DISH aetiology is not known. The underlying cause of ossification in DISH is multifactorial, including metabolic, hormonal and genetic factors. Correlation has been described with centripetal obesity and primarily type 2 diabetes mellitus, a hyperinsulinaemia [5] or glucose tolerance disorder [6]. Frequent disorders, of lipid metabolism (hypercholesterolaemia, hypertriacylglycerolemia) and also hyperuricemia have been reported. Prevalence of diabetes in adults and glucose tolerance disorder reaches in patients with DISH 17–60 %, prevalence of DISH in adults with type 2 diabetes mellitus is about 13–50 % [7]. The current findings show that the main contributing factor in aetiopathogenesis of this disease is hyperinsulinaemia, or insulin resistance [8]. No correlation has been found between the degree of hyperglycaemia and bone changes in the skeleton.

Another important factor is hormonal effect particularly the effect of the growth hormone (GH). Similar hyperostotic bony bridges as in DISH occur also in acromegaly, which only proves the GH role in this process [9]. Age-related hormonal changes also play a role in GH regulation. It is assumed that decline in gonadotropins, androgens in particular, is responsible for increased production of estrogens as natural growth hormone stimulators. In hyperinsulinaemia, IGF-I secretion is stimulated, which increases osteosynthetic potential. This could explain the correlation between osteoproductive changes and incidence of diabetes mellitus and obesity. The increase rates of obesity in patients with DISH might imply a role of adipokines in the pathogenesis of DISH, as leptins and osteoblast inhibitor Dickkopf -related protein-1 (DKK-1) [10, 11].

An important role in aetiopathogenesis of DISH as well as ossification of the posterior longitudinal ligament (OPLL) is played also by genetic factors, due to familial incidence of DISH is described. Association with the metabolic syndrome. Correlation, however, has not been so far proven within the HLA systems which, as a result, cannot be used yet to distinguish mainly DISH and ankylosing spondylitis. Authors of extensive Slovak studies expressed a hypothesis about DISH autosomal dominant inheritance, which would explain also association between the above-mentioned metabolic disorders and DISH [12].

Based on analysis of bone turnover markers and their impact on bone formation and bone resorption, as well as the findings related to the role of hyperglycaemia and insulin resistance, associated with DISH, these parameters were examined in the patients with DISH treated in the Institute of Rheumatology in Prague. The results were published in details in the journal Česká revmatologie. Another study conducted by the same institute included 64 patients with DISH and focused on bone turnover markers (ALP, osteocalcin, vitamin D, pyridinoline, deoxypyridinoline) and local factors affecting the activity of bone cells (IGF-I, free IGF-I and their binding proteins IGFBP2, IGFBP3). The authors focused on bone metabolism in this group of patients and deviations, if any, either in favour of bone formation or bone resorption or a change in the activity of local factors. Results of the examinations, however, did not reveal any correlation with or a trend to a change in values of bone turnover markers or local growth factors in the bones of the group of patients with DISH [13].

As a result, it seems that the process of heterotopic bone formation in DISH has no impact on the values of bone turnover markers and the changes are rather of local nature.

Sometimes the course of the disease is quite asymptomatic, and the diagnosis is established accidentally, e.g. during chest radiography, or sometimes as late as during autopsy; but often it is a cause of multiple serious disorders. An important clinical symptom is onset of pain after the age of 40, often associated with gradual limitation of mobility of the spine, accompanied by rigidity especially in the mornings and evenings and sometimes resulting from inactivity or weather changes, similarly as in osteoarthritis. A typical feature is segmental involvement of the spine, with only sporadic cases of flowing ossification of all vertebrae in the same extent and with the bamboo-like pattern as seen in ankylosing spondylitis. Some patients report marked pain of the axial skeleton, mainly the cervical and lumbosacral spine, which may lead to development of radicular syndromes, but in general the pain is usually temporary, mild, associated with physical activity and accompanied by gradual limitation of mobility. Quite often, DISH may be combined with enthesopathic pain at the periphery (38 %). It affects predominantly heels, knees, shoulders, and elbows, sometimes with palpable bone appositions. Pain in these locations often occurs in combination with inflammatory irritation or overloading. Enthesopathic manifestations in pelvis develop usually in the late phase.

Other clinical manifestations of DISH may include also dysphagia caused by compression of the oesophagus by cervical ossification formations [14], which exacerbates in extension of the spine and gets milder in with its flexion. Pain in the chest may result from ossification of sternocostal and costovertebral articulations, and in elderly patients, it may be confused with heart disorders. A DISH-like process is ossification of the posterior longitudinal ligament (OPLL) in the cervical spine. This entity occurs most often in Asia; its prevalence in Japan is reported at about 1.5 % [15]. OPLL may be responsible for a number of neurological complications, particularly cervical myelopathy, paresthesia, motor disorders and other neurological symptoms.

DISH is quite often clinically and radiographically confused with ankylosing spondylitis (AS). DISH has a typical radiographic finding, with absence of SI joint involvement.

The most frequent and one of the first locations of typical radiographic changes is Th spine. The changes are usually asymmetrical, located on the right side, which is explained by pulsation of the aorta on the left side. Initially ossifications are incomplete and only gradually fuse. Intervertebral space is as a rule preserved. Involvement of C and LS spine develops later and is less frequent.

Unlike in DISH, in AS, there develops secondary metaplasia of inflammatory tissues along the anterolateral aspects of vertebrae and ligaments, with gradual ossification of the peripheral part of the fibrous ring of intervertebral discs and the adjacent ligaments. As compared to DISH, it is an inflammatory condition of the spine. AS typically affects apophyseal, costovertebral and sacroiliac joints (sacroiliitis); syndesmophytes can be found in the pelvic region. Patients with AS often have a history of iridocyclitis or arthritis and are HLA-B27 positive (Table 23.2).