Diffuse idiopathic skeletal hyperostosis (DISH), also known as ankylosing hyperostosis and Forestier’s disease, has been traditionally considered to be a non-inflammatory systemic condition with underlying metabolic derangement that results in new bone formation. It is characterized by the formation of new bone along the anterolateral spinal column affecting at least four contiguous vertebraes.

DISH in spine most commonly occurs on the right side of thoracic spinal segment(proposed to be caused by the pulsatile aorta's protecting impact on the left thoracic spine.), though it also affects cervical lumbar spine, ossification at the location of entheses in the peripheral skeleton may also be present, for example at the shoulders, elbows, wrists, pelvis, hips, knees, and ankles.

Although the disease is significantly prevalent, its awareness is still limited among clinicians, which could lead to misdiagnosis, unnecessary investigations and treatment.

DIAGNOSTIC CRITERIA :

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In 1976, Resnick and Niwayama published their strict radiographic criteria to diagnose DISH:

1. The presence of ‘flowing’ calcification and ossification along the anterolateral aspects of at least four contiguous vertebral bodies with or without associated localized pointed excrescences at the intervening vertebral body-disc junctions.
2. A relative preservation of disc height in the involved areas and the absence of extensive radiographic changes of ‘degenerative’ disc disease.
3. Absence of apophyseal joint bony ankylosis and sacroiliac joint erosion, sclerosis or bony fusion.

ETIOLOGY :

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Various risk factors have been identified in literature including Gout, hyperlipidemia, and DM. HLA-B8 is common in both DISH & DM.
- Epidemiological associations

PATHOLOGY :

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The pathogenesis of DISH is currently unknown.

On a microscopic level, the new bone forms a bone bridge from one vertebral body to the adjacent vertebral body. The new bone is in continuum with the upper and lower vertebral body and contains mostly cortical and some cancellous bone. Woven bone is present, suggesting an ongoing remodeling in the new bone formation.

HISTORY & PHYSICAL FINDINGS :

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•  Patient is generally asymptomatic, and is diagnosed incidentally.
•  Aged over 40 years and above . Male > Female.
•  Back pain and stiffness that increases with inactivity, wet weather and cold temperature.
•  Decreased ROM may be present.
•  Minor trauma can result in spinal fracture and instability, it can lead to neurological abnormality.
•  Dysphagia, hoarseness, sleep apnea, dyspnea and upper airway obstruction ( in case of cervical involvement)

TREATMENT :

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•  NSAIDs & Analgesics
•  Physiotherapy treatment aims at reducing pain and improving ROM, secondary aim is to improve strength in muscles.
•  Light exercise and localized application of heat has been found to be effective in several studies.

Differentiating features between AS & DISH

	DISH	AS
Age/Gender	Middle aged people Male > Female	Younger people Male > Female
Genetic factor	Associated with HLA-B8 (common in patients with DISH & DM)	Associated with HLA-B27
Lab findings	ESR, CRP, Rh factor, Antinuclear antibody- Normal	ESR, CRP, Rh factor, Antinuclear antibody- Elevated
RADIOGRAPHIC FEATURES:
Syndesmophytes	Nonmarginal	Marginal
Vertebral bodies	"Flowing" ossification and hyperostosis; large osteophytes; bony ankylosis frequent radiographically, less frequent pathologically	Thin syndesmophytes; osteitis with "squaring"; extensive bony ankylosis radiographically and pathologically
Intervertebral discs	Normal or mild decrease in height	Normal or convex in shape
Apophyseal joints	Normal or mild sclerosis; occasional osteophytes	Erosions, sclerosis and bony ankylosis
Sacroiliac joints	Para-articular osteophytes	Erosions, sclerosis and bony ankylosis
Peripheral skeleton	"whiskering", para-articular osteophytes; ligament calcification and ossification, hyperotosis	"whiskering", arthritis