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For diagnostic lumbar punctures, it is recommended that the stylet is fully reinserted into the spinal needle before needle removal [1]. In a randomised trial of 600 patients by Strupp et al, post-dural puncture headache (PDPH) occurred in 16% of patients after lumbar puncture using a 21-G Sprotte needle without reinsertion of the stylet, compared with 5% when the stylet was reinserted [2].

Filaments of arachnoid mater extend into the subarachnoid space, and it has been postulated that these contribute to the aetiology of PDPH by entering the needle lumen during outward flow of cerebrospinal fluid, and extending through the arachnoid mater when the needle is withdrawn. Cerebrospinal fluid may then continue to flow along the ‘wick’ created [2, 3]. Reinserting the stylet is thought to displace arachnoid filaments back into the subarachnoid space, and prevent this effect.

This concept has been thought not to occur during spinal anaesthesia [4], consequent to filament displacement during administration of the injectate. However, close examination reveals that reverse, outward flow occurs at the end of injection, resulting from negative ‘suction’ pressure created by elastic recoil of the deformed syringe plunger (Fig. 5). Strategies that might prevent such flow include the application of continuous pressure to the plunger until the spinal needle has been withdrawn from the subarachnoid space, and reinsertion of the stylet before the needle is withdrawn.

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Figure 5.  Schematic representation of reverse flow, demonstrating plunger deformation (a), and recoil (b).

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Although the clinical significance of reverse flow is unknown, its simple prevention using these techniques would seem prudent to reduce further the low incidence of PDPH that occurs after spinal anaesthesia [5].

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