Darragh Halpenny MBBCH, MRCPI, Presenter: Nothing to Disclose

Katie O' Sullivan MBBCh, Abstract Co-Author: Nothing to Disclose

William Torreggiani, Abstract Co-Author: Nothing to Disclose

The prevalence of obesity has significant implications for radiology departments. Image guided lumbar punctures (LP) are frequently requested in obese patients. We assessed the percentage of patients in whom the standard LP needle (9cm) will not reach the subarachnoid space, due to overlying fat. We also assessed clinician knowledge of needle size & the potential to use longer needles in obese patients.  

The first 400 patients to undergo abdominal computed tomography (CT)in our institution in 2011 formed our study group. The distance from the skin to the subarachnoid space at all lumbar intervertebral spaces was calculated using sagittal images from abdominal CTs. Intra abdominal & subcutaneous fat & abdominal girth were calculated. Data is presented as mean±standard deviation, differences between groups were evaluated with ANOVA & correlations determined by Spearman rank.

400 patients were assessed. In 66(16.5%), the distance from skin to subarachnoid space was > 9cm with distance greatest at the L4/5 interspace. Those with a distance > 9cm had greater abdominal girth (89.8±1.4cm Vs 116.2±3.1cm, P<0.001), greater intra-peritoneal fat (9.9±0.3cm Vs 11.9±0.7cm, P=0.013) & greater subcutaneous fat (1.7±0.1cm Vs 3.1±0.3cm, P<0.001) than those with back fat < 9cm. The distance to the subarachnoid space positively correlated with abdominal girth (r2=0.782, P<0.001), intra-peritoneal fat (r2=0.348, P=0.001) & subcutaneous fat (r2=0.673, P<0.001). A survey regarding LP technique was given to residents in 2 teaching hospitals, with 60 responses. 41 (68.3%) had failed to obtain an LP in obese patient. 43 had referred a patient for image guide LP (71.6%). 60 (100%) routinely use 9cm spinal needles to perform LPs. 13 (21.6%) were aware of the existence of longer spinal needles & 8 (13.3%) had used one. Of those that had failed to obtain a LP in an obese patient 7 (17.1%) had used a longer needle. 

Increased body fat may impact on the ability to sucessfully perform LPs. Lack of clinical awareness of spinal needle length may lead to increased amounts of failed LPs. Clinicians should consider a longer spinal needle in obese patients when a LP fails with a standard needle.

In 16.5% of patients, LP will not be possible with standard length needles, combined with poor clinician awareness of LP needles, this may lead to LP failure in obese patients.

Halpenny, D, O' Sullivan, K, Torreggiani, W, Does Obesity Preclude Lumbar Puncture with Standard Spinal Needles? The Use of Computed Tomography to Assess the Distance from the Skin to the Lumbar Subarachnoid Space and Correlation with Physicians Recognition of this Issue.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11034306.html