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Refresher Courses - Saturday June 9 |
From the Department of Anesthesia, University of Iowa, Iowa, USA..
Address correspondence to: Dr. Michael Todd, Department of Anesthesia, The University of Iowa, 200 Hawkins Drive 6546 JCP, Iowa City, IA 52242-1009 USA. Phone: 319-356-2633; Fax: 319-356-2940; E-mail: michael-todd{at}uiowa.edu
ANESTHESIOLOGISTS manipulate the cervical spine (Cspine) every day of their lives (during endotracheal intubation and patient positioning) and they deal frequently with patients having Cspine disease. Nevertheless, few anesthesiologists clearly understand the anatomy and biomechanics of the Cspine, its motion during airway manipulation, the clinical implications of various disorders etc. The primary purpose of this presentation is to briefly review the anatomy and motion of the normal Cspine and the movements that occur during routine direct laryngoscopy and intubation. In addition, I will also discuss several relevant disease states, and close with some comments regarding stabilization methods.
Anatomy
It is conceptually easiest to divide the neck into two portions: 1) the subaxial spine (below the "axis" of C2) and 2) the upper Cspine (which includes the skull base). The vertebrae of the subaxial spine are anatomically similar to other "normal" vertebrae, having a distinct vertebral body, lamina, spinous process, tranverse elements etc. The vertebrae of the upper Cspine are quite different. Its almost impossible to describe their structure in words, or adequately show them in 2-D images, but a few key structures should be noted.
C1 (the Atlas): Basically, C1 is a simple ring, with large superior and inferior articular surfaces, which interact with the skull base above and C2 below respectively. It has no vertebral body and no spinous process.
C2 (the Axis): A very unusual structure. It's most unusual feature is a long, thumb-like extension of its vertebral body, which extends upward to pass through the anterior arch of C1. This is the dens or the odontoid process.
A labelled, high resolution lateral x-ray of a normal upper cervical spine is shown in Figure 1
. Several items are notable.
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The small, lucent space between the anterior aspect of the ondontoid and the back of the anterior arch (not marked). This space is termed the "anterior atlanto-dental interval" or AADI. A "reciprocal space" is from the back of the odontoid to the anterior aspect of the posterior arch of C1. This is the "posterior atlanto-dental interval" or PADI. The spinal cord moves through the PADI. Note also: Since C1 is a rigid ring, if the AADI gets larger, the PADI must get smaller. We will come back to this when we talk about atlanto-axial instability.
The distance between the posterior arch of C1 and both the occiput and the laminar elements (arch, spinous process) of C2. This is a neutral position film.
Motion
The cervical spine is designed to support the head and to permit maximal motion in three dimensions without damaging the spinal cord. The three axes of motion are: flexion/extension (floor to ceiling), lateral bending (shoulder-to-shoulder) and axial rotation (turning side-to-side). There is some general reduction in mobility with age, but in normal, middle aged individuals, our maximal head-shoulder ranges of motion are as follows.
Flexion/extension: 130–140°
Lateral bending:85–90°
Axial rotation:160–170°
This motion is not distributed uniformly along the entire Cspine. A few points are relevant:
O-C1 joints/ligaments are very "tight" and allow relatively little motion (remember this later): there is effectively no axial rotation, only 5–10 of lateral bending, and 20 of extension; there are only 5 of flexion. Basically, the only movement at O-C1 is extension (look up). This means that the space between the skull base and the posterior arch of C1 (in Figure 1
) can get small - but it can't get larger.
The joint capsules at C1-C2 are much looser and hence there is more freedom of motion. Almost all axial rotation occurs at C1-C2 , with the odontoid process forming the pivot point: when you turn someone's head to the side, nearly all the motion is at C1-C2. Watch someone with an O-C2 fusion. They can look up and down - but can't rotate very well.
Flexion and extension are roughly equal at C1-C2 (about 10 of each from neutral) but there is little side-to-side bending.
At C2-C3 and below, motion is more uniformly distributed; however, maximal motion occurs at C4-C5-C6, an observation which coincides with the much more common occurrence of injury at these levels.
Lateral bending (head on shoulder) is the result of roughly 5–10 of motion per segment below C2 (pure lateral bending is unusual; most commonly lateral bending is accompanied by some axial rotation).
There is very little anterior-posterior (AP) translational motion anywhere in the Cspine (sliding front to back).
Movement with intubation
Since the 1940's, we've read about the idea that to visualize the glottis and introduce an endotracheal tube, it is necessary to "align the pharyngeal and laryngeal axis". More recent work by Fred Adnet has clearly shown that this is a fallacy and was based on some erroneous (false?) observations. Nevertheless, to place a tube via direct laryngoscopy, you still must be obtain a "line of sight" view between your eye and enough of the glottis to allow placement of the endotracheal (ET) tube. This requires a complex series of movements. The following Figure 2 is taken from a series of cinefluoroscopic images we've obtained during direct laryngoscopy (DL) and intubation.
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There are a couple of clinical correlates of this observation. Most importantly, any intervention that impedes this combination of extension and flexion will make it more difficult to visualize the glottis. For example, surgical O-C1-C2 fusions make it extremely difficult (impossible) to successfully perform a DL. External "stabilization" methods (traction, manual stabilization, collars etc) may reduce movement during DL - but they will also make visualization more difficult.
Instability at C1-C2: rheumatoid disease and Down syndrome
Instability is defined as excessive translational or rotational motion of any vertebra. Instability can occur anywhere - and may be exceptionally difficult to diagnose. Because of the unique motion associated with DL, one form of instability is of particular interest. This is atlanto-axial instability, which means that the odontoid process is no longer firmly held against the back of the anterior arch of C1, due either to disruption of the transverse ligament, or from damage to the ondontoid process itself (e.g., fracture across the basis on the odontoid). Because of the unique forces/motion seen during DL, this disorder is particularly important to anesthesia.
The most common situation encountered by anesthesiologists is in patients with severe rheumatoid arthritis (RA) and patients with Down syndrome. In patients with severe/longstanding RA, there is destruction of multiple joints in the neck and to the transverse ligament. Roughly 30% of patients with severe disease will have some instability at C1-C2, although surgical correction is needed in relatively few. It is recommended that all patients with severe RA have periodic flexion/extension x-rays, certainly prior to any surgery. Similarly, roughly 15% of patients with Down syndrome have laxity in the transverse ligament. It is recommended that all Down's patients have x-rays at ages three, 12 and 18, before any surgical procedure that requires DL or extensive neck manipulation, or prior to engaging in vigorous sports.
The mechanical problem with AA instability relates to: a) the fact that C1 is relatively rigidly affixed to the base of the skull; and b) C1 is a rigid ring. If the transverse ligament is damaged, lifting the skull and C1 will result in an increase in the AADI - and hence a decrease in the PADI. In other words, C2 remains "fixed" while C1 slides anteriorly, with the cord becoming compressed/trapped in the space behind the odontoid (small arrows, below, Figure 3).
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Traumatic injuries do not occur with equal frequency at all locations; they are much more common at C5 and C6. One of our greatest worries is the failure to diagnose an injury in a neurologically intact individual. This has resulted in a series of "rules" about when Cspine diagnostic studies are needed in patients with blunt trauma (ALL patients with penetrating trauma require films). The general "rule" is that films are needed when any one of five criteria are met:
The value of these rules have recently been confirmed in a huge prospective study of over 30,000 emergency patients with blunt trauma (see reference Hoffman JR, et al.). Of those patients with one or more of the above-noted criteria, serious Cspine abnormalities were found in about 2%. Conversely, abnormalities were found in only 0.03% of patients (one of 4,309) with no such symptoms (and there were no sequelae in that patient). The problem, of course, is that positive findings are still uncommon; 100 x-rays or computed tonography scans are needed to detect two serious injuries. Nevertheless, given the consequences of undetected injury, this is reasonable.
Cspine injuries pose several problems for the anesthesiologist. The first is management of the patient with a known injury. The second concerns management of the patient in whom no films were ever obtained, due to surgical urgency. The third is the patient in whom films have been taken, but whose neck, for a variety of medical or bureaucratic reasons, has not been "cleared".
Under ideal situations, patients in all three categories would simply be managed by flexible fiberoptic laryngoscopy and intubation; no neck motion would ever be required. Unfortunately, this is not feasible in many situations. Instead, we are confronted by the combative or intoxicated patient with the potential full stomach in whom an awake fiberoptic intubation is not feasible, and in which an asleep intubation - which in most hands takes at least a minute or more - is not deemed acceptable. We are hence constantly asking "in which patient is it acceptable to do a DL and is there anything that can be done to minimize the risk of Cspine injury during such a DL".
There has been a reasonable amount of work concerning the impact of "stabilization" techniques including soft collars, hard (Philadelphia) collars, manual external stabilization, axial traction (hand-held or with Gardner-Wells tongs), and complete halo-vest (four post) fixation. It is impossible to review this literature entirely. However, a few points are worth mentioning:
Soft and hard collars are of minimal (no?) value in preventing Cspine motion during DL, certainly motion of the upper Cspine.
Manual stabilization will reduce movement to some degree - at the expense of more difficult intubation.
Axial traction also reduces extension at 0-C1 - but again impedes visualization. More important, even small amounts of traction may: a) distract a complete injury; and b) actually augment AP translational motion at this injury segment.
Near perfect stabilization can be achieved with halo-fixation. However, intubation via DL may be near impossible.
What about alternative airway management techniques? There has been great interest in the use of airway management methods that do not require Cspine movement - or at least which don't require the same motions as DL. They include the esophageal combitube, the laryngeal mask airway (LMA) and the intubating LMA (ILMA). There is relatively little information available regarding the use of these in the presence of severe Cspine injuries. It is probably incorrect to assume that, since head extension/flexion is not required for placement, that these devices are safe. For example, inflation of the large pharyngeal cuff of the combitube exerts a great deal of pressure against the vertebral bodies of C2, C3 and/or C4, depending on placement. Similarly, insertion of the LMA or ILMA results in pressure against C2/C3. Manipulation of the ILMA to facilitate ET insertion also may (theoretically) create problems. Unfortunately, while we know of cases in which DL results in spinal cord injury, experience with these alternative methods is extremely limited - and hence no data exists to determine whether or not they offer advantages. Each of these methods may be deemed appropriate in certain situations - but beware of the unfounded belief that they are "better" than any other method.
Summary comments
There are no easy answers regarding the management of the patient with severe cervical spine disease/damage. If fiberoptic laryngoscopy is possible (using either a flexible scope, Bullard/Wu scope or other method) it is probably indicated. If this is not possible, then the technique with which the anesthesiologist is most familiar is indicated. Manual in-line stabilization is probably medico-legally advisable - but will not prevent motion of a severely disrupted spine. Moreover, all stabilization methods make DL/intubation more difficult (anything which limits extension at the upper spine and flexion of the lower will make visualization more difficult).
Further Reading
Penning L. Normal movements of the cervical spine. Amer J Roentgenology 1978; 130: 317–26.
Majernick TG, Bieniek R, Houston JB, Hughes HG. Cervical spine movement during orotracheal intubation. Ann Emerg Med 1986; 15: 417–20.[Medline]
Crosby ET, Lui A. The adult cervical spine: Implications for airway management. Can J Anaesth 1990; 37: 77–93.[Abstract]
Hastings RH, Marks JD. Airway management for trauma patients with potential cervical spine injuries. Anesth Analg 1991; 73: 471–82.[Medline]
Hastings RH, Kelley SD. Neurologic deterioration associated with airway management in a cervical spine- injured patient. Anesthesiology 1993; 78: 580–3.[Medline]
Hastings RH, Wood PR. Head extension and laryngeal view during laryngoscopy with cervical spine stabilization maneuvers. Anesthesiology 1994; 80: 825–31.[Medline]
Hastings RH, Vigil AC, Hanna R, Yang BY, Sartoris DJ. Cervical spine movement during laryngoscopy with the Bullard, Macintosh, and Miller laryngoscopes. Anesthesiology 1995; 82: 859–69.[Medline]
Sawin P, Todd MM, Traynelis V, et al. Cervical spine motion with direct laryngoscopy and orotracheal intubation: an in vivo cineflouroscopic study. Anesthesiology 1996; 85: 26–36.[Medline]
Watts AD, Gelb AW, Bach DB, Pelz DM. Comparison of the Bullard and Macintosh laryngoscopes for endotracheal intubation of patients with a potential cervical spine injury. Anesthesiology 1997; 87: 1335–42.[Medline]
Cervical spine research society. Clarke CR (Chair). The Cervical Spine. 3rd ed. Philadephia: Lippincott- Raven, 1998: (see in particular chapters 3–4).
Taggard DA, Menezes AH, Ryken TC. Instability of the craniovertebral junction and treatment outcome in patients with Down's syndrome. Neurosurgical Focus 1999; 6: article 3.
Keller C, Brimacombe J, Keller K. Pressures exerted against the cervical vertebrae by the standard and intubating laryngeal mask airways: A randomized, controlled, cross-over study in fresh cadavers. Anesth Analg 1999; 89: 1296–1300.[Abstract/Full Text]
Brockmeyer D. Down syndrome and craniovertebral instability. Pediatr Neurosurg 1999; 31: 71–7.[Medline]
Kihara S, Watanabe S, Brimacombe J, Taguchi Y, Yamasaki Y. Segmental cervical spine movement with the intubating laryngeal mask during manual in-line stabilization in patients with cervical pathology undergoing cervical spine pathology. Anesth Analg 2000; 91: 195–200.[Abstract/Full Text]
Lennarson PJ, Smith D, Todd MM, et al. Segmental cervical spine motion during orotracheal intubation of the intact and injured spine with and without external stabilization. J Neurosurg 2000; 92: 201–6.[Medline]
Brimacombe J, Keller C, Kunzel KH, Gaber O, Boehler M, Puhringer F. Cervical spine motion during airway management: A cinefluoroscopic study of the posteriorly destabilized third cervical vertebrae in human cadavers. Anesth Analg 2000; 91: 1274–8.[Abstract/Full Text]
Hoffman JR, Mower WR, Wolfson AB, Todd KH, Zucker MI, for the National Emergency X-Radiography Utilization Study Group. Validity of a set of clinical criteria to rule out injury to the cervical spine in patients with blunt trauma. New Engl J Med 2000; 343: 94–9.[Abstract/Full Text]
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