Evaluation of Functional Dimensions of Macintosh Blade During Laryngoscopy in Patients With Normal Mouth Opening: Development of a Feasibility Model for Facilitation of Laryngoscopy in Cases of Mouth Opening Restriction Due to Submucous Fibrosis

Upper airway access is one of the essential components of general anaesthesia, and more often than not, it is gained by direct laryngoscopy followed by tracheal intubation. While direct laryngoscopy depends on laryngoscope blade design and the anesthetists' skill set, the ease of tracheal intubation is influenced by the line-of-view to the glottis gained by the laryngoscopy maneuver. In patients with normal mouth opening (≥4.0 cm), the act of conventional airway access is not difficult given that the neck extension and the TMJ joint mobility is within normal limits. Conversely, it becomes difficult in patients who have mouth opening restriction, and hence they are adjudicated as 'anticipated difficult airway' (ADA). The ADA, attracts several issues that, not uncommonly, converges on to time inefficiency, heightened equipment cost, operator anxiety, and unfortunately, patient morbidity. There are many causes of restricted mouth opening, defined technically as reduced inter-incisor distance (IID) including trauma, TMJ movement limitation, intra/extra-oral pathology resulting in pain, or the most common pathology in the Indian subcontinent, oral submucosal fibrosis (SMF). While the pathology itself is a forerunner of oral mitosis, it is the associated restricted mouth opening component that put conventional airway access efforts of the anaesthesiologist in spot, and consequently, a whole, sometimes unnecessary rigmarole has to be activated under the heading of 'advanced approach to ADA'.

Oral submucous fibrosis (SMF) is a well-recognized, potentially malignant progressive condition afflicting the oral cavity that is characterised by a reduced mouth opening. The reduction in mouth opening occurs due a vertical reorientation of collagen fibrils that are normally haphazardly arranged. Of the currently available multiple classifications to identify disease stage/severity, the functional classification based on the inter-incisor distance (IID) seems to be most relevant.

IID forms an important component of airway examination and a value of less than 3-cm is suggestive of difficulty in airway management. However, despite the above, SMF is largely unrecognized as a cause of airway difficulty, and even when recognized, no set protocol is available to ensure (as far as possible) a favourable outcome in terms of least morbidity to the patient, and stress to the anaesthesiology team in question.

Need Assessment for the Study Generally, in a tertiary level centre, with all facilities at hand, cases with a reduced IID are normally tackled using a fiberoptic bronchoscope guided tracheal intubation. However, in India, the burden of this disease lies to a great extent in rural/suburban settings where access to a fiberoptic bronchoscope is unlikely. As a result these cases are referred to 'higher' centres for management. This imposes undue burden (logistics/ financial) onto patients and their families. Hence, there is a great need for anaesthesiologists working at the primary/secondary level healthcare set up to get oriented to this problem and be able to manage it with conventional options (direct laryngoscopy & intubation) available to them.

Scientific Contention Direct laryngoscopy remains the foremost among the various techniques available to access the upper airway and the Macintosh blade has been the most preferred blade type. In order to improve direct laryngoscopy, many modifications of blade design have been tested/ compared with the Macintosh blade, but apparently, the clarity as to which laryngoscope blade is most suited for a given airway remains low. For the majority of practicing anaesthesiologists, the availability of different laryngoscope designs has only increased the hit-and-trial option for laryngoscopy.