Are High Lumbar Punctures Safe? An MRI Morphometric Study of the Conus Medullaris

A high lumbar puncture (LP) at L2‐L3 or above is often necessary to consider on technical grounds, but complications of conus medullaris (CM) damage during high LP are potentially concerning. We hypothesized that a high LP might be safer than previously thought, by accounting for movements of the CM upon patient positional changes. We retrospectively reviewed standard normal supine lumbar spine MRIs of 58 patients, and used electronic calipers on axial images at the T12‐L1, L1‐2, and L2‐3 disc levels to measure the transverse diameter of the CM relative to the size of the dorsal thecal sac space (DTSS) through which a spinal needle could be inserted. On 142 axial images, the means for CM diameters were 8.2 mm, 6.0 mm, and 2.9 mm at the three levels, respectively. We then used known literature mean CM displacement values in the legs flexed and unflexed lateral decubitus positions (LDP), to factor in CM shifts to the dependent side. We found that at all three levels, the likely positional shift of the CM would be too small and insufficient to displace the entire CM out of the DTSS. However, if needle placement could be confined to the midsagittal plane, an LP in the unflexed LDP would theoretically be entirely safe at both L1‐L2 and L2‐L3, and almost so at L2‐L3 in the legs flexed LDP. Thus, high LPs at L1‐L2 and L2‐3 are in theory likely safer than considered previously, more so in the legs unflexed than in the flexed LDP.

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