Are collagenous and lymphocytic colitis different aspects of the same disease?

Objective. Collagenous colitis (CC) and lymphocytic colitis (LC) are two subtypes of microscopic colitis (MC). Even though they most often are described as different entities they share many clinical and histological features. The aim of this study was to investigate the occurrence of conversion between CC and LC in a larger cohort of patients. Materials and methods. All 664 patients in our Pathology register with a diagnosis of CC and LC were scrutinized and those where additional endoscopies had been carried out were included, and their biopsies were re-examined. Results. Sixty-five patients (55 women, 10 men, median age 58 years; range 29-86) fulfilled our criteria for inclusion. The primary diagnosis was CC in 47 patients (39 women, 8 men, median age 58 years; range 29-86) and LC in 18 patients (16 women, 2 men, median age 58 years; range 33-74). Conversion occurred in nine of the 65 patients (14%, all women, median age 59 years; range 41-72), three from CC to LC and six from LC to CC. Conclusion. This study has found that patients can show histological features consistent with both CC and LC over time. These patients could represent a subgroup with a true conversion between two separate entities. Alternatively, MC could be a spectral disease where the varying histological features are manifestations of the natural fluctuation. A third possibility could be that the histological changes reflect different manifestations during the disease course and consequently, the diagnostic criteria could be too vague.