To the Editor:—
Manual in-line stabilization (MILS) is employed during direct laryngoscopy in patients with known or potential cervical spine instability to try to stabilize the spine. A recent article by Santoni et al. evaluated how MILS affected the pressure against the tongue and jaw during direct laryngoscopy.1Pressure was measured with sensors attached to the upper surface of a Macintosh 3 blade. In nine anesthetized paralyzed patients, institution of MILS increased the pressures during laryngoscopy almost two-fold, as compared with pressures measured without MILS. Although MILS is intended to stabilize the cervical spine during laryngoscopy, the authors proposed that “secondary increases in pressure application with MILS have the potential to increase pathologic cranio-cervical motion.”
In the absence of MILS, upward and forward force exerted on the airway will be transmitted in some part to the cervical spine and the spine will move, as Dr. Todd and his group have demonstrated.2The force will also compress the tongue, contributing to exposure of the vocal cords with laryngoscopy.3However, it is not clear how much movement-generating force, if any, will be applied to the spine if MILS is instituted as described. Santoni et al. explain that MILS is performed by an assistant holding the patient's occiput and applying “forces equal and opposite to those created by the anesthesiologist.”1One might expect that matching laryngoscopy force with an equal and opposite force would result in no net force on the head, thus reducing the force and movement of the cervical spine, as compared with the situation with no MILS. In fact, Santoni et al. list the goal of the MILS as preventing or minimizing head and neck movement.
How MILS actually works in practice is another issue. The assistant performing the task is guided by feel without any measurement of force. Thus, MILS may not balance the force of laryngoscopy and may not minimize movement. The Santoni group may be correct that MILS does not accomplish the objective. However, they only measured the pressure on the laryngoscope and did not evaluate the force exerted by MILS . Thus, they do not know what the net force was and cannot say one way or another from this body of research whether MILS had the potential to reduce or increase the craniocervical motion.
The observation that MILS worsened glottic visualization, a finding also reported by other investigators, is interesting and could be an outcome of the way the MILS was executed. The increased force on the tongue with MILS should lead to greater compression of the tongue, increase the space in the airway and, if anything, improve the glottic view rather than impairing it. In a study examining simulated cervical spine precautions, we showed several years ago that having an assistant hold a patient's head firmly against the table during laryngoscopy significantly reduced the amount of head extension necessary to expose the vocal cords compared to the state with no head stabilization.4We suggested that less head extension was needed because the downward pressure on the head allowed the laryngoscopist to lift more forcibly and achieve greater displacement of the tongue from the field of view. Some additional factor must be operative in the Santoni study to worsen the view with MILS. Perhaps the main effort of the assistants was to resist head extension, rather than balance the upward force of laryngoscopy. Limited head extension would make laryngoscopy more difficult and could contribute to higher grade views. In our 1991 paper, the assistants did not oppose head extension.
We congratulate Santoni et al. on their highly relevant study. The degree to which MILS is beneficial in patients with potential cervical spine injury is an important and timely issue.5The application of physics and engineering principles to medical problems will help answer clinical questions such as this. We would welcome further research about how laryngoscopy and MILS impact the forces exerted on the spine and airway, and how best to implement MILS.
*VA San Diego Healthcare System and University of California, San Diego, California. firstname.lastname@example.org