ANATOMY OF THE SPINE AND PERIPHERAL NERVES

Although not exclusive to obstetric anaesthesia, a sound knowledge of the anatomy pertinent to epidural and spinal anaesthesia is fundamental to obstetric anaesthetists because of the importance of these techniques in this field. In addition, knowledge of the relevant peripheral nerves is important in order to differentiate central from peripheral causes of neurological impairment.

The structures involved in obstetric neuraxial anaesthesia comprise the vertebrae and sacral canal, vertebral ligaments, epidural space, meninges and spinal cord. The important peripheral aspects are the lumbar and sacral plexi and the muscular and cutaneous supply of the lower part of the body.

Vertebrae (Fig. 10.1)

The vertebral column has two curves, with the cervical and lumbar regions convex anteriorly and the thoracic and sacral regions concave. Traditionally, T4 is described as the most posterior part (most dependent in the supine position), although T8 has been suggested by recent imaging studies. L3–4 is the most anterior part (uppermost in the supine position), although this curve may be flattened by flexing the hips. In the lateral position, the greater width of women's hips compared with their shoulders imparts a downward slope from the caudal end of the vertebral column to the cranial end.