Introduction

Stenosis of the thoracic spine is a relatively rare condition when compared to stenosis of the cervical or lumbar spine. Because it is unusual, a full understanding of the condition’s epidemiology and clinical presentation is limited, yet like stenosis in other spinal regions, its causes are numerous. These include ossification of the ligamentum flavum (OLF) (Figure 48-1) or posterior longitudinal ligament (OPLL) (Figures 48-2 and 48-3), herniation of intervertebral discs (Figures 48-4 and 48-5), and spondylosis. Other causes include neoplastic lesions, facet cysts, vascular malformations, and fracture. Stenosis of the thoracic spine typically presents with a variable combination of three main symptoms: back pain, radiculopathy, and myelopathy.

FIGURE 48-1 Ossification of the ligamentum flavum (OLF) of T8-9 with severe cord compression and progressive paraparesis in a 61-year-old male. OLF is more common in Asian males and in the lower segments of the thoracic spine.

FIGURE 48-2 Sagittal plain CT scan demonstrating ossification of the posterior longitudinal ligament (OPLL) of the upper thoracic segments extending to the cervicothoracic junction in a 56-year-old female.

FIGURE 48-3 Multisegment laminectomy, fusion, and instrumentation was required for cord decompression and junctional stabilization. The dorsal dura is extensively ossified.

FIGURE 48-4 Thoracic disc herniation, T8-9, with symptomatic cord compression and T2 signal change of the cord.

FIGURE 48-5 Axial view demonstrates significant midline cord compression. This lesion requires a direct, ventral approach and was resected by costotransversectomy.

Much of what is known about thoracic stenosis has been derived from the experience of Japanese practitioners. OLF is cited as the most common cause of thoracic spinal stenosis. Although case reports of OLF with thoracic myelopathy in whites and North Americans have been published,¹² patients of Asian descent are most frequently affected. Up to 20% of Asians older than 65 years of age have some degree of thoracic stenosis due to OLF.³ Aizawa et al, in a retrospective study of 265 Japanese patients, found OLF to account for more than half of all cases of thoracic myelopathy.⁴ Middle-aged men had the condition more frequently than women. It is not yet clear why this condition has a gender discrepancy and why it tends to occur in younger patients than those with cervical or lumbar stenosis.

Because OLF and OPLL are unusual among Westerners, much of the European and North American literature focuses on thoracic intervertebral disc disease as the primary etiology of thoracic spinal stenosis. As with OLF and OPLL, thoracic disc herniation is overall an unusual cause of thoracic back pain, radiculopathy, and myelopathy. Studies suggest than it affects males most frequently and tends to occur between the fourth and sixth decades.⁴

Pathology

A number of anatomical features make the thoracic spinal cord particularly vulnerable to injury. Unlike the cervical region, where the spinal cord takes up approximately 25% of the cross-sectional area of the canal, the thoracic cord constitutes 40% of the canal. Due to this anatomical difference, space-occupying lesions in the thoracic spine may cause more rapid and profound impingement and impairment of the cord. The thoracic kyphosis also creates a relative “bowstring” effect with the spinal cord draped across the posterior longitudinal ligament, the intervertebral discs, and the vertebral bodies. This positions the ventral cord in close apposition to compressive pathology of these structures.

The thoracic spinal cord has a more tenuous blood supply than the lumbar and cervical neurological segments. Ventral perfusion derives from the main feeding vessel, the artery of Adamkiewicz, which variably supplies the thoracic spinal cord. Intrinsic blood supply comes from the midline anterior spinal artery and two posterior vessels that are smaller than their counterparts in other regions of the spine. Intercostal arteries make up the extrinsic blood supply and are smaller and fewer in number than those in the cervical and lumbar spine. This vascular arrangement creates a relative watershed area between T4 and T9 that makes the region vulnerable to ischemic injury.

OLF occurs as a normal part of the aging process and rarely leads to stenosis. Histologically, the normal ligamentum flavum is composed of significant amounts of elastin that, with aging and degeneration, is progressively replaced by collagen, fragments of bone, cartilage, and fibrous tissue.⁵ Pathologic ossification is characterized by extreme progression of these processes leading to overgrowth and, ultimately, canal and foraminal stenosis. The precise mechanisms of pathologic OLF have not yet been clearly determined. It has been suggested that high mechanical stress of the thoracolumbar junction leads to degeneration of the facets and intervertebral discs, initiating progressive injury of the ligamentum flavum in this region.⁶ Ossification then proceeds in response to repetitive injury. This may explain why OLF occurs more frequently in the lower thoracic spine. Although plausible, this theory has been questioned because the cervical and lumbar spinal regions are more mobile than the thoracic spine, yet ossification in these locations is less common.⁷ It is also rare for OLF lesions to extend across multiple levels. Medical comorbidities such as diabetes mellitus, abnormalities in calcium metabolism, hypoparathyroidism, and Paget disease may play a significant role and have been associated with pathologic OLF.^8,9 The higher incidence of OLF in the Japanese population clearly suggests a genetic etiology.

OLF associated with thoracic myelopathy most typically occurs in the lower thoracic spine.¹⁰ In their epidemiologic study, Aizawa et al found that T11-12 was the most common site of compression, followed by T10-11 and T9-10. When OPLL was the cause, T1-2 was affected most commonly, followed by T2-3 and T3-4.¹¹

As with OLF and OPLL, isolated traumatic injury to thoracic intervertebral discs is rare. The splinting effect of the rib cage as well as the vertical orientation of the thoracic facets serves to reduce the forces on thoracic discs compared to those in the lumbar spine. This is thought to decrease the incidence of discal injury in the thoracic area. Degeneration of thoracic discs due to aging can occur as well, being most common in the fourth through the sixth decades of life, with males affected more frequently than females. Thoracic disc herniation tends to occur in the midline or just lateral to the midline, and predominates in the lower thoracic levels (Figures 48-6 and 48-7).¹² Wood et al reviewed 90 MRI scans of asymptomatic patients and demonstrated that, often, thoracic disc herniations exist without symptoms.¹³ An additional study by Wood et al showed that these herniations do not frequently progress.¹⁴

FIGURE 48-6 Left-sided T7-8 HNP with predominantly radicular symptoms, refractory to conservative care.

FIGURE 48-7 Minimal cord displacement and lateral lesion position make this situation optimal for transpedicular or transfacet discectomy.

Clinical Presentation

The clinical presentation of thoracic stenosis ranges from simple back pain to frank myelopathy. Thoracic back pain is the most common presentation of a disc herniation, with patients describing the pain as “passing through their chest.” When lower thoracic discs are affected, the pain may radiate to the abdomen, flank, or groin. Paresthesias, numbness, intercostal neuralgias, unsteady gait, and fatigue while walking may also be present. When myelopathy occurs, it often presents itself as trunk and lower extremity weakness (especially of proximal musculature), spasticity, and sensory loss. The upper extremities should be spared. Alterations in bladder control can occur. Because of the similarities in presentation, and the infrequency of thoracic spinal stenosis, thoracic myelopathy can often be confused with lumbar and/or cervical stenosis, leading to delays in diagnosis.

Diagnosis

Physical findings of patients with thoracic myelopathy are similar to those seen in lumbar and cervical stenosis. Lower extremity weakness, hyperreflexia, decreased sensation, sphincter malfunction, loss of abdominal reflexes, and gait instability can be present. In addition, patients may localize their pain and hypesthesia in a thoracic distribution. Complete paraplegia due to thoracic spinal cord compression has also been documented.¹⁵

Patients with symptoms of thoracic spinal stenosis often may have findings on plain x-rays that suggest the etiology of the condition. Destructive lesions such as tumors and vascular malformations may be directly visualized, as can some fractures. When there is OLF, beak-like bony densities are characteristically seen projecting into the posterior aspect of the spinal canal on the lateral x-ray.¹⁶ Asymptomatic disc herniations are poorly visualized with plain x-ray, unless there is a significant amount of ossification.¹⁷ Unless there is an underlying metabolic abnormality, laboratory studies are typically normal.

Studies have compared the utility of CT and MRI in the diagnosis of thoracic spinal cord compression due to ossification of spinal ligaments.¹⁸ Both CT and MRI scanning may play a role in the diagnosis of thoracic spinal cord compression. Plain CT provides detailed information regarding the bony anatomy and degree of ossification of the spinal ligaments, as well as ossification of intervertebral discs and facet hypertrophy. MRI, on the other hand, helps to define the extent of spinal cord injury and identify facet cysts. In the setting of OLF, CT myelography is unnecessary, as it adds little to the data collected by MRI, and the contrast injection occasionally exacerbates symptoms of stenosis. If MRI is contraindicated in a patient, such as with a pacemaker, CT myelography is the study of choice.

Treatment

Thoracic myelopathy due to OLF or OPLL is not well treated by conservative methods such as nonsteroidal antiinflammatory medications and physical therapy. Surgical decompression is often required when stenosis results in myelopathy or debilitating radiculopathy. The best mode of surgical treatment for stenosing OLF is not well defined in the literature. Depending on the extent of compression, laminoplasty, partial or total laminectomy, circumferential decompression, and decompression with fusion have been proposed. The proponents of laminoplasty suggest that this procedure may produce less instability than other modes of decompression.¹⁹ Prognosis after surgery depends on the extent of compression and neurological deficit. Recurrent stenosis has been documented, making routine follow-up necessary.²⁰

For thoracic stenosis due to OPLL, multiple treatment options have been described, including laminectomy, laminoplasty, resection of the PLL, anterior decompression and fusion via thoracotomy, and posterior decompression and fusion.^21,22 Anterior decompression with fusion has been supported by a recent long-term study.²³Fujimura et al reviewed the data of 33 patients followed for more than 5 years after anterior decompression and fusion was performed for myelopathy in the thoracic spine due to ossification of the PLL. Their results suggested that anterior decompression with fusion can lead to favorable long-term results. They indicated, however, that patients with OPLL that affects multiple levels and those with concurrent OLF may not do as well in long-term follow-up.

Posterior surgical approaches provide an alternative treatment option for myelopathy due to OPLL, particularly when there is also OLF. It is suggested that, for OPLL, posterior decompression alone may not be sufficient due to draping of the spinal cord over the kyphotic thoracic spine. Some advocate a circumferential decompression through an isolated posterior approach.²⁴ High complication rates have been documented with this treatment, however, particularly when more than five levels are decompressed. In a study by Masahiko et al, as many as 33% of patients had deterioration in their neurological status following surgery. Both the relatively avascular nature of the thoracic spinal cord and adhesion of dural sac to the PLL are thought to contribute to these complications. Even higher rates of neurological deterioration have been seen with laminectomy alone, while lower rates have been reported with posterior decompression and fusion.²⁵ In theory, performing a laminectomy without fusion further destabilizes the spine and subjects the injured spinal cord to additional stresses that lead to a further decline in neurological function. Masashi et al suggested that in the neurologically intact patient, resection of the PLL is an acceptable treatment, but for patients with preoperative spinal cord injury, removal of the PLL increases the risk of paralysis.²⁶

Many patients with thoracic disc herniations are asymptomatic. When symptoms do exist, unlike patients with OPLL and OLF, conservative treatment and time can be sufficient to treat the majority of herniations. Modification of activities, physical therapy, and the careful use of nonsteroidal antiinflammatory drugs are the mainstays of conservative care. The literature suggests that less than 2% of thoracic discs require surgical treatment.²⁷ When conservative care fails to alleviate symptoms after 4 to 6 weeks, a neurological deficit progresses, or there is evidence of worsening myelopathy, surgical treatment is warranted.

Operative care of patients with thoracic disc herniations requires assessing several factors to determine operative risk and approach. Depending on the location and nature of the disc herniation, anterior, thoracoscopic, lateral, or posterior approaches may be used, the details of which are largely beyond the scope of this chapter. In general, anteriorlyoriented lesions require a direct ventral approach such as a transthoracic, thoracoscopic, or posterolateral (e.g., costotransversectomy, extracavitary) technique for safe resection that avoids cord manipulation, with bony reconstruction as necessary. Pulmonary function tests should be performed on patients with questionable pulmonary reserve if thoracotomy is considered. Discal calcifications, which are more common in the thoracic spine,²⁸ are associated with a greater degree of dural adhesion or associated dural calcification. Such information is important for the treating surgeon in deciding how much disc to remove from the dura, and whether concomitant dural resection needs to be considered. Once in the operating room, it is essential to identify the correct disc for resection. Sagittal CT reconstructions or MR images are essential to determine the appropriate level, and plain radiography or fluoroscopy in the operating room is standard practice for localization.

For most lateral soft disc herniations, the preferred approach is posterior and usually involves a pediculofacetectomy, typically perfomed by transpedicular or transfacet technique rather than laminectomy. Significant complications have been reported with posterior laminectomy alone,²⁹ including cord contusion and lack of improvement of symptoms. The need for fusion following thoracic disc removal remains controversial but generally depends on the assessed degree of stability of the region following decompression. It may be beneficial to resect a significant amount of vertebral body in cases of multilevel disc disease, or when the disc is in the lower thoracic spine, which may render the spine unstable and require secondary fusion.

Other Causes For Thoracic Spinal Stenosis

PROGNOSIS

An understanding of the long-term results of surgical treatment of thoracic spinal stenosis due to OLF is limited due to the lack of sufficient study. The literature suggests that a number of factors may make surgical outcomes less favorable. These include a longer duration of symptoms prior to decompression,³⁶ the presence of a proximal stenotic lesion,³⁷ and a greater degree of stenosis.³⁸ Another study looking at OLF did not show an association between preoperative symptom duration and results of surgery, although only 24 patients were evaluated.³⁹ As with the importance of the duration of symptoms, the data are also not clear on how preoperative neurological status affects surgical outcome. A meta-analysis by J. Anamasu et al suggests that the larger, more recent studies indicate a positive association between preoperative neurological function and surgical outcomes.⁴⁰ Other studies have highlighted some of the long-term complications of thoracic decompressive surgery, including the development of late kyphotic deformities and spondylosis.^41,42

Treatment of thoracic stenosis due to disc herniation has poorer outcomes for those patients with advanced age, extended preoperative duration of the condition, and greater degrees of myelopathy. Studies of patients with nonligamentous causes of thoracic stenosis have questioned the long-term prognosis following surgical decompression. Palumbo et al reviewed 12 patients with thoracic stenosis.⁴³ In 11 of the patients, stenosis was due to spondylosis. Their results showed that 5 of 12 patients had initial improvement that later declined. They suggested that deterioration was due to development of delayed stenosis, instability, or deformity.

Conclusions

Although thoracic spinal stenosis is a relatively rare condition as compared to stenosis of the cervical or lumbar spine, the treating physician must consider it in the differential diagnosis of the older patient with thoracic back pain or symptoms or signs of myelopathy. This awareness may prevent delay in the diagnosis of this condition due to its similarity to the presentation of lumbar stenosis. Most patients with a discogenic etiology for their pain can be treated conservatively. Both CT and MRI aid in defining the pathology and determining the most appropriate treatment. CT myelography should be avoided when possible in patients with OLF, as it can exacerbate myelopathy. When surgery is indicated, the approach should be dictated by the location of the disc. Patients with OLF or posterior longitudinal ligament or neoplasm will most likely require both surgical decompression and fusion.

Circumferential decompression carries a high risk of neurological deterioration. Patients with neurological complications due to metastatic lesions in the thoracic spine benefit from surgical decompression in addition to radiation. Larger, longer-term studies need to be performed to better define the importance of preoperative duration of symptoms, neurological status, and mode of decompression to overall prognosis. This task is made difficult by the relative rarity of thoracic spinal stenosis.