BMJ  2006;333 (18 November), doi:10.1136/bmj.39031.354942.68

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Smaller atraumatic needles are probably best for diagnostic lumbar punctures

Abstract

Research question What is the best way to do a diagnostic lumbar puncture?

Answer Use a small gauge atraumatic needle and replace the stylet before removal

Why did the authors do the study? To determine the best way of doing a lumbar puncture safely in adults suspected to have meningitis and to assess the accuracy of commonly used analyses of cerebrospinal fluid.

What did they do? They searched systematically through the Cochrane Library, Medline, and Embase for randomised trials in any language evaluating different techniques for diagnostic lumbar puncture. The authors did a similar search for studies of cerebrospinal fluid analysis for diagnosing bacterial meningitis in adults. Two people independently selected studies for inclusion in the review using prespecified quality criteria. Differences were resolved by consensus.

What did they find? They found 15 randomised trials evaluating different lumbar puncture techniques. Pooled data from five trials in 587 patients suggested that atraumatic needles helped reduce the incidence of headache after a lumbar puncture, although the difference was not significant (absolute risk reduction with an atraumatic needle 12.3%, 95% CI –1.72% to 26.2%; odds ratio 0.46, 0.19 to 1.07). One trial in 600 patients reported a lower risk of headache if the operator reinserted the stylet before removing the atraumatic needle (ARR 11%, 6.5% to 16%), and one trial in 100 patients reported that a 26 gauge standard needle caused significantly fewer headaches than a 22 gauge standard needle (ARR 26%, 11% to 40%).

The pooled results from four trials comparing bed rest with immediate mobilisation were inconclusive (odds ratio for headache after immediate mobilisation 0.84, 0.6 to 1.16).

Gram staining is an accurate way of ruling in, but not ruling out, bacterial meningitis according to three studies, the biggest of which (n=2635) reported a likelihood ratio of 737 (230 to 2295) for a positive result. Other results that make the diagnosis more likely include a white blood cell count 500 µ/l (one study: 15, 10 to 22), a ratio 0.4 between glucose concentrations in the cerebrospinal fluid and blood (two studies: 18, 12 to 27); and a lactate concentration 3.5 mmol/l in the cerebrospinal fluid (three studies: 21, 13 to 35). For all three tests, the likelihood ratios for a negative result were between 0.1 and 0.3.

What does it mean? The evidence suggests that the best way to prevent headaches is to use a smaller gauge atraumatic needle and reinsert the stylet before you remove it. Patients probably don't need bed rest afterwards, although this remains to be confirmed. The best position for the patient is still unclear, and few studies looked at other outcomes such as number of failed attempts. The authors found no studies on the potentially important effect of operator experience.

Biochemical tests on cerebrospinal fluid are more useful when the results are positive than when they are negative but are unlikely to be used in isolation in the real world. A likelihood ratio of 21, for example, means that patients with bacterial meningitis are 21 times more likely to have a lactate concentration 3.5mmol/l than patients without bacterial meningitis.

References

Straus SE How do I perform a lumbar puncture and analyze the results to diagnose bacterial meningitis? JAMA 2006;296:2012-22[Abstract/Free Full Text]

Rapid Responses:

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It depends how much CSF you want

Neville W Goodman
bmj.com, 23 Nov 2006 [Full text]