Thoracic Discectomy

Clinical Presentations

Thoracic disc herniation (TDH) is an important degenerative pathology that may cause significant disability, including back pain, and neural symptoms, including radicular pain and myelopathy. The use of computed tomography (CT) and magnetic resonance imaging (MRI) has resulted in a significant increase in the detection and diagnosis rates of these lesions. With the improved diagnostic methods using MRI and CT scanning, studies have reported that the prevalence of TDHs ranges from 11.1% to 14.5%. Symptomatic TDH is very rare, accounting for 0.25% to 0.75% of all disc herniations reported in the literature. ^, Thoracic radicular pain is uncommon, as compared with lumbar or cervical back pain. In 75% of patients with TDH, the affected level is below T8 and is mostly at T11–T12.

The distribution of TDHs may reflect the fact that the lower thoracic level is more mobile, and the posterior longitudinal ligament at this level is relatively weaker when compared with other levels. TDHs are more common in males than in females and tend to occur between the fourth and sixth decades of life. Most cases are central (two-thirds), and the rest are paracentral (one-third). In most patients, trauma is an initiating factor, including accidental axial loading such as falling on the hips, a prolonged flexion posture, incorrect posture, or heavy lifting. The diagnosis of TDH may be difficult owing to the lack of a characteristic clinical presentation. Because of variability in clinical presentations, TDH is often overlooked in patients with back pain, and a diagnosis of TDH is generally delayed until an average of 15 months after symptom onset. ^,

Patients with TDH may have a wide variety of symptoms, the most common of which is continuous or intermittent back pain, which is usually described as burning or stabbing. Patients with symptomatic TDH can be divided into three groups: axial pain, radicular pain, and myelopathy. ^, Axial thoracic pain is usually localized from the middle to the lower thoracic region but, under some circumstances, may radiate to the middle to the lower lumbar area. It is sometimes confused with cardiac, pulmonary, or abdominal pathology. Radicular pain is often described as an anterior chest bandlike discomfort in a dermatomal distribution and is the second type of presentation. Thoracic radiculopathy often presents with severe chest or abdominal discomfort, which often misleads primary physicians into prescribing extensive medical workup. ^, High TDH (T2–T5) may be rarer but can mimic cervical disc disease and present with symptoms of upper arm pain, radiculopathy, and paresthesia along the sclerotome. Myelopathy is the most severe symptom of TDH. In mild myelopathy, patients complain of paresthesia and axial back pain without definite functional impairment. Lower extremity weakness, ataxia, paresthesia, and bladder and bowel dysfunction are severe presentations of myelopathy. Paraplegia can result from severe compression of the spinal cord or occlusion of the anterior spinal artery. Bladder and bowel dysfunction are found in approximately 15% to 20% of patients with symptomatic TDH. ^{, ,}

Indications for Surgery

Atypical clinical symptoms resulting from the lack of a characteristic presentation delay diagnosis of TDH. Surgical treatments have been associated with suboptimal outcomes in part because of diagnostic delay. ^{, ,} No consensus has been reached regarding the indications for disc removal except for severe myelopathy. Most surgeons generally avoid prophylactic surgery. Patients who present with sustained intractable pain but do not respond to intensive but conservative treatment must be considered for surgical decompression. Symptomatic relief after selective epidural block is evaluated to confirm the diagnosis. Diagnostic block is particularly useful for diagnosing nonspecific somatic pain.

Preoperative Planning and Imaging

Plain radiography is simple and inexpensive but limited by the inability to diagnose neural compression. The relationship between the disc space and the rib head at each vertebra is important for the lateral transthoracic approach. The direction of the pedicle and the height of the neural foramen and the superior articular process are important for the transforaminal endoscopic approach. CT is useful for detecting calcification of the hernia, ossification of the ligamentum flavum (OLF), ossification of the posterior longitudinal ligament (OPLL), and vacuum disc. CT scanning shows better images of bony anatomy and lateral pathologies such as foraminal stenosis. CT myelography allows an accurate evaluation of the degree of spinal cord compression in patients who cannot undergo MRI. MRI is the most useful diagnostic technique for soft tissue and neural structures. Measurement of bone marrow signal intensity is a noninvasive evaluation method for differential diagnosis of infection, fracture, and metastasis. Gadolinium-enhanced MRI is required in cases of recurrent herniation, infection, and tumorous condition. On T1-weighted MRI, a herniated disc shows an intermediate signal intensity and contiguity with the parent disc, and the posterior osteophyte shows a low signal intensity similar to that of the vertebral cortex. Meanwhile, thickened posterior longitudinal ligaments also show low signal intensity on T1-weighted imaging.

Surgical Strategies

The clinical symptoms of TDH often manifest as various types of severe, incapacitating pain as a result of any surgical approach. ^, This makes the choice of the least morbid surgical approach for thoracic discectomy even more significant. However, the rarity of symptomatic thoracic discogenic conditions, combined with relative surgical inexperience in dealing with the unique anatomy of the thoracic region, has hampered the development of a universally accepted gold standard; the choice of a thoracic discectomy technique is mostly based on the surgeon’s training, clinical experience, and personal preference. Table 138.1 and Fig. 138.1 compare different surgical approaches with their indications.

The transthoracic approach is indicated for (1) central or centrolateral disc herniation, (2) severe spinal cord compression causing myelopathy, (3) calcified disc herniation with severe dural adhesion, (4) concomitant OPLL, and (5) multilevel disc herniation. ^, The posterior or posterolateral approach is indicated for (1) paramedian or foraminal disc herniation, (2) mild spinal cord compression or myelopathy, (3) soft disc herniation or partial calcification without dural adhesion, (4) concomitant OLF, and (5) migrated herniation. ^, For example, calcified central disc herniations with myelopathy are indicated for the transthoracic approach, and lateral disc herniations are more preferably treated with the posterolateral approach, while reserving posterior approaches for small, noncalcified paracentral disc herniations. ^, Fig. 138.2 is a flowchart showing the treatment of symptomatic TDHs.

Given the unique surgical challenges presented by thoracic discectomy, the several surgical techniques that have been described are not accepted universally. Minimally invasive techniques have revolutionized the management of cervical/lumbar spinal conditions, and these techniques have been applied to the thoracic region as well. Endoscopic surgery is an advanced minimally invasive approach for TDH. The thoracoscopic approach has a similar indication as the open transthoracic approach (anterior approach), and the transforaminal endoscopic approach is appropriate for soft paramedian to lateral herniation causing intractable pain without significant myelopathy. ^, Both the thoracoscopy and posterolateral endoscopy techniques are similar, utilizing endoscopic visualization to address compressive disc pathology with lower approach-related morbidity. However, thoracoscopy is associated with greater surgical insult, with the use of general anesthesia, larger working channels, and retraction of the thoracic viscera. ^, Transforaminal thoracic endoscopic discectomy offers the advantages of a more natural access to the dorsal intervertebral disc space, with minimal removal or destruction of surrounding anatomical structures, using smaller working channels that facilitate midline decompression of the ventral thecal sac pathology under direct vision without any retraction or contact of the thecal sac itself.