Patients

Between March 1998 and September 2000, 26 patients with a recent onset of myositis were identified at the rheumatology unit at the Karolinska Hospital, Stockholm, which has a referral area (northern part of Stockholm County) with a population of about 900 000 inhabitants. Patients with inclusion body myositis (n = 2), malignancy (n = 1), and those with another defined inflammatory connective tissue disease (CTD) or antibody profile indicating another CTD (n = 6) were excluded from this study. Thus 17 patients were included. The diagnosis of PM/DM was based on the criteria suggested by Bohan and Peter.^11,12 Furthermore, the PM diagnosis was confirmed by the presence of non-necrotic fibres invaded by mononuclear inflammatory cells in seven patients. Inclusion body myositis was diagnosed according to diagnostic criteria proposed by Griggs et al.¹³ The local ethics committee approved the study, and informed consent was obtained from all patients.

Clinical features

Age, sex, ethnicity, smoking habits, and the presence of muscle weakness, myalgia, skin rash, dysphagia, arthritis, arthralgia, Raynaud’s phenomenon, swollen hands, cough, and dyspnoea were recorded at the time of diagnosis.

Laboratory studies

Biochemical analyses included serum levels of creatine kinase (CK), alanine and aspartate aminotransferase (ALT, AST), and lactate dehydrogenase (LD). Serological studies included rheumatoid factor (RF) measured by nephelometry, and antinuclear antibodies (ANA) analysed by indirect immunofluorescence using Hep-2 cells. Antiribonucleic protein (RNP), anti-Sm, anti-Ro/SSA, anti-La/SSB, and anticentromere antibodies were detected by enzyme linked immunosorbent assay (ELISA). Antihistidyl-tRNA synthetase (anti-Jo-1) was detected by ELISA or immunodiffusion. Tests were performed at the Departments of Clinical Chemistry and Clinical Immunology, Karolinska Hospital, Stockholm.

Radiology

A chest x ray examination was performed by the Fuji digital phosphor-plate technique and included anteroposterior and lateral views. Computed tomography (CT) examinations of the lungs were performed with a Siemens Somatom Plus scanner (Erlangen, Germany) and included both spiral CT and HRCT. All examinations were performed in maximal inspiration without intravenous contrast medium enhancement. The spiral CT was performed with a 10 mm slice thickness and pitch 1.0–1.2, depending on the length of the lungs. The HRCT was performed with a 2 mm slice thickness and 20 mm interspaces covering the entire lung. Both the 2 and 10 mm images were reconstructed with a high spatial frequency algorithm (defined for the actual scanner as AB 7541).

An experienced thoracic radiologist (ER), who was unaware of the clinical details, visually evaluated the radiological findings. The radiologist determined the presence and degree of a reticular pattern caused by a combination of interlobular lines and irregular thickening of interlobular septa as well as a reduction of the lung volumes. An arbitrary scale with a range of 0–3 was created: 0 = absence, 1 = mild, 2 = moderate, and 3 = severe degree of reticular pattern.

Pulmonary function tests

Total lung capacity (TLC) and vital capacity (VC) were determined in a body plethysmograph (Autobox 2800, Gould Electronics, The Netherlands). The single breath transfer factor for carbon monoxide (Tlco) was determined according to a modified Krogh procedure¹⁴ using the above spirometry system, and was corrected for alveolar volume. The results from lung function investigations were expressed as a percentage of the predicted normal using standard reference values¹⁵; values below 80% of predicted were considered to be abnormal.

Interstitial lung disease

A diagnosis of ILD was defined as the occurrence of radiographic signs of ILD on chest x ray examination or HRCT and/or restrictive ventilatory defect (reduction in lung volumes).

Statistics

Statistical analyses of the data were performed using Student’s t test (age, lung function, and time since onset of symptoms) and maximum likelihood χ² test (sex, PM/DM diagnosis, and the prevalence of pathological laboratory tests and symptoms). Significance was established at the p<0.05 level.

Clinical features

Of the 17 patients included, nine had PM and eight had DM. Table 1⇓ summarises the main characteristics of the patients at presentation. All patients but one were white. At the time of diagnosis four patients (PM = 2, DM = 2) were current smokers, six were ex-smokers who had stopped smoking 0.5–49 years ago. Pulmonary symptoms, including dyspnoea and/or cough, were present in 71% of the patients. There was a significant difference between PM and DM for the presence of arthritis and Raynaud’s phenomenon (p<0.05 for both).

Laboratory findings

Table 1⇑ summarises the laboratory findings. CK was increased in all patients; range 5– >76.8 μkat/l (reference value <2.5 μkat/l for women and <3.3 μkat/l for men). AST was increased in all patients. ALT was increased in all but four, who had a value slightly below the upper reference value. Positive RF was detected in 7 (41%) patients, and ANA with a speckled pattern were present in 9 (53%) patients. Anti-Jo-1 antibodies were present in 4/9 (44%) patients with PM and in none of the patients with DM (p<0.05). Anti-Ro/SSA antibodies were detected in 4/9 (44%) patients with PM and 2/8 (25%) with DM. Anti-La/SSB antibodies were detected in 2/9 (22%) patients with PM and in none of the patients with DM. Anti-RNP, anti-Sm, anticentromere, and Scl-70 antibodies were negative in all patients.

Radiographic findings

Radiological signs of ILD on chest x ray examination were present in eight patients: five with degree 1 and three with degree 2 abnormalities. HRCT of the lungs was performed in 15 patients, and radiological signs of ILD on HRCT were present in nine (table 2⇓). The most common abnormalities consistent with ILD on HRCT were interstitial thickness (60%) and volume reduction (33%).

Pulmonary function tests

Pulmonary function tests were performed in 11 patients, either before (seven patients) or within 3 weeks after (four patients) starting corticosteroid treatment. VC and TLC were reduced (<80% of the predicted values) for six patients, respectively. The mean VC and TLC were 83% and 86% of predicted values, respectively. Tlco was reduced (<80% of the predicted values) in all tested patients but one; mean 63% of predicted value (table 2⇑).

Interstitial lung disease

Eleven patients (65%) demonstrated objective signs of ILD. Table 3⇓ presents the characteristics of the patients with and without ILD. Patients with ILD had significantly lower VC and TLC than those without (p<0.05 for both). Three of 11 patients with ILD had neither cough nor dyspnoea, and among the six patients without any objective signs of ILD, four reported respiratory symptoms. There was no statistically significant difference between patients with and without ILD for the presence of cough or dyspnoea. All the men, but only 5 of the 11 women, showed signs of ILD (p<0.05). Arthritis was more common in patients with ILD than in those without (p<0.05). Anti-Jo-1 antibodies were found only in patients with PM (p<0.05), and all had ILD. There was no difference between those patients with and without ILD for the other autoantibodies. ILD was more often present in patients with PM than in those with DM, although this difference was not significant. The average time from onset of myositis symptoms until the diagnosis was shorter in the patients with ILD than in those without (p<0.05). There was no significant difference between the groups for the presence of dysphagia, arthralgia, myalgia, Raynaud’s phenomenon, or increased levels of CK, LD, ALT, or AST.