Thoracic Discogenic Pain Syndrome 

Thoracic disc herniation (TDH) is an uncommon and underreported entity that is often challenging to diagnose because of a relative paucity of examination findings and because of its nonspecific presentation. The number of patients with objective neurologic findings due to thoracic disc herniation is low, and most patients can be treated with a conservative approach without surgical intervention.^{[1, 2]}

Frequency

United States

Asymptomatic thoracic disc herniations are relatively common in the general population. Autopsy studies have shown that the prevalence rate ranges from 7-15%. The prevalence of asymptomatic disc herniations found radiographically varies with the imaging modality used. Awwad et al showed that 11-13% of asymptomatic subjects were found to have thoracic disc herniation on compute tomography (CT) myelograms,^[3] whereas Wood et al showed 37% of such individuals were found to have thoracic disc herniation on magnetic resonance images (MRIs).^[4]

Despite the relatively high frequency of asymptomatic disc herniations, symptomatic disc herniations occur in a range from 1 in 1000 to 1 in 1 million persons. The number of patients with objective neurologic findings due to thoracic disc herniation is thought to be closer to 1 in 1 million annually.

Although the frequency of thoracic discectomies is increasing, they are still performed much less frequently than discectomies in the cervical or lumbar regions. These procedures represent approximately 0.13-0.15% of admissions for disc disease and from 0.2% to 4% of all discectomies.

The thoracic region of the spine is relatively inflexible and functions primarily to provide erect posture and assist in weight bearing of the trunk, head, and upper extremities during daily activities. The vertebral bodies are taller posteriorly than anteriorly, resulting in an anterior concavity and normal thoracic kyphosis.

In the thoracic spine, the addition of the sternum, the ribs, and their associated ligamentous structures provide additional support and rigidity. The 10 most superior ribs articulate anteriorly with the sternum and posteriorly with the transverse processes and vertebral bodies. These ribs are oriented vertically, with slight medial angulation in the coronal plane. This arrangement provides the thoracic spine with relatively good stability in the midsagittal plane. However, it also affords less stability in the lateral and rotational planes. Biomechanical studies have shown that thoracic intervertebral discs are most susceptible to injury when torsional and lateral forces are applied in tandem.

Several features of the thoracic spine increase its susceptibility to spinal cord compression associated with thoracic disc herniation, as follows:

• The ratio of the spinal canal to the thoracic spinal cord is smaller than that found in the cervical and lumbar regions. Although the cross-sectional diameter of the thoracic cord is smaller than that of its cervical or lumbar counterparts, the diameter of the spinal canal is proportionally even smaller. Thus, the ratio of the spinal cord to the canal in the thoracic spine is 40%, whereas this ratio in the cervical spine is only 25%.
• The dentate ligaments situated between the spinal cord and the nerve roots restrict posterior movement of the spinal cord within the canal. This makes the thoracic spine prone to vertical compression from anterior disc and bony prominences.
• The natural kyphosis of the thoracic spine places the spinal cord in close proximity to the posterior longitudinal ligament and the posterior aspects of both the vertebral bodies and the discs in the thoracic region. This makes the thoracic cord especially susceptible to ventral compression from herniations.

Normal discs and disc degeneration

The 3 basic structures of normal vertebral discs are the nucleus pulposus, the annulus fibrosus, and the vertebral endplates. The nucleus pulposus is the gelatinous core of the disc and is composed mostly of water and proteoglycans. The annulus fibrosus surrounds the nucleus pulposus and is composed primarily of water and concentric layers of collagen. The vertebral endplates lie on the superior and inferior aspect of the discs adjacent to the vertebral bodies and aid in the diffusion of nutrients into the discs. As a normal part of aging, the water content of the discs decreases, leading to decreased disc height and impaired capability to absorb the axial loads of the spine. Disc herniations, annular tears, and endplate degeneration all can occur.

Location of thoracic disc herniation

Thoracic disc herniation are generally classified into 4 categories. These are central thoracic disc herniations, centrolateral thoracic disc herniations, lateral thoracic disc herniations, and intradural thoracic disc herniations. Central and centrolateral protrusions are the most common and are found in 70% of cases. Intradural herniations are rare and are found in less than 10% of cases. Clinical presentations vary, but the following generalizations are appropriate:

• Central protrusions may cause spinal cord compression, and patients may present with myelopathic symptoms, such as increased muscle tone, hyperreflexia, abnormal gait, and urinary/bowel incontinence.
• Centrolateral protrusions may result in a presentation resembling Brown-Sequard syndrome, with ipsilateral weakness and contralateral pain or sensory disturbances.
• Lateral herniations may cause nerve root compression, and patients may present with a radiculopathy.

Intraosseous disc herniations

Thoracic intervertebral discs can herniate into the spinal canal as well as through vertebral endplates, directly into the adjacent vertebral bodies. The resulting herniations are called Schmorl nodes or cartilaginous nodes. These can occur in association with osteoporosis, tumors, metabolic diseases, congenital weak points in the endplates, or degenerative endplate changes. Although Schmorl nodes often do not cause symptoms, an inflammatory, foreign body–type reaction can occur, resulting in severe pain.

Scheuermann disease, or juvenile kyphosis, is a disorder of childhood in which these types of changes are particularly pronounced. Children with this disorder generally present at age 8-16 years with rigid thoracic kyphoses. Although the exact etiology is not known, endplate degeneration and avascular necrosis of the ring apophysis result in the development of multilevel Schmorl nodes and vertebral wedging. This may cause the patient to have a severe kyphotic posture and pain in the early teenage years.

Annular tears

Tears in the annulus fibrosis may contribute to thoracic discogenic pain (TDP), even in the absence of an associated disc herniation. The outer third of the annulus fibrosis is innervated by the sinuvertebral nerve, which relays sensory information, including pain, to the dorsal root ganglion. Tears in this region, particularly radial tears, may be clinically significant. A study by Schellhas et al evaluated the results of 100 patients with thoracic discographies.^[5] The study found that greater than 50% of painful discs had annular tears with no evidence of significant herniation.

Calcification

Calcification is also a common finding in thoracic disc herniations, particularly in those discs that are herniated as a result of degeneration. The terms “hard” disc herniations and “soft” disc herniations are used throughout the literature to indicate disc herniations with and without calcification, respectively. The presence and extent of calcification is also important in surgical planning.

In patients with symptomatic thoracic disc herniations for which trauma is implicated as the cause, a twisting or torsional movement is often involved. Participation in any sport that involves axial rotation of the spine can potentially increase the risk of disc herniation. These types of forces may be observed in sports such as golf, in which axial rotation of the spine is required at the top of the backswing, with subsequent uncoiling and hyperextension observed through the downswing and follow-through.

Minimizing forces on the spine through proper mechanics in specific sporting activities is important. Additionally, the dynamic stabilizers of the spine should also be strengthened to counteract the significant forces exerted on the spine during certain athletic activities.