobility. These methods are quickly and easily performed at the bedside, but, unfortunately, their sensitivity and specificity for accurate prediction of difficulty with airway management is not very robust.  The ultrasound (US) has been in clinical use since the early 1900s, and its use in airway management has been published since then.  Ultrasonography (USG) of the upper airway is capable of providing detailed anatomic information and has numerous potential clinical applications.  It can be used to identify airway pathology and may assist other methods in prediction of difficulty with airway management. 
The term "airway" in its daily usage refers to the upper airway which may be defined as an extrapulmonary air passage, consisting of the nasal oral cavities, pharynx including nasopharynx, oropharynx and hypopharynx, larynx, trachea, and large bronchi.  A "difficult airway" is one in which there is a problem in establishing or maintaining gas exchange via a mask, an artificial airway or both.  This article reviews various preoperative airway assessment approaches by the use of US. The scientific material presented here was hand searched from textbooks and journals, electronically from PubMed, and Google scholar using text words.
| Technique of Upper Airway Ultrasound Imaging and Basic Principles|| |
Accurate interpretation of US images involves important steps like proper transducer selection, reviewing the relevant anatomy of the region being imaged, considering the physics of the US beam, and then identifying the target structures. , The important airway structures visible on US are the mouth, tongue, larynx, vocal cords, cricothyroid membrane, cricoid cartilage, trachea, esophagus, stomach, lungs, and pleurae. 
There are two approaches to upper airway ultrasounding: The transcutaneous approach and the intraoral technique.
As done in some studies,  a systematic sonographic examination of the upper airway from the floor of the mouth to the suprasternal notch can be conducted using either a linear or curved transducer oriented in one of three planes: Sagittal (longitudinally in the midline), parasagittal (longitudinally lateral to the midline), and transverse (transversely across the anterior surface of the neck). Standard 7.5 MHz linear probe and 5 MHz curved array probe are commonly used for visualization of superficial and deeper structures of the airway, respectively. The curved low frequency transducer (~4 MHz) is most suitable for obtaining sagittal and parasagittal views of the tongue and structures in the submandibular and supraglottic regions.  The patient should be placed in the supine sniffing position with a pillow under the occiput to achieve optimum head extension and neck flexion.  Bony structures (e.g., the mandible and hyoid) will be brightly hyperechoic with an underlying hypoechoic acoustic shadow. Cartilaginous structures (e.g., the epiglottis, thyroid cartilage [TC], cricoid cartilage, and tracheal rings) will appear hypoechoic and their intraluminal surface will be outlined by a bright hyperechoic linear air-mucosa interface [Figure 1]. The vocal cords can be readily visualized through the TC and are seen forming an isosceles triangle with a central tracheal shadow. , The true vocal cords appear as two triangular hypoechoic structures [Figure 2]. The false vocal cords lie parallel and cephalad to the true cords and are more hyperechoic in appearance. ,
|Figure 1: Parasagittal section between the thyroid and hyoid cartilages showing the thyroid cartilage (TC), epiglottis (E), and base of the tongue (BOT)|
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|Figure 2: Axial section at the level of the vocal cords showing the thyroid cartilage, the true vocal cords (TVC), and the false vocal cords (FC)|
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The epiglottis in the transverse and parasagittal views through the thyrohyoid membrane is visible as a hypoechoic curvilinear structure. , It can be easily identified in almost all individuals in the transverse plane with a varying cephalad or caudal angulation of the linear transducer. Due to acoustic shadowing by the hyoid bone it is not easily visualized in the parasagittal plane. Extended submandibular sagittal view using a curved transducer can also identify it. Identification of the epiglottis can also be facilitated by tongue protrusion and swallowing.  In a study, epiglottis could be visualized in 90% cases in transverse view and 70% in parasagittal view  [Figure 3].
|Figure 3: Axial section between thyroid and hyoid bone showing the thyrohyoid membrane (THM), epiglottis (E), and the oropharynx (PH)|
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Both transverse and sagittal views can be used for visualization of the hyoid bone. It is visible on the transverse view as a superficial hyperechoic inverted U shaped linear structure. The cricoid cartilage has an oval hypoechoic appearance in the parasagittal view. The cricothyroid membrane is seen on sagittal and parasagittal views as a hyperechoic band. The tracheal rings (T1, T2, and T3) resemble a string of beads on the parasagittal and sagittal views and form an inverted U on the transverse view , [Figure 4].
|Figure 4: Sagittal section showing the tracheal rings (R) which have a "string of beads" appearance|
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The esophagus is seen in the oblique transverse view at the level of the suprasternal notch posterolateral to the trachea. 
In the transverse view, the thyroid gland can be visualized anterolateral to the trachea  [Figure 5].
|Figure 5: Axial section at thyroid level showing trachea (TR) in the center, both lobes of thyroid (left lobe and right lobe) on either side and the thyroid isthmus (IST) anteriorly|
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Intraoral sublingual approach
This technique may overcome the difficulty of maintaining good probe-skin contact over the uneven and curved surface of the anterior neck by the transcutaneous approach. A small footprint, high frequency curved array probe is placed into the sublingual fossa, intraorally. A longitudinal view of the larynx can be obtained by placing the probe sagittally and longitudinally under the patient's tongue. Excellent probe-tissue contact is achieved. The images of the larynx can be optimized further by tilting the probe upward and pointing it caudally. The technique is challenging to perform in uncooperative patients (e.g., young and pediatric) and is being evaluated as an adjunct to airway assessment in adults. 
| Limitations of Upper Airway Ultrasonography|| |
- Airway structures are filled with air. Because of high acoustic impedance, intraluminal air does not transmit US signals properly and produces a poor US image. Air will appear as hyperechoic artifacts which in turn can obscure US visualization of any structure posterior to it.  Hence, we can view partial to complete anterior and lateral wall of the airway structures, but cannot view the posterior wall.  (e.g., the posterior pharynx, posterior commissure and posterior wall of trachea), and posterior wall of trachea.
- The epiglottis is suspended in air. So, visualization of the entire epiglottis is difficult.  Singh et al. could visualize all the airway structures except the epiglottis in a feasibility study. 
- Intraluminal air and acoustic shadowing of hyoid bone can obscure visualization of the epiglottis. ,
- Calcification of the TC, cricoid arch, and larynx prevents visualization of the vocal cords, especially in males and old age people. ,
- US cannot show the resolution of specific muscles of the lateral pharyngeal wall (LPW). 
- The fat deposit in the neck region cannot be differentiated from other soft tissue structures. Hence, it may not be possible to evaluate the association of fat deposits with obstructive sleep apnea (OSA) by USG. 
- The correct interpretation of USG images requires a sound knowledge of sonographic anatomy, otherwise the acoustic artifacts can be mistaken for abnormal structures. The superficial structures can be interpreted quite easily, but the interpretation of deeper structures can still be quite challenging.  US visualization is indirect and images are subject to individual interpretation depending on the equipment being used and individual experience and training. Anesthesiologists may encounter considerable difficulties during identification of target structures when learning US techniques which may lead to frustration and failure. 
- During US examination, the neck of the patient has to be slightly extended in order to allow more space for scanning with the transducer. This expands the acoustic window for visualization of the internal carotid artery and the LPW. This extended neck position can make the subject's head slightly elevated, and this might inadvertently affect the degree of narrowing of the airway. This could affect the measurement of the adjacent soft tissue structures. 
- USG is an operator-dependent technique. 
- There are only a few studies on sonography of the airway structures for the purpose of airway assessment.  Prediction of the difficult airway by the use of US imaging is a research area. Further studies are needed in this area.  At present, the exact sensitivity and specificity of US for prediction of difficulty with intubation is not known.
| Clinical Applications of Upper Airway Ultrasounding in the Preoperative Period|| |
1. US can be used to quantify anterior neck soft tissue. US quantification of the anterior neck soft tissue has been used to predict difficult intubation.  The distance from the skin to the anterior aspect of the trachea is measured at three levels: Vocal cords, thyroid isthmus, and suprasternal notch. The amount of soft tissue at each level can be calculated by averaging the amount of soft tissue in millimeters obtained in the central axis of the neck and 15 mm to the left and right.  As concluded in a study in 50 morbidly obese patients, an abundance of pretracheal soft tissue at the level of the vocal cords as quantified by US is a good predictor of difficult laryngoscopy in obese patients. , The measurement of pretracheal soft tissue at the level of the vocal cords was found to be a good predictor of difficult laryngoscopy in the Middle Eastern obese patients in Israel. 
- A pilot study showed that laryngoscopy was difficult in patients with increased sonographic thickness of the anterior neck soft tissue at the level of the hyoid bone and the thyrohyoid membrane.  However, few authors have reported that pretracheal soft tissue thickness measured via USG is not an indicator of difficult laryngoscopy in morbidly obese patients. 
- USG can be used to obtain certain airway measurements preoperatively to predict difficult intubation. Several sonographic parameters are useful as predictors of difficult laryngoscopy and intubation.  An oblique transverse US view of the airway in the preoperative holding area can help to obtain measurements like the distance from the epiglottis to the midpoint of the distance between the vocal folds and depth of the preepiglottic space. Both these measurements were found to have a strong correlation with the Cormack-Lehane grading for airway assessment thus suggesting that noninvasive USG can be used to supplement presently available modalities of preanesthesia airway assessment including the Mallampati classification.  Hyomental space measurements using USG can be useful in predicting difficult intubation.  In a study on five obese patients and seven morbidly obese adults, a submandibular sonographic examination was performed in the supine position with a curved, low frequency transducer. The hyomental distance was measured while the head was placed in the neutral position, and the ratio of the two distances (hyomental distance ratio) was calculated. The sonographic hyomental distance ratios in the difficult intubation group were in the 1-1.05 range and those in the easy intubation group were in the 1.12-1.16 range, thus showing that a preoperative assessment of the hyomental distance ratio may predict difficult laryngoscopy resulting in difficult intubation. 
- USG can be useful in the,  evaluation of soft tissue masses before intubation. Pharyngeal or laryngeal pathology, such as tumors, abscesses, or epiglottitis which may have a significant effect during airway management, are detected by the use of the US scan. ,,,,
- OSA is often a contributor to difficulties in managing the upper airway.  In patients with a history of sleep apnea, upper airway USG has been used to visualize approximation of the tongue base posteriorly and inferiorly toward the hypopharynx to cause airway obstruction.  US measurement of the width of the tongue base has been found to correlate with the severity of sleep related breathing disorders.  Enlargement of the soft tissue structures, particularly the LPW is associated with an increased likelihood of OSA among patients presenting to sleep disorder centers.  The thickness of the LPW can be measured in an oblique coronal plane by sonography. Patients with OSA have significantly thicker LPWs on USG measurements.  A study on patients with OSA showed that sonographic measurement of LPW thickness is a novel and reliable technique to assess the severity of OSA. 
- USG can be used to assess the diameter of the upper airways. It is a reliable tool for determining the narrowest diameter in the cricoid lumen.  In healthy patients, the narrowest diameter of the subglottic upper airway is the width of the air column at the level of the cricoid cartilage. This diameter governs the selection of the endotracheal tube (ETT) size, as excessive tube diameter may damage the tracheal mucosa leading to post extubation stridor or subglottic stenosis.  Preoperative measurement of subglottic diameter can avoid intubation related complications of either trauma or ineffective ventilation.  Airway USG can be used to predict and calculate the appropriate size (diameter) of endotracheal, endobronchial, double lumen, or tracheostomy tubes. ,, Measurement of the outer tracheal width by USG can be a useful method for predicting the diameter of the left main bronchus and for selecting a left side double lumen bronchial tube.  A study in 19 healthy adults found that USG allows measurements of the air column width at the level of the cricoid cartilage with a precision of 0.33 mm and that the transverse cricoid lumen diameter is smaller than the anteroposterior diameter thus showing that  USG is a reliable tool to assess the smallest diameter of the cricoid lumen that is, its transverse diameter. The results suggested that USG may help to select the proper ETT size, particularly in infants and children.  US may be useful to evaluate patients with subglottic stenosis, a common disorder in neonatal or pediatric anesthesia and intensive care. ,, However, in a study on nine patients aged 1-20 years, USG was compared with the ETT sizing method and it was noted that USG was unable to provide an accurate measurement of the subglottic diameters.  A study in children aged 1-72 months undergoing general anesthesia showed that anterior neck USG measured outer diameter-ETT at the subglottic diameter was in good agreement with the actual outer diameter-ETT, thus suggesting that USG measured subglottic diameter helps to choose the appropriate ETT diameter for the child's neck.  In some studies in children, US measurement of subglottic upper airway diameter better predicted appropriately sized ETT than traditional formulas using age and height. , However, USG measures only the transverse diameter of the trachea at one level. Also, the external diameter of the tracheal tubes varies according to the manufacturer, and thus the tracheal tube size used must be assessed on an individual basis. Also, the diameter measurements are subjected to variation and are time consuming. These shortcomings can lead to inappropriate tube selections made by USG in 40% of cases. 
- In all difficult airway patients, it is important that the anesthesiologist should identify the cricothyroid membrane preoperatively because, in all airway algorithms, the final escape if everything fails is always oxygenation through the cricothyroid membrane. Localization of the cricothyroid membrane is often difficult or impossible by inspection and palpation (eg: a huge thyroid swelling, Ludwig's angina, and history of radiation or pathology in the neck).  Airway USG can be used to identify the cricothyroid membrane in such cases. It allows preparation for emergency cricothyroidotomy, oxygen insufflation, distribution of local anesthetics, or retrograde intubation. 
| Advantages of Ultrasound Technique for Upper Airway Imaging|| |
The technique is reliable, fast, repeatable, and transportable.  It has no claustrophobic effects, is relatively safe, simple, noninvasive, and readily available. , Easily transportable US machines are now-a-days available. The technique is safe to use in pregnancy.  Laryngeal USG as a bedside tool in children and infants, especially, does not require strict immobility as opposed to magnetic resonance imaging (MRI) or computed tomography (CT) scan for which infants are often anaesthetized and intubated. The ETT may deform the laryngeal structures and, therefore, disturb the measurements in these cases. 
The technique is simple to learn. A total of approximately 15 procedures are required for operators to obtain reliable and reproducible measurements.  Recently some researchers have created a low-cost gel phantom training model qualitatively consistent with human anatomy to impart training in US airway guided assessment of the airway. 
| Comparison of Ultrasound with Other Techniques for Upper Airway Imaging|| |
Because of the superficial location of the larynx, USG offers images of higher resolution than CT or MRI.  Studies comparing different modes of the depiction of the airway have found an overall very good concordance between USG and direct cadaveric dissection.  Prasad et al. found that USG could visualize all of the upper airway structures as reliable as CT. 
Lakhal et al. compared the diameter of the cricoid lumen measurements using USG and MRI in healthy, young adults, and concluded that the diameter of the upper airway can be reliably measured using USG. 
Neck US measurements are as accurate as MRI for the quantification of fat depth.  A study suggested that upper airway USG can provide anatomic information that would not be evident on clinical screening indices used in the assessment of a difficult laryngoscopy. 
Some authors have found a strong correlation between tracheal widths as measured by USG and tracheal width and left main bronchus width as measured by CT. 
| Future Scope|| |
Currently, airway US imaging is a research area of great interest with promising preliminary findings.  Large, randomized studies are required to compare US with established imaging techniques such as CT, MRI, and with other gold standard techniques like direct laryngoscopy for airway assessment. 
| Conclusion|| |
USG is a simple, yet challenging technique useful as a tool for upper airway imaging. It has the potential soon to become an important part of routine preoperative airway evaluation.
The authors thank Dr. Ranjana Raja for all the technical help in preparing the manuscript.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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