Diffuse idiopathic skeletal hyperostosis
Overview
- Definition: A systemic "bone-forming diathesis", non-inflammatory condition characterised by calcification and ossification of ligaments and entheses.
- Epidemiology:
- Age: Elderly (> 60 years). Rare < 50.
- Gender: M > F (2:1).
- Associations: Strongly linked to metabolic syndrome (obesity, type 2 diabetes mellitus, hypertension, dyslipidaemia).
- Clinical presentation:
- Often asymptomatic or mild stiffness (disproportionately mild compared to imaging).
- Dysphagia (large cervical osteophytes).
- Myelopathy (if associated with OPLL).
Diagnostic criteria (Resnick and Niwayama)
Strict adherence to these criteria is tested in exams to differentiate from AS and DJD.
- Flowing ossification along the anterolateral aspect of at least 4 contiguous vertebral bodies.
- Preservation of disc height in the involved segments (and absence of severe degenerative disc changes).
- Absence of facet joint ankylosis and absence of sacroiliac (SI) joint erosion/fusion.
- Crucial: If the SI joints are fused, it is not DISH.
Imaging features
Spine
- Thoracic spine (Classic location = T7-T11):
- "Melting candle wax" appearance: Flowing, undulating ossification of the Anterior Longitudinal Ligament (ALL).
- Right-sided predominance: The pulsation of the descending aorta inhibits ossification on the left side.
- Exception: In patients with situs inversus, changes occur on the left.
- Cervical spine:
- Thick anterior osteophytes (C4–C7).
- Can cause mechanical dysphagia or difficult intubation.
- Lumbar spine:
- Thick flowing ossification, often bridging.
Extraspinal ("whiskering")
- Pelvis:
- "Whiskering" of the iliac crests, ischial tuberosities, and trochanters (enthesophytes).
- Para-articular bridging of the SI joints (superior margin) without intra-articular fusion.
- Patella: Anterior surface ossification (quadriceps insertion).
- Heel: Calcaneal spurs (Achilles/plantar fascia).
Complications (check areas)
- Fracture (Chalk stick / Carrot stick fractures):
- The fused spine acts as a long lever arm. Minor trauma can cause catastrophic hyperextension fractures.
- Highly unstable; high risk of cord injury/epidural haematoma.
- Action: CT whole spine is mandatory if a DISH patient presents with back pain after minor falls.
- OPLL (Ossification of posterior longitudinal ligament):
- Strong association with DISH.
- Can cause spinal canal stenosis and myelopathy.
- Dysphagia: Due to mass effect from cervical osteophytes.
Differential diagnosis
| Feature | DISH | Ankylosing spondylitis | Degenerative disc disease |
|---|---|---|---|
| Ossification type | Flowing / bulky (ALL) | Thin / vertical (Syndesmophytes) | Horizontal / claw-like (Osteophytes) |
| Disc height | Preserved | Preserved (until late) | Reduced (Vacuum phenomenon) |
| SI joints | Normal (or superior bridging only) | Sacroiliitis (Erosions/Fusion) | Normal / Degenerative |
| Facet joints | Normal | Fused / Ankylosed | Degenerative |
| Age | Elderly (> 60) | Young adult (< 40 onset) | Increases with age |
High-yield exam pearls
The "aorta shield" effect
DISH is strictly right-sided in the thoracic spine.
- If you see flowing ossification on the left side of the thoracic spine, look for:
- Situs inversus (check the gastric bubble/heart).
- Psoriatic arthritis or Reactive arthritis (bulky paramarginal syndesmophytes can be asymmetric, but usually look different).
The trauma trap
A fracture in a DISH spine is often trans-discal (through the calcified disc space).
- It can be subtle on X-ray.
- Clinical rule: DISH + Minor Trauma + Pain = CT Spine (don't stop at X-ray).
DISH vs AS
The most common viva question regarding DISH is distinguishing it from Ankylosing spondylitis.
- Look at the SI joints.
- If the SI joints are open and visible
DISH. - If the SI joints are fused/eroded
AS.
