Skin to cervical epidural space distances as read from magnetic resonance imaging films: consideration of the "hump pad.".

ABSTRACT

STUDY OBJECTIVE: To measure the distances from the skin to the epidural space (DSES) of the lower cervical and upper thoracic intervertebral spaces. DESIGN: Retrospective review of films of the cervical spine as obtained by magnetic resonance imaging (MRI). SETTING: Health care facility that provides diagnosis and treatment of patients with chronic pain. MEASUREMENTS AND MAIN RESULTS: MRI sagittal films of 100 patients, who had diagnostic studies for chronic headaches and cervicobrachial radiculopathy, were reviewed. Measurements were made of DSES, the dural sac, and the spinal cord by centimeter ruler. Estimates were also made of the width of the epidural space by measuring the distance from the ligamentum flavum to the dural sac. The longest DSES were noted at C6-7 and C7-T1 levels, with a mean of 5.7 cm, but they decreased to a mean of 5.4 cm at the T1-2, and to 4. 7 cm at the T2-3 intervertebral spaces. One of the major factors in this variability was the presence of an accumulation of fatty tissue along the lower cervical and upper thoracic area, which the authors named the "hump pad." This accumulation appears to be thicker in obese patients, with a slight correlation coefficient with the patient's weight. The distances from ligamentum flavum to dural sac, representing the depth of the epidural space, averaged 0.3 cm, 0.4 cm, 0.5 cm, and 0.4 cm, respectively. CONCLUSIONS: In the cervical spine, DSES varies from space to space. In obese individuals, the fat pad may increase DSES at the lower cervical intervertebral spaces. The longest mean distances from the ligamentum flavum to the dural sac and to the spinal cord were found at the T1-2 and T2-3 levels, precisely where DSES is shorter. All things being equal, the upper thoracic intervertebral spaces appeared to provide a greater margin of safety for insertion of epidural catheters to treat cervicobrachial radiculopathies.