Diffuse Idiopathic Skeletal Hyperostosis and Ossification of the Posterior Longitudinal Ligament

Etiology

The cause of DISH is uncertain. DISH has been tenuously associated with several risk factors, most notably diabetes mellitus and hyperinsulinemia, hyperuricemia, obesity, and other endocrine abnormalities. Other issues of possible importance include mechanical, trauma, dietary, environmental, and genetic factors.

The precise cause of OPLL is also unknown. Trauma, fluoride or vitamin A toxicity, diabetes mellitus, inherited factors, and prior infection have been proposed as causative, but no single factor has been definitively confirmed.

Prevalence and Epidemiology

DISH is common in the elderly. In one large population study, 25% of men and 15% of women older than 50 years showed radiographic changes of DISH based on chest radiographs. In men older than 80 years, the prevalence was 35%. The prevalence is lower in African Americans, Native Americans, and Asians. Because prevalence data are usually based on radiographic criteria relating to the spine, it is likely that the overall prevalence of DISH is higher than reported.

OPLL was initially believed to have a striking predilection in Japanese persons, thought to occur with a frequency of 1.0% to 1.7% among those with cervical spine disorders. More recently, reports indicate a more widespread ethnic and geographic distribution of this disease. The diagnosis of OPLL is usually established between 50 and 60 years of age. OPLL is more common in men than women. It is a relatively common occurrence in patients with conditions associated with hyperostosis of the spine, especially DISH.

Clinical Presentation

For multiple reasons, establishing strong causal associations between radiographic changes and clinical symptoms in DISH is difficult. In the elderly, minor or nonspecific symptoms are sometimes disregarded as age-related or attributed to other musculoskeletal diseases that frequently coexist with DISH. Spine-related symptoms, such as pain and stiffness, are usually mild with DISH and may be no greater than in the general population. Symptoms relating to DISH involving the peripheral skeleton are also ambiguous. Local pain, tendinosis, and swelling are considered common peripheral symptoms by Resnick. Bone changes of DISH in the pelvis may be asymptomatic. Although uncommon, cervical dysphagia, stridor, and aspiration are potentially serious symptoms attributed to DISH, affecting the cervical spine. Extensive DISH in the cervical spine may lead to difficult intubation. The most severe potential complication of DISH is fractures through the fused levels, which have been reported after minor trauma. Most common in the cervical spine, such fractures may displace markedly with disastrous consequences. Painful pseudarthrosis has been reported, particularly in the thoracic and lumbar regions.

Patients with OPLL may be asymptomatic or experience neck pain and/or neurologic symptoms, including lower or upper extremity motor and sensory abnormalities. Neurologic symptoms are more common in cases in which the ligament thickening decreases the spinal canal by greater than 60% or the sagittal diameter less than 8 mm. Neurologic symptoms may progress slowly or arise precipitously after trauma.

Pathophysiology

Resnick characterizes the pathology of thoracic spine bone formation in DISH into three types. Type I are ligamentous calcifications within fibers of the anterior longitudinal ligament, independent of disk abnormalities. Type II changes occur in association with disk abnormalities, especially involving the anulus fibrosus. These changes are identical to those found in spondylosis deformans. Type III changes are early ossification at the enthesis of the anterior longitudinal ligament along the midanterior aspect of the vertebral body. Ossification eventually bridges the disk spaces, producing flowing ossification of variable thickness. Advanced spinal changes consist of massive flowing ossification, frequently with large, irregular bony masses at the level of the disk. Peripheral pathologic changes of DISH include hyperostosis at entheses, para-articular osteophytosis, and conspicuous ligament ossification. Adjacent tendons show degenerative changes of tendinosis.

The bony growth in OPLL consists of mature cortical bone, most predominantly along the midline of the posterior vertebral bodies. It is uncertain whether the mechanism of bone formation involves endochondral ossification versus some other initiator of bone formation. Ultimately, the ligament ossification encroaches on the adjacent spinal cord, producing cord compression, infarction, and demyelination.

Imaging Techniques