Diffuse idiopathic skeletal hyperostosis (DISH) is a non-inflammatory process characterized by hyperostosis at tendon insertions and around joint capsules and ossification of the anterior longitudinal ligament of the spine. The prevalence of DISH is approximately 10% in persons aged >50 years.¹⁾ The condition is diagnosed based on the following radiological criteria: the presence of ≥4 consecutive fused vertebral bodies with contiguous ligamentous ossification, preservation of the intervertebral disk space, and absence of gross degeneration in the apophyseal and sacroiliac joints.²⁾ Although various established orthopedic problems are reportedly associated with DISH, including vulnerability to trauma and risk of fractures,^3-8) few studies have reported DISH as a cause of restrictive respiratory impairment.^9-11) Here, we report two cases of DISH that led to type 2 (hypercapnic) respiratory failure by limiting the respiratory movement of the chest wall. To our knowledge, this is the first report describing patients with severe type 2 respiratory failure associated with DISH, which necessitated non-invasive positive-pressure ventilation therapy (NIPPV).

The flexibility of the spinal column is reduced in patients with DISH owing to the ossification of the anterior longitudinal ligament. The coexistence of ankylosis of the costovertebral joints with ossification further restricts thoracic motion, reducing the mechanism of lung expansion to diaphragmatic contraction only.^9,10) In ankylosing spondylitis (AS), which shares the same features of spinal ankylosis with DISH, ankylosis of the joints reduces the compliance of the thorax, fixing the thorax in the extended position.^12,13) These changes lead to decreased vital capacity and relatively increased RV with preserved TLC. Decreased alveolar ventilation volume leads to increased PaCO₂. These features were observed on spirometry in both of our cases. In Case 1, the preserved minute volume during respiratory function tests with a compensatory increase in respiratory rate indicated no apparent loss of strength in the respiratory muscles themselves. However, there was a limit to the increase in respiratory rate during continuous movement. The patient could not compensate for increased oxygen demand owing to persistent tachypnea; thus, his oxygen saturation (SpO₂) decreased while walking and other daily activities. We also observed increased RV/TLC with relatively preserved TLC along with severe restrictive respiratory impairment in Case 2. The ossification of the anterior longitudinal ligament observed in the two patients was not as bulky as previously reported in patients with DISH;¹⁴⁾ however, there were no signs of sacroiliitis, which is the hallmark sign of AS. Additionally, both patients denied experiencing back pain when they were young. Considering these facts and their age, we concluded that DISH rather than AS was the underlying condition for restrictive respiratory impairment. Severe upper respiratory tract obstruction caused by cervical osteophytes is a mechanism of respiratory failure related to DISH.¹⁵⁾ However, neither patient had lesions obstructing their upper respiratory tracts. The present cases indicate that restrictive ventilatory function caused by thoracic lesions may lead to severe respiratory failure in patients with DISH. In line with previous reports,^9,10) we assume that ankylosis of the costovertebral joints contributes to severe respiratory dysfunction. The patients did not take medications that may have caused drug-induced hypercapnia, such as tranquilizers or painkillers.

The pattern of spirometry was similar to that observed in patients with amyotrophic lateral sclerosis (ALS).¹⁶⁾ The two patients described here were evaluated in our neurology department for suspected motor neuron disease with respiratory onset. However, needle electromyography did not show a progressive denervation process, which is the hallmark of ALS,¹⁷⁾ in either patient. Although neuromuscular diseases are major extrinsic factors of restrictive spirometric patterns, physicians should also pay attention to abnormalities in vertebral bodies, especially after ruling out common restrictive causes. In Case 2, reduced FEV₁% was also observed, although no signs of emphysema were noted on chest CT. Based on the patients’ smoking history and right heart strain indicated by echocardiography, he might have had non-emphysematous chronic obstructive pulmonary disease. However, the use of bronchodilators and inhaled corticosteroids did not relieve his respiratory symptoms. Moreover, signs of asthma, such as persistent coughing and wheezing sounds, were not observed. Based on a study reporting the association of DISH with reduced FEV₁,¹⁸⁾ we speculate that DISH affected the patient’s expiratory ability and vital capacity.

To our knowledge, these are the first two reported cases of DISH associated with severe type 2 respiratory failure, although the prevalence rate of thoracic DISH is high among older adults. Along with skeletal problems limiting thoracic motion, the low BMI in this population might be related to adverse outcomes because physical frailty is a known aggravator of respiratory diseases.¹⁹⁾ However, despite the severity of their respiratory failure, the muscle strength in the extremities, including grip strength, in these patients remained normal on MMT, and they did not report unintentional weight loss at the time of diagnosis. Thus, we concluded that DISH restricting thoracic motion rather than physical frailty was the leading cause of their restrictive ventilatory function. NIPPV allowed the patients to perform their activities of daily living.

Although DISH is a relatively common condition among older adults, its effect on respiratory function is underestimated. Herein, we illustrated the importance of focusing on thoracic movement and explicitly evaluating DISH when physicians encounter cases with restrictive respiratory impairment of unknown etiology. Restrictive respiratory impairment caused by the disease can follow a severe life-threatening clinical course, as described in the present report.