Lumbar Puncture: Adult

Lumbar puncture in the adult may be performed with the patient either sitting or lying on the side on a firm, flat surface. In each position, the lumbar spine must be flexed as far as possible to separate the vertebral spines maximally and expose the ligamentum flavum in the interlaminar window (Fig. 7.2). A line between the highest points of the iliac crests intersects the vertebral column just above the palpable spine of L4. With the spines now identified, the skin is anaesthetized and a needle is inserted between the spines of L3 and L4 (or L4 and L5). Exact identification of the level by palpation is difficult (Broadbent et al 2000). The soft tissues the needle will ultimately traverse should also be anaesthetized, but care should be taken to avoid injection of an excessive amount of local anaesthetic, which can compromise one’s appreciation of the structures being traversed. These include the subcutaneous fat and supraspinous and interspinous ligaments down to the ligamentum flavum itself. The lumbar puncture needle is then inserted in the midline or just to one side and angled in the horizontal and sagittal planes sufficiently to pierce the ligamentum flavum in or very near the midline (Fig. 7.3).There is a slight loss of resistance as the needle enters the epidural space, and careful advancement pierces the dura and arachnoid to release CSF.

Fig. 7.2 Lumbar interlaminar window in extension and flexion.

Fig. 7.3 Position of the needle in lumbar puncture.

Lumbar Puncture: Neonate and Infant

At full term (40 weeks) the spinal cord usually terminates somewhat lower than the adult level, sometimes reaching the body of L3. The supracristal plane intersects the vertebral column slightly higher (L3–4). By the second postnatal month, the level of cord termination has usually reached its permanent position, level with the body of the first lumbar vertebra. The lower end of the subarachnoid spine is found at sacral level 1 or 2. These differences must be borne in mind when identifying landmarks before undertaking lumbar puncture in neonates and infants.

A lumbar puncture is performed by placing the baby in a position, either lying or ‘sitting,’ that gives maximum convex curvature to the lumbar spine. A needle with trocar is inserted into the back between the spines of the third and fourth lumbar vertebrae and into the subarachnoid space below the level of the conus medullaris. The space between L3 and L4 is approximately level with the iliac crests, and the needle and trocar are usually inserted into the intervertebral space immediately above or below the iliac crests.

Cisternal Puncture

In cisternal puncture, the cisterna magna is entered by midline puncture through the posterior atlanto-occipital membrane. Further details of this difficult specialist technique are beyond the scope of this book.

Lumbar Epidural

For access to the lumbar epidural space, the approach is as for lumbar puncture. The intention in epidural injection is to avoid dural puncture, so it is best to enter the epidural space in the midline posteriorly, where the depth of the space is greatest. Techniques for entering the epidural space rely on the appreciation of loss of resistance to injection of the chosen medium (usually air or saline) as the space is entered. There is very little distance between the ligamentum flavum and the underlying dura on either side of the median plane.

Caudal Epidural

The route of access to the caudal epidural space is via the sacral hiatus. The space is thus entered below the level of termination of the dural sac (S2). With the patient in the lateral position or lying prone over a pelvic pillow, the sacral hiatus is identified by palpation of the sacral cornua (Fig. 7.4). These are felt at the upper end of the natal cleft approximately 5 cm above the tip of the coccyx. Alternatively, the sacral hiatus may be identified by constructing an equilateral triangle based on a line joining the posterior superior iliac spines: the inferior apex of this triangle overlies the hiatus. After local anaesthetic infiltration, a needle is introduced at a 45-degree angle to the skin to penetrate the posterior sacrococcygeal ligament and enter the sacral canal. Once the canal is entered, the hub of the needle is lowered so that the needle may pass along the canal (Fig. 7.5). If the needle is angled too obliquely it will strike bone; if it is placed too superficially it will lie outside the canal. The latter malposition can be confirmed by careful injection of air while palpating the skin over the lower sacrum.

Fig. 7.4 Palpation of the sacral cornua for caudal epidural injection.

(With permission from Ellis, H., Feldman, S.A. 1997. Anatomy for Anaesthetists, 7th ed. Blackwell Science, Oxford.)

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