Subarachnoid space needle manipulations for successful block.

Sir,

We often encounter subarachnoid block failures in spite of cerebrospinal fluid (CSF) dripping at spinal needle's hub. Although rare anatomical abnormalities like spinal subdural hygroma should be considered, technique-related spinal failures technical are more common and can be easily corrected.

Anatomically, ligamentum flavum is Y-shaped structure bordering epidural space.[1] Loss of resistance technique appreciates epidural space that does not always have CSF-filled subarachnoid space anteriorly [Figure 1]. Due to gravity-related CSF-pooling in lumbar cistern, the dural sac's volume is largest at third and fourth lumbar interspaces in flexed sitting position.[2] Due to fixity to stable conus medullaris, floating cauda equina roots have been shown to cluster anteriorly and medially (away from the posterior pool of CSF in lumbar cistern) in flexed body position.[345]

Figure 1

Schematic diagram to explain the understanding behind calculated re-directioning technique for spinal anesthesia/dural puncture

If CSF is not flowing freely and is difficult or impossible to aspirate, it indicates Whitacre/Pencan®(pencil point tip) needle's small eyelet (and not Quincke needle's larger-sized “eyelet” with cutting tip) is in thick dural-arachnoid sidewall [Figure 1]. To assess the location of the eyelet, the needle is rotated 90° counter-clockwise. If this rotation improves CSF flow/aspiration, the eyelet is in right sidewall and needle needs to be withdrawn a couple millimeters into the epidural space before re-directing just left to sidewall puncture. Alternatively, if 90° counter-clockwise rotation decreases or completely stops CSF flow/aspiration, the needle is in left sidewall and the re-directed dural puncture should be just right to sidewall puncture. The withdrawal of needle and re-insertion after redirection avoids delivery of anesthetic medications in the dural-arachnoid sidewall and ensures the injection of spinal anesthetics in the central CSF pool within the posterior half of lumbar cistern, away from peripherally-and-anteriorly seated cauda equina nerve roots within lumbar cistern in flexed-spine sitting position.

Similar technique can be employed during dural puncture with combined spinal epidural (CSE) needles set wherein after withdrawing the spinal needle, the epidural needle also needs to be withdrawn few millimeters out of the epidural space for re-insertion per calculated redirection (calculation elicited from sidewall-seated spinal needle's counter-clockwise rotation). Alternatively, if the spinal needle is atraumatically brushing dural-arachnoid sidewall during true “dry tap” CSE technique, the sidedness of sidewall (right or left) can only be judged by patients’ paresthesias’ lateralization and subsequently following the above-mentioned epidural needle's minimal withdrawal before calculated re-direction technique. If the patient does not complain lateralized paresthesias for true “dry tap” CSE technique, sidewall's sidedness cannot be judged and calculated re-direction technique cannot be employed.

In summary, irrespective of the theoretical risk for increased post-dural-puncture headache with two dural puncture holes (sidewall and center), the benefit of successful subarachnoid block with calculated re-directioning (vs. commonly performed random re-directioning involving n number of dural puncture holes) outweighs the risks of the subarachnoid space needle manipulations.