UMEM Educational Pearls

We all dread performing lumbar punctures on the obese patient. The traditional standard length spinal needle (9 cm) is becoming increasingly inadequate in reaching the subdural space in our overweight society.

Abe et al developed a formula for selecting the proper needle length to reach the middle of the spinal canal from the skin using retrospective CT data from 178 patients.

Length of needle (cm) = 1+ 17 x Weight (kg)/ Height (cm)

Given the average height of the American woman (163 cm or 5’4’’) our standard length spinal needle will FAIL to reach the mid-thecal space if a woman weighs more than 170 lb (75 kg)!!!

Paul Blart Mall Cop, and King of Queens star Kevin James (5’8’’, 285 lb) would require a 13.7 cm spinal needle. This means even our long spinal needles (12.7 cm) would FAIL by 1 cm.

Note that this formula resulted in selection of needles too small (6%) and too long (31%) of the time. Abe’s linear correlation had an R value of 0.81, (p<.001)

Bottom-Line: Consider use of a long spinal needle (12.7 cm) or IR guided LP in overweight /obese patients and the above formula to guide your depth of insertion


Abe KK et al. Lumbar puncture needle length determination.  2005 Oct;23(6):742-6