Differences in lumbar dural sac dimension in supine and lateral positions in late pregnancy: a magnetic resonance imaging study

• Changes in position alter the dimension of the dural sac in late pregancy.
• The axial section area of the dural sac is smaller in the supine than the lateral position.
• The transverse diameter of the dural sac is smaller in the supine than the lateral position.
• These changes may affect the spread of spinal anesthesia when changing patient position.

BackgroundThis study was designed to quantitatively investigate differences in lumbar dural sac dimensions between the lateral and supine positions in late pregnancy.MethodsTen healthy volunteers with singleton pregnancies at 28–39 weeks of gestation were included. Magnetic resonance imaging was performed in random order while subjects were in the left lateral and supine positions. Lumbosacral axial scans were obtained at the L1–2, L2–3, L3–4, L4–5 and L5–S1 intervertebral disc levels. The axial section area, anteroposterior maximum diameter and transversal maximum diameter of the dural sac were measured and differences between these parameters in the lateral and supine positions were compared.ResultsThe axial section areas of the dural sac at L1–2 (P<0.001), L2–3 (P=0.001), L3–4 (P<0.001) and L4–5 (P=0.005) and the transversal maximum diameter of the dural sac at L1–2 (P<0.001), L2–3 (P<0.001), L3–4 (P<0.001) and L4–5 (P=0.001) were greater in the lateral position compared with the supine position. The anteroposterior maximum diameter of the dural sac at L4–5 was greater in the lateral position compared with the supine position (P=0.019) but there were no significant differences at other levels. The magnitude of the differences in axial section area and transverse maximal diameter were similar among the levels studied.ConclusionsThe axial section area and the transversal maximum diameter of the dural sac in the lumbar area are reduced in the supine compared with the lateral position in late pregnancy.