Diffuse idiopathic skeletal hyperostosis

Diffuse idiopathic skeletal hyperostosis (DISH), also known as Forestier disease, is a common condition characterized by bony proliferation at sites of tendinous and ligamentous insertion. It primarily affects the spine of older individuals. On imaging, it manifests as flowing bridging anterior vertebral osteophytes, most prominent in the thoracic spine, and enthesopathy particularly around the pelvis. The presence of flowing anterior ossification spanning at least four vertebral levels with preserved disc spaces is often used to define the disease.

Epidemiology

DISH most commonly affects the elderly, especially in the 6^th to 7^th decades ³. The estimated frequency in the elderly is around 10% ⁶, with a male predominance. The prevalence appears higher in developed countries.

Associations

Recognized associations include:

• ossification of the posterior longitudinal ligament

  • accompanies around 50% of cervical DISH cases ¹³

• metabolic syndrome, obesity and diabetes mellitus

• ossification of the stylohyoid ligament and Eagle syndrome ¹⁵

  • DISH and ankylosing spondylitis patients both have measurably longer styloid process than controls

Clinical presentation

DISH is often identified as an incidental finding when imaging for other reasons. However, spine stiffness and decreased mobility similar to ankylosing spondylitis may be a presenting complaint. In the cervical spine, dysphagia may occur from direct impression upon the hypopharynx and upper esophagus. Spinal fractures, following even low impact trauma, may be a presenting scenario.

Pathology

The etiology of DISH is still unknown but likely involves an imbalance of growth factors and inhibitors leading to excessive bone formation. Histopathological features of spinal DISH include ⁵:

• focal and diffuse calcification and ossification of the anterior longitudinal ligament

• ossification of the paraspinal connective tissue and annulus fibrosus

• degeneration of the peripheral annulus fibrosus fibers

• anterolateral extensions of fibrous tissue

• hypervascularity

• chronic inflammatory cellular infiltration

• periosteal new bone formation on the anterior surface of the vertebral bodies

The rate at which the ossification progresses is reported to be around one vertebral space bridged every 10 years ¹⁴. The bone outgrowth begins at the anterior vertebral body above and below the disc.

Location

• thoracic spine (most common)

  • right anterolateral ossifications: particularly T7-11 ⁵

  • descending aortic pulsation prevents it on the left (opposite occurs if there is situs inversus)

• cervical spine (common)

  • anterior ossification, maximal at lower half of vertebral body

• lumbar spine (less common)

  • implies advanced/late disease

• sacroiliac joints

  • bridging osteophytes located ventrally

• extra-spinal

  • enthesophytes around the pelvis, knee, elbow, ankle

Radiographic features

The Resnick and Niwayama criteria remain the most widely used in the imaging diagnosis of DISH requiring: ¹⁹

• flowing osteophytes over at least four contiguous vertebrae of the thoracic spine

• preservation of intervertebral disc space without extensive degenerative intervertebral disease

• absence of facet and costovertebral joint ankylosis

• absence of sacroiliac joint erosion, sclerosis or fusion

This, however, essentially represents end-stage DISH and others have suggested using a vertebral span of three or even two levels in the setting of prominent extra-spinal enthesopathy.

Plain radiograph and CT

Spinal features

• flowing ossifications: florid, flowing ossification along the anterior or right ⁷ aspects of at least four contiguous vertebrae. The left lateral aspect is usually spared in the thoracolumbar spine, thought due to aortic pulsation inhibiting ossification ⁵.

• the osteophytes are largely paraspinal with a gap often seen between the vertebral body cortex and the osteophyte. This helps distinguish DISH from degenerative, marginal osteophytes from the vertebral endplate, and from the syndesmophytes of ankylosing spondylitis which involve Sharpey’s fibers of the annulus fibrosis ²¹.

• disc spaces are usually well preserved

  • osteophytes/ossification is out of proportion to the degree of disc degeneration

• resultant ankylosis and rigidity is more common in the thoracic than cervical or lumbar spine

  • frequently incomplete

  • can have interdigitating areas of protruding disc material in the flowing ossifications

• no facet joint of costovertebral ankylosis although entheseal bridging may be present ¹⁷

• chunky ossifications from the lower anterior half of the vertebral body in the cervical spine have been described as being like a candle flame or parrot-beak ²²

Sacroiliac features

• no sacroiliitis or erosions, however bridging osteophytes anterior to the SI joints, similar to flowing osteophytes in the spine, are common

• posterior bridging osteophytes occur less commonly

• the bridging osteophytes may mimic joint ankylosis on x-ray

Extraspinal features

• enthesopathy of the iliac crest, ischial tuberosities, and greater trochanters

• prominent pelvic enthesophytes are strongly associated with DISH ¹⁶

• exuberant spur formation in the appendicular skeleton (olecranon, calcaneum, patellar ligament) frequently present

• 'whiskering' enthesophytes

MRI

• findings correspond to those on CT

• typically true ossification with marrow signal

• fat metaplasia at the vertebral corners, and to a lesser degree bone marrow edema, may occur in DISH, but this is less common than in ankylosing spondylitis ¹⁸

Treatment and prognosis

DISH is generally managed clinically with analgesics and non-steroidal anti-inflammatory drugs when pain and stiffness are related. Managing comorbid conditions like obesity and diabetes is important. Possible complications may require specific treatment: 

• acute spinal fractures in low impact trauma

  • inspect the ossified ligaments for disruption, and compare with prior imaging where available to discern fracture from a region without ossification

  • note that fractures can also occur above or below the rigid segment due to altered biomechanics

  • chalk stick fracture

• dysphagia is reported in ~28% of cases involving the cervical spine ¹², which may be caused by mechanical compression due to anterior cervical bone production ⁸

• cervical DISH can make intubation more difficult ¹¹

• lumbar DISH can cause radiculopathy ¹¹

Differential diagnosis

• ankylosing spondylitis

  • syndesmophytes: thinner, form over the annulus, and are vertically oriented ("bamboo spine") 

  • true sacroiliitis involving the synovial joint (inferior two-thirds) with erosions, adjacent marrow edema/fat metaplasia and eventual intra-articular ankylosis, as opposed to the para-articular bridging osteophytes seen in DISH

  • osteoporosis is prominent

• degenerative spine disease

  • disc degenerative changes, disc height loss

  • osteophytes arise from the endplates rather than anterior cortex/paraspinal location

  • anterior longitudinal ligament uncommonly ossified in degenerative disease alone ⁹

  • prominent facet and apophyseal joints degenerative changes in addition

• retinoid arthropathy 

  • patients using retinoid acid for skin diseases

  • skeletal hyperostosis 

  • predominantly involves the cervical spine

• fluorosis

  • fluorite intoxication due to long-term ingestion

  • can cause paraspinal ligament calcification

  • will have accompanying osteosclerosis, or trabecular blurring/haziness

• if seen in a child, consider juvenile idiopathic arthritis (JIA)