A challenge in treating subglottic stenosis is assessment of airway caliber before and after management. At present, surgeons lack a simple, efficient, and precise method of measuring subglottic stenosis intraoperatively. We present a novel, easily reproducible tool for measurement of the diameter, location, and length of subglottic stenosis during suspension laryngoscopy.
Methods and Observations
A set of 5 Kirschner wires (30-cm length and 1.6-mm diameter) were bent 90° at both ends to produce a series of 10 short ends designed to measure airway diameter (0.3- to 2.1-cm length with 2-mm intervals). Short, bent ends of the measuring sticks were designed to measure airway diameter. Hash marks at 2-mm intervals were created along the long axis of the measuring sticks to measure subglottic stenosis length and location relative to the vocal cords. The measuring stick was tested in 10 adult patients undergoing suspension microlaryngoscopy for endoscopic treatment of subglottic stenosis between September 2012 and July 2013. The accuracy of the measuring stick was evaluated using an airway phantom. The measuring stick enabled easy and precise quantification of subglottic stenosis diameter (82.5% agreement with reference; interobserver agreement, r = 0.995; P < .001), length (72.5%; r = 0.995; P < .001) and location during suspension laryngoscopy.
Conclusions and Relevance
The measuring stick is inexpensive and simple to construct. It allows for safe, accurate, and practical measurement of subglottic stenosis diameter, length, and location during suspension laryngoscopy.
Subglottic stenosis (SGS) remains a diagnostic and therapeutic challenge for the otolaryngologist. Patients with SGS require long-term follow-up, and many cases require repeated interventions to maintain airway patency.1,2 A key component of diagnostic evaluation, assessment of treatment efficacy, and longitudinal follow-up of SGS involves gauging the severity of stenosis using quantitative measurements of airway dimensions.
Numerous methods to measure SGS intraoperatively exist. Myer et al3 proposed the first airway stenosis grading system using endotracheal tube dimensions as a measurement reference. Tubes that fit through the most stenotic segment of the airway and tolerated normal leak pressures (10-25 cm of water) were compared in diameter with the patient’s expected, age-appropriate endotracheal tube to determine the percentage of obstruction. This method allowed for estimation of airway diameter only. Measurement of airway luminal dimensions using quantitative endoscopic techniques, such as videobronchoscopy, has also been described.4,5 However, obtaining accurate measurements is challenging, and image scales must be corrected to account for endoscope optics and visuospatial distortion.6,7 Additional methods of intraoperative airway measurement include optical coherence tomography and ultrasonography, which can be limited by image artifact and shadows.8,9
In 2012, a research group at Washington University, St Louis, Missouri, described a mechanical device (Sliding Extension Measurement Tool) that measures subglottic diameter during suspension laryngoscopy.10 After placement of the distal end of the tool through a laryngoscope to the region of stenosis, airway diameter is measured by correlating an umbrellalike extension of a measuring spoke at the distal end with linear displacement (along the long axis of the instrument) of a user-controlled switch proximally. This measuring tool, while novel in design, is complex and may be difficult to duplicate.
Currently, surgeons lack an inexpensive, easily reproducible and efficient technique to measure SGS in 3 dimensions (3-D) during suspension laryngoscopy. This is critical because the surgeon’s real-time estimation of airway dimensions often drives selection of operative tools. This process may require interchanging and expenditure of balloons until an appropriate dilator is selected, increasing intraoperative time and cost. To answer this need, an inexpensive and practical instrument, referred to as the “measuring stick,” was created for measurement of SGS diameter, length, and location and under direct laryngoscopy.
Study subjects included 10 adult patients (ages ≥18 years) who underwent suspension microlaryngoscopy for endoscopic treatment of SGS performed by the senior author (S.P.V.) in a university voice and swallowing center between September 2012 and July 2013. Approval for this study was obtained from the institutional review board at the University of California, Irvine. All patients provided written informed consent for participation.
Construction of the measuring stick involved adaptation of 5 single trocar Kirschner (K) wires (KI-71-153; Key Surgical) measuring 22.9 cm in length and 1.6 mm in diameter. The K-wires were bent 90° at both ends to produce a series of 10 short ends ranging in length from 0.3 to 2.1 cm at 2-mm increments (Figure 1A). Bending of the K-wires resulted in a smooth 90° curve between the long axis and short end of the instrument. After bending, the total length of the short ends were measured and adapted to include the 90° curve and K-wire diameter along the long axis (Figure 2). If necessary, a wire cutter was used to clip the short end to the desired length.
A, Set of 5 Kirschner wires with a series of 10 bent short ends (ranging 0.3-2.1 mm in length) for measuring airway cross-sectional diameter. B, Hash marks at 2-mm intervals for measuring location and length of stenosis. Scale is in centimeters
The length of each short end is inclusive of the 90° bend and K-wire diameter (1.6 mm). The opposite end of the stick (9-mm short end) is not shown.
The ends of the K-wires were filed down to eliminate sharp edges. Hash marks at 2-mm intervals were created along the long axis of the K-wires using a razor blade (Figure 1B). All short end lengths and hash marks were measured with a ruler to ensure accuracy. Instruments were preoperatively sterilized using conventional steam sterilization methods. After sterilization, all short end lengths and hash mark distances were verified by ruler to ensure the measuring sticks had not undergone dimensional change secondary to saturated steam and pressure. The total cost for the set of 5 K-wires was approximately $17.00.
An operating room simulation was prepared, consisting of an operating table, endoscopy tower, and suspension laryngoscopy instruments. An airway phantom was constructed using a rigid polyvinyl chloride (PVC) pipe with a length of 10-cm and an inner diameter of 3.5-cm. A section of the inner surface of the PVC pipe was lined 360° with modeling clay to form a constricted lumen and simulate SGS. The phantom and a microlaryngoscope were mounted in 3-prong clamps to simulate suspension laryngoscopy. The clay thickness and length along the long axis of the pipe were manipulated by a researcher (G.K.S.) to create various degrees of SGS diameter and length, respectively. The narrowest anteroposterior diameter and total length of the clay within the phantom were measured by the researcher, as a gold standard, using a ruler under direct visualization. Twenty different SGS diameters and 20 different SGS lengths were created in the phantom. Two observers (A.F. and E.S. [listed in “Additional Contributions”]), blind to the reference measurements and to each other’s measurements, independently inserted a Hopkins rod telescope and measuring sticks through the laryngoscope to measure the anteroposterior diameter and length of stenosis. Each observer measured the diameter and length once for each of 20 unique SGS geometries. Reliability was assessed using the intraclass correlation. Statistical power was calculated using a 2-sided Fisher z test with a significance level of .05 and a null hypothesis of r = 0.8 (Pearson correlation coefficient). With a sample size of 20, the power to detect r = 0.995 (as observed) is greater than 99%. Statistical analysis was performed using SYSTAT v13.0 (Systat Software).
Following induction with general anesthesia, patients underwent suspension laryngoscopy with a microlaryngoscope (Ossoff-Pilling). Supraglottic jet ventilation was performed by the anesthesiologist. The surgeon’s (S.P.V.’s) nondominant hand guided a Hopkins rod telescope for visualization, while the surgeon’s dominant hand inserted the measuring stick through the laryngoscope. The measuring stick was advanced past the vocal folds so that the short, bent end of the wire was positioned at the region of the stenosis. When necessary, the measuring stick was swiftly interchanged until the length of the short end of the stick matched the diameter of the stenotic lumen (Figure 3A-B). Once an appropriately sized measuring stick was selected, lumen size was measured using the known length of the short end of the measuring stick. The tilt of the instrument relative to the long axis of the airway was manually adjusted under endoscopic visualization to ensure the short end was perpendicular to the airway axis. This method was used to measure both anteroposterior and transverse diameters.
Under endoscopic visualization, the total length of stenosis and the distance between the true vocal folds and the proximal limit of stenosis were measured using the hash marks along the long axis of the instrument. In the setting of nonuniform stenosis, luminal dimensions at any location along the long axis of the airway were measured by interchanging measuring sticks of different lengths. Following airway measurement, SGS was treated by microlaryngoscopic laser lysis and balloon laryngoplasty. Airway diameter measurements were repeated postoperatively (Figure 3C).
The level of exact agreement between reference (ruler) and observed (K-wire) measurements of SGS dimensions in the airway phantom was high for SGS diameter (82.5%) and SGS length (72.5%). The intraclass correlation coefficients for agreement with reference were for SGS diameter were r = 0.996 (P < .001) and r = 0.998 (P < .001) for observers #1 and #2, respectively (mean, r = 0.997). Intraclass correlation coefficients for agreement with reference for SGS length were r = 0.995 (P < .001) and r = 0.995 (P < .001) for observers #1 and #2, respectively. Interobserver agreement was r = 0.995 (P < .001) for SGS diameter and r = 0.995 (P < .001) for SGS length.
Patients with SGS often require long-term management with repeated interventions for relief of stenosis. At present, there is no gold standard algorithm for the management of SGS. The decision to treat and the type of intervention is individualized and largely dependent on clinical symptoms and, if applicable, response to prior treatments. While clinical presentation and office-based endoscopy guide management decisions, the precise treatment plan for patients meeting indications for endoscopic intervention is often determined intraoperatively following real-time estimation of SGS geometry. Furthermore, quantitative longitudinal data on SGS dimensions may help surgeons estimate the rate of disease progression for patients undergoing a repeated intervention.
Assessment of the airway in a controlled operative setting allows for safe and precise measurement of SGS. In contrast, SGS measurement in an outpatient setting often requires additional cost and a separate appointment for imaging and yields imprecise measurements that may not correlate with actual SGS dimensions at the time of intervention. Office-based transnasal tracheoscopy is a practical and inexpensive airway evaluation, which, however, does not allow for quantification of airway dimensions.11 High-resolution computed tomography can diagnose SGS and measure the airway lumen to within 1 mm.12 However, patients are exposed to ionizing radiation, and accuracy is limited by method of lumen segmentation and angulation of the airway axis relative to the scanning plane.13,14 Magnetic resonance imaging has inadequate spatial resolution for precise millimeter-scale measurements.15 Hence, a method for real-time, intraoperative airway measurement is valuable to guide surgical therapy based on precise, objective data.
Intraoperative measurement of airway stenosis allows the otolaryngologist to select appropriate operative tools and efficiently compare pretreatment and posttreatment airway diameters. Knowledge of the SGS diameter helps minimize operative time by preventing selection of an inappropriately sized balloon which, at present, can cost over $800 (INSPIRA AIR Balloon Dilation System; Acclarent Inc). Current options for intraoperative measurement of SGS have limitations. The Cotton-Myer grading system does not measure the length or location of stenosis.3 This technique is also associated with risk of mucosal injury from multiple intubation attempts and a reduced sensitivity of leak pressures in the event of nonconcentric stenosis. Construction of the SEMT requires expertise, approximately 8 weeks for material sourcing and labor, and high costs (estimated $400 for a disposable tool and $1200 for a nondisposable tool), since it is not commercially available.10 While optical coherence tomography and ultrasonography can potentially measure airway dimensions intraoperatively, these procedures are time consuming and require expensive equipment and operational proficiency.
Given its minimalist design, the measuring stick described in this study is inexpensive, easily reproducible, and simple to use. The tool is safe, with no sharp edges or loose parts; can be effectively sterilized for multiple uses; and has a low-profile design. In the event of high-grade or nonconcentric stenosis, the smallest measuring stick (0.3-cm and 0.5-cm bent ends) can be inserted completely through the stenotic segment to measure SGS length and distance from the vocal cords. A measurement lower limit of 3 mm underscores the applicability of the measuring stick in both pediatric and high-grade adult SGS cases. The normal subglottic diameter is 4.5 mm to 5.5 mm in the full-term neonate and approximately 3.5 mm in premature infants.2 A subglottic diameter of 4 mm or less in a full-term neonate or 3 mm or less in a premature infant is considered to be stenotic.2 Therefore, our measuring sticks would provide accurate measurements for most neonatal SGS cases. Additional measuring sticks may be constructed with a similar design using smaller gauge K-wires and shorter bent ends to allow for measurement of neonatal airway diameters less than 3 mm. However, further validation testing would be necessary for such instruments.
A potential drawback of the tool is that a measuring stick with greater than a 1.5-cm bent end was unable to be passed through the body of a standard Ossoff-Pilling laryngoscope. Although measurements that large are rarely necessary in our SGS cases, a laryngoscope that is less restrictive or 2 stacked measuring sticks with shorter bent ends can be used in this situation. In addition, the depth of subglottic scar tissue cannot be measured using the measuring stick. However, knowledge of scar thickness has less utility for guiding endoscopic intervention as opposed to the potential for guiding open laryngotracheal reconstruction (eg, deciding between slide tracheoplasty and tracheal resection). An imaging modality such as optical coherence tomography, which yields high-resolution cross-sectional images, may serve as a more effective diagnostic tool to evaluate scar thickness.
Intraoperative sizing of SGS helps the otolaryngologist make informed decisions and minimizes resource consumption. The use of the measuring stick bypasses the need for costly diagnostic imaging, which requires expert image analysis. Furthermore, airway measurements in the immediate postdilatation state and at future interventions can be compared to determine efficacy of intervention and rate of SGS progression, as well as be used to formulate individualized treatment plans.
The measuring stick is inexpensive and easy to duplicate. This simple instrument allows for safe, practical, and real-time intraoperative measurement of SGS diameter, location, and length.
Submitted for Publication: August 10, 2014; final revision received November 11, 2014; accepted November 20, 2014.
Corresponding Author: Sunil P. Verma, MD, University Voice and Swallowing Center, Department of Otolaryngology–Head and Neck Surgery, University of California Irvine, 62 Corporate Park, Ste 115, Irvine, CA 92606 (firstname.lastname@example.org).
Published Online: February 19, 2015. doi:10.1001/jamaoto.2015.29.
Author Contributions: Drs Sharma and Verma had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: All authors.
Acquisition, analysis, or interpretation of data: All authors.
Drafting of the manuscript: All authors.
Critical revision of the manuscript for important intellectual content: All authors.
Statistical analysis: Sharma, Foulad.
Administrative, technical, or material support: Foulad.
Study supervision: Foulad, Verma.
Conflict of Interest Disclosures: Dr Verma is a consultant for Acclarent, Inc, which, however, did not provide financial support for this study.
Previous Presentation: This study was presented as a poster at the 135th annual American Laryngological Association meeting at the Combined Otolaryngology Spring Meetings; May 14, 2014; Las Vegas, Nevada.
Additional Contributions: Karen Lopez, RN, and Rosa Aya Yamamoto, RN, at the University of California Irvine (UCI) Medical Center operating rooms assisted in the sourcing, sterilization, and storage of materials. Erica Su, BS, assisted with operating room simulation and instrument validation testing. Kathryn E. Osann, PhD, assisted with statistical analysis. Financial compensation was not provided.
Sharma GK, Foulad A, Verma SP. A Novel Device for Measurement of Subglottic Stenosis in 3 Dimensions During Suspension Laryngoscopy. JAMA Otolaryngol Head Neck Surg. 2015;141(4):377-381. doi:10.1001/jamaoto.2015.29