The thoracic paravertebral space (TPVS), when viewed in transverse cross-section is triangular-shaped (red triangle in figure below). The base is formed by the posterolateral aspect of the vertebral body / intervertebral discs / intervertebral foramina / articular processes. The anterolateral border is formed by the parietal pleura, whilst the posterior border is formed by the superior costotransverse ligament. This ligament extends from the inferior aspect of the transverse process above to the superior aspect of the rib tubercle below. Lateral to this ligament (and continuous with it) is the internal intercostal membrane, which is the aponeurotic continuation of the internal intercostal muscle, and thus runs between the upper and lower border of adjacent ribs.12 The apex of the triangular TPVS communicates with the intercostal space laterally. The cephalad limit of the TPVS has not been defined. It has been shown that solution injection into the TPVS can spread caudad into the abdominal and lumbar region, through the medial and lateral arcuate ligaments of the diaphragm. It is generally accepted, however, that the caudad limit of the paravertebral space is at the origin of the psoas muscle at L1.9
The TPVS contains mainly fatty tissue, and is traversed by the intercostal or spinal nerves, intercostal vessels, dorsal rami, rami communicantes, and the sympathetic chain. The spinal nerves do not have a fascial sheath in the TPVS, which explains their susceptibility to local anesthetic blockade.
The space is divided into an anterior and posterior compartment by a fibroelastic membrane, the endothoracic fascia. The endothoracic fascia is the deep investing fascia of the thoracic cavity. It blends medially with the periosteum of the vertebral body; and laterally, is closely applied to the ribs. Caudally, it is continuous with the transversalis fascia of the abdominal cavity and this may explain why solutions injected in the TPVS may spread to the lumbar region. The spinal nerves have been described as running through the compartment posterior to the endothoracic fascia.2 This however is controversial,10,11 as the precise anatomy of the endothoracic fascia, and its relationship to the spinal nerves in particular, remains ill-defined. It has been shown that injection closer to the spinal nerves (using a nerve-stimulator-guided technique) is more likely to result in longitudinal spread of the injectate in the TPVS.11