• Darragh Halpenny, Katie O'Sullivan, John P Burke, and William C Torreggiani.
• Department of Radiology, Adelaide and Meath Hospitals incorporating the National Children's Hospital, Tallaght, Dublin, Ireland.
• Eur Radiol. 2013 Nov 1;23(11):3191-6.

ObjectivesFailed 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.MethodsImages 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.ResultsFour 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.ConclusionsA 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.

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