The safest approach to the CSF is to enter the lumbar cistern of the subarachnoid space in the midline, well below the level at which the spinal cord normally terminates (see Fig. 7.1). The fine needle employed is unlikely to damage the mobile nerve roots of the cauda equina. This procedure is called lumbar puncture. It is also possible to access the CSF by midline puncture of the cerebellomedullary cistern (cisterna magna); this is called cisternal puncture.

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.

Access to the Epidural Space

The epidural ‘space’ lies between the spinal dura and the wall of the vertebral canal. It contains epidural fat and a venous plexus. Access to this space, usually in the lumbar region, is required for the administration of anaesthetic and analgesic drugs and for endoscopy. The caudal route is used mainly for analgesic injections.

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.)

Fig. 7.5 Position of the needle in caudal epidural injection.

Thoracic and Cervical Epidurals

It is possible to access the epidural space at the thoracic and cervical levels, but the specialist techniques required are outside the scope of this book. The principles are the same as those for lumbar epidurals, but the special anatomy of the vertebral spines at the other levels requires the angle of approach to be modified.

Vertebral Column

The vertebral column is a curved linkage of individual bones or vertebrae (Figs 7.6, 7.7). A continuous series of vertebral foramina runs through the articulated vertebrae posterior to their bodies and collectively constitutes the vertebral canal, which transmits and protects the spinal cord and nerve roots, their coverings and vasculature. A series of paired lateral intervertebral foramina transmit the spinal nerves and their associated vessels between adjacent vertebrae. The linkages between the vertebrae include cartilaginous interbody joints and paired synovial facet (zygapophyseal) joints (Fig. 7.8), together with a complex of ligaments and overlying muscles and fasciae. The muscles directly concerned with vertebral movements and attached to the column lie mainly posteriorly. Several large muscles producing major spinal movements lie distant from the column and have no direct attachment to it, such as the anterolateral abdominal wall musculature. The column as a whole receives its vascular supply and innervation according to the general anatomical principles considered later in this chapter.

Fig. 7.6 Vertebral column. A, Anterior aspect. B, Lateral aspect. C, Posterior aspect.

Fig. 7.7 A, Sagittal MRI of the thoracolumbosacral spine. B, Sagittal MRI of the cervicothoracic spine.

(Courtesy of Dr. Justin Lee, Chelsea and Westminster Hospital, London.)

Fig. 7.8 High-resolution computed tomogram through the posterior abdominal wall at the level of the body of the fourth lumbar vertebra, showing zygapophyseal joints between the fourth and fifth lumbar vertebrae.

Vertebral column morphology is influenced externally by mechanical and environmental factors and internally by genetic, metabolic and hormonal factors. These all affect its ability to react to the dynamic forces of everyday life, such as compression, traction and shear. These dynamic forces can vary in magnitude and are influenced by occupation, locomotion and posture.

The adult vertebral column usually consists of 33 vertebral segments. Each presacral segment (except the first two cervical) is separated from its neighbour by a fibrocartilaginous intervertebral disc. The functions of the column are to support the trunk, protect the spinal cord and nerves and provide attachments for muscles. It is also an important site of haemopoiesis throughout life. The total length of the vertebral column is approximately 70 cm in males and 60 cm in females. The intervertebral discs contribute about one-quarter of this length in young adults, although there is some diurnal variation in this contribution. Approximately 8% of overall body length is accounted for by the cervical spine, 20% by the thoracic, 12% by the lumbar and 8% by the sacrococcygeal regions. Although the usual number of vertebrae is 7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4 coccygeal, this total is often variable, with reports of between 32 and 35 bones. The demarcation of groups by their morphological characteristics may be blurred. Thus, there may be thoracic costal facets on the seventh cervical vertebra, giving it the appearance of an extra thoracic vertebra; lumbar-like articular processes may be found on the lowest thoracic vertebra or the fifth lumbar vertebra may be wholly or partially incorporated into the sacrum. As a result of these changes in transition between vertebral types, there may be 23 to 25 mobile presacral vertebrae.

Anterior Aspect

The anterior aspect of the column is formed by the anterior surfaces of the vertebral bodies and of the intervertebral discs (see Fig. 7.6A). It has important anatomical relations at all levels and should be considered in continuity. It forms part of several clinically significant junctional or transitional zones, including the prevertebral–retropharyngeal zone of the neck, the thoracic inlet, the diaphragm and the pelvic inlet. The anterior aspect of the column is covered centrally by the anterior longitudinal ligament, which forms a fascial plane with the prevertebral and endothoracic fascia and with the subperitoneal areolar tissue of the posterior abdominal wall. Infection and other pathological processes may spread along this fascial plane.

Lateral Aspect

The lateral aspect of the vertebral column is arbitrarily separated from the posterior by articular processes in the cervical and lumbar regions and by transverse processes in the thoracic region (see Fig. 7.6B). Anteriorly, it is formed by the sides of vertebral bodies and intervertebral discs. The oval intervertebral foramina, behind the bodies and between the pedicles, are smallest at the cervical and upper thoracic levels and progressively increase in size in the thoracic and upper lumbar regions. The lumbosacral (L5–S1) intervertebral foramen is the smallest of the lumbar foramina. The foramina permit communication between the lumen of the vertebral canal and the paravertebral soft tissues (a ‘paravertebral space’ is sometimes described), which may be important in the spread of tumours and other pathological processes. The lateral aspects of the column have important anatomical relations, some of which vary considerably between the two sides.