Does obesity preclude lumbar puncture with a standard spinal needle? The use of computed tomography to measure the skin to lumbar subarachnoid space distance in the general hospital population.

OBJECTIVES: Failed lumbar puncture (LP) is a common indication for referral for radiologically guided LP. This study aims to evaluate what percentage of the hospital population would fail an LP using a standard 9-cm needle because of obesity and a skin to subarachnoid space distance greater than 9 cm.
METHODS: Images of 402 consecutive patients undergoing computed tomography of the abdomen and pelvis were reviewed. Skin to subarachnoid space distance was calculated using sagittal images. A survey was conducted among junior hospital doctors to assess their experience of performing lumbar puncture in obese patients.
RESULTS: Four hundred patients were included. Fifty-five patients (13.8 %) had a skin to subarachnoid space distance greater than 9 cm. Intra-abdominal fat, subcutaneous fat and abdominal girth correlated with distance between the skin and subarachnoid space. Among junior doctors, 68.3 % (n = 41) reported LP failure on an obese patient; 78.4 % (n = 47) were unaware of the existence of a longer needle and 13.3 % (n = 8) had experience using a longer needle.
CONCLUSIONS: A significant proportion of the hospital population will fail LP with a standard length spinal needle. Selecting a longer needle may be sufficient to successfully complete LP in obese patients. KEY POINTS: • Lumbar puncture failure commonly leads to referral for an image-guided procedure • Standard lumbar puncture may fail in 13.8 % of patients due to obesity • 78.4 % of trainee doctors are unaware of the existence of longer spinal-needles • Using longer spinal needles may allow successful LP in obese patients.