DIAGNOSTIC CRITERIA
The first diagnostic criteria were described by Resnick and Niwayama [2]. These criteria consisted of the presence of continuous ossification of the right anterolateral aspect of four consecutive vertebral bodies in the absence of signs of degenerative disc disease, ankylosis or joint erosions.
In 1985, Arlet and Maziére similar criteria reducing the number of vertebral bodies affected by bony bridges to three. They also added ossification of the iliolumbar, sacroiliac and apical axis ligaments to their diagnostic criteria [11].
Utsinger et al. developed the first criteria for DISH that included diagnostic probability ranges (definite, probable and possible DISH) [7].
In 2019, Kuperus et al. developed diagnostic criteria aimed at diagnosing early DISH, with a sensitivity of 93% and a specificity of 83% [12].
DIFFERENTIAL DIAGNOSIS
DISH raises the differential diagnosis with two other entities: ankylosing spondylitis and lumbar osteoarthritis. DISH can coexist with both of them.
Ankylosing spondylitis is a seronegative spondyloarthropathy typically found in young males. It associates musculoskeletal and systemic symptoms such as anterior uveitis or inflammatory bowel disease. The axial skeleton is the most common site of involvement, specifically the sacroiliac joints in the form of sacroiliitis. Bony bridges between vertebral bodies through the Sharpey fibres of the annulus fibrosus appear in the spine (syndesmophytes).
Osteoarthritis or lumbar degenerative disease is a very common entity whose prevalence increases with age. Joint space reduction, subchondral sclerosis, osteophyte formation and degenerative disc disease stand out as radiological landmarks.
IMAGING FINDINGS
1) Axial
1.1 Spine
Spinal involvement is the most frequent finding in DISH. The most common location is at the lower thoracic spine, although all segments can be affected. Incidental diagnosis from a lateral chest X-ray is quite common.
In early stages of the disease it may go unnoticed, with only bony outgrowths of the anterior portion of the vertebral plates. These progress to bony bridges, initially incomplete and eventually complete. These bony bridges have been described in the classic literature as a "candle wax" morphology.
Another possible feature in the lower lumbar spine is the ossification of the iliolumbar ligament.
Cervical spine involvement is common too, and can be a cause of restricted mobility, neck pain and dysphagia.
1.2 Pelvis and sacroiliac joint
The involvement of the pelvis in DISH is very varied, being present in 55-100% of cases and therefore being the second most frequent site. Typical findings are joint fusion and anterior or posterior bony bridges. In the rest of the pelvic bone structure, findings such as bony bridging at the symphysis pubis and ossifications of the hamstring entheses, iliac crests and cotyloid ridges have been described.
1.3 Rib cage
Few scientific studies refer to DISH findings in the rib cage, although series describe it as being present in up to 20-80% of patients. Findings include bony outgrowths with cortical hypertrophy of the costovertebral joints, which may also present bony bridges. These lesions may raise the differential diagnosis with Paget's disease of bone, especially on scintigraphy.
2) Peripheral
2.1 Knee
Knee ossifications in DISH context have been described in 67-88% of cases. The patella, quadricipital tendon and patellar tendon at both proximal and distal insertion are involved. Ossification of the bony insertion of the anterior cruciate ligament, the proximal tibioperoneal joint and enthesitis of the fabella have also been reported.
2.2 Foot
Calcaneal exostosis at the Achilles tendon insertion and plantar fascia is characteristic. Other less known but described findings are ossification of the insertion of the peroneus brevis muscle at the base of the fifth metatarsal and metatarsophalangeal para-articular ossifications.
2.3 Shoulder, elbow and hands
Radiological findings in the upper limbs are less frequent than in the lower limbs. However, in the shoulder, exostoses of the inferior border of the acromion, coracoid process, greater humeral tuberosity and deltoid tuberosity have been described. In the elbow, olecranial and at both epicondyles exostoses have been described. In the hands, specifically in the fingers, there can be quadrature of the phalanges with widening of the joint space and para-articular ossifications.
COMPLICATIONS
1. Vertebral fractures
Although DISH may be asymptomatic, the complications described within this disease reach significant severity. These include a predisposition to suffer vertebral fractures, even in the setting of low-energy trauma. These fractures are more frequent at the dorsal and cervical level. The more severe the axial involvement of DISH is, the more likely it is that vertebral fractures will lead to a spinal cord injury.
2. Cervical compressive symptoms
Anterior growth of the bony bridges creates can compromise other cervical structures. Complications can occur in the form of dysphagia due to extrinsic compression and, more rarely, dyspnoea. Therapeutic options include surgical resection of major bony excrescences.
3. Heterotopic ossifications
Patients with DISH are thought to be up to three times more predisposed to heterotopic ossifications around any external insult. A higher frequency of heterotopic ossifications following hip arthroplasty procedures has been reported in the literature.