Trismus and the limits of laryngoscopy

The i-scoop showed improved performance over other laryngoscopes in Raymondos et al.'s study of intubation in ‘normal’ and ‘difficult’ manikins 1. The authors adapted their manikin to be ‘difficult’ by imposing ‘retrognathia and an immobile cervical spine’. These imposed difficulties contribute to the anatomical set, termed ‘limited spatial conditions’ 2 that are associated with difficult airway management using laryngoscopes. We write to emphasise that there is another member of this set, not reported by Raymondos et al., which may have profound implications for laryngoscopy whatever device is chosen: access through the mouth. Use of any available laryngoscope requires blade insertion (with the accompanying sheath) or ‘guiding bar’ (for the i-scoop) through the mouth, which then traverses the inter-dental gap. Poor mouth opening imposes severe limitations on the function of any laryngoscope. Devices with a smaller cross-sectional diameter, such as optical stylets or flexible fibrescopes, also have a limit, albeit lower. Moreover, the mouth opening must not only accommodate the device, but also the tracheal tube, whose external diameter (rarely specified in reports) may be larger than the device blade, bar or insertion cord. In Raymondos et al.'s study, mouth opening, as reflected by the distance between the teeth, was reported as 5.9 cm. The lower limit at which the oral or dental gap becomes truly limiting for their device is not discussed. We think this is important for two reasons: first, mouth opening is a fundamental consideration for effective laryngoscopy; and second, opening is often associated with other members of the anatomical set, such as prominent incisors with retrognathia, to form a common ‘constellation of difficulty’. A variety of conditions limit mouth opening. In some conditions, such as noma (canrum oris), high-grade or absolute trismus are common, rendering any technique requiring oral access either difficult or impossible. Indeed, for noma survivors: “in some cases the inter-incisor gap was ascribed a negative value, because the jaw tends to fix in a position with the incisors overlapped” 3. As anaesthetists for the Facing Africa charity, which provides free facial reconstructive surgery for survivors of noma (see www.facingafrica.org), we manage airways for many patients with trismus. In the series reported by Coupe el al., (also Facing Africa anaesthetists) 11/26 (42%) of their patients had a gap of ≤ 20 mm 3. In our experience, a reasonable limit to reliably successful blade insertion and subsequent tracheal intubation is an oral gap (imposed by tissue or teeth) of 20 mm. We usually use either the Glidescope® (Verathon Medical, High Wycombe, UK) or Mcgrath MAC® (Aircraft Medical, Edinburgh, UK) devices for these patients. For others, flexible fibreoptic management is our first choice. For industrialised nations, dental abscess is associated with trismus and airway difficulties 4. Raymondos et al. list limitations for their study. We agree with them that blood or secretions can degrade any laryngoscopic view, but extend this to emphasise that severe oro- pharyngeal contamination, especially bleeding, can render laryngoscopy impossible. Futile attempts may be a dangerous form of error fixation in some circumstances. We also agree with the authors that the i-scoop is a promising new device but manikin-based studies have limitations 5. This report is one stage in the evaluation process as suggested by Cook 6.