Lumbar puncture may be undertaken with the subject sitting or lying in the left lateral position (figure 53a,b). In the latter, ask the patient to draw their legs up to their chest, to keep their back vertical and near the edge of the bed, to ensure that the needle is advanced horizontally in the midline. The entry point is above or below the spine of the fourth lumbar vertebra, this is identified in the supracristal plane (passing between the upper borders of the iliac crests). A lumbar puncture needle of approximately 5 cm long, and is advanced through the supraspinous and interspinous ligaments into the extradural space. Various techniques are used to show the sudden release of pressure on a syringe, as it enters the space, but still lies outside the spinal meninges.
An epidural anesthetic is delivered into the extradural space, a catheter is left in situ to provide long-term pain relief postoperatively. Further anterior advancement of the needle is felt to hit the dura and then, on piercing it, cerebrospinal fluid can be withdrawn (page 369). Injection of anesthetic at this level produces a spinal anesthetic. The patient’s shoulders are slightly raised, to retain the anesthetic around the nerves of the cauda equina. NB the dosage of anesthetic required is less for spinal anesthesia, and therefore differentiation of the site for extradural and spinal anesthesia is essential to avoid an overdose.
In a caudal block, anesthetic is placed in the sacral canal, through the sacral hiatus. It is undertaken in the left lateral position. The surface marking of the hiatus is the apex of an equilateral triangle, based on the two posterior superior iliac spines (the dimples of Venus). The hiatus is also palpable and is covered by the sacrococcygeal membrane. The needle is passed through the skin and the membrane, the latter being felt as a slight click. The needle (or cannula) is then advanced for a centimeter cranially. The dural sac ends at S2, but aspirate to exclude both CSF and blood before injection. There should be minimal resistance; observe and palpate to exclude a subcutaneous delivery.