Identification of thoracic intervertebral spaces by means of surface anatomy: a magnetic resonance imaging study

Background: Successful thoracic epidural analgesia depends on the sensory blockage of specific dermatomes following appropriate placement of the epidural catheter. This study aimed to ascertain how accurately anaesthesiologists identify thoracic intervertebral spaces, and whether counting from the prominent vertebra is easier than using the iliac crest as an anatomical landmark. Methods: Five anaesthesiologists attempted to locate one out of five consecutive intervertebral spaces (Th-7-Th-8 to Th-11-Th-12) on patients referred for magnetic resonance imaging of the vertebral column. The intended thoracic interspace and the counting reference point (C-7-Th-1 or L-3-L-4) were marked with oil capsules. The body mass index, gender and position of the patient were recorded. The exact capsule positions were determined by a radiologist after the study. Results: In 92 patients, 26.7% of the thoracic interspaces were correctly identified. The counting reference point was the only variable studied with a significant influence on error. The accuracy increased when the iliac crest was used as an anatomical landmark rather than the prominent vertebra (odds ratio, 0.29). The majority (76.4%) of all the incorrectly placed capsules were found cephalad to the intended level. Conclusion: We recommend that the caudal of two to three possible interspaces should be used when placing an epidural catheter in the thoracic spine. Because of the inaccurate localization of the thoracic intervertebral spaces, documentation should state the site of puncture as being in the upper or lower thoracic spine instead of claiming to be in an exact interspace.