The word “rheumatism” can be split into two words. And, “rheu” and “matism”. On changing the spelling we realise, the word rhymes with kangroo, as “roo” and “Matiz”

Rheumatism → roo + Matiz

The second part “Matiz” begins with a capital letter because it is the first word of the line. In the poem also, we see, evry line starts with a capital letter, even if it is in continuation with the previous line.

Therefore, the letter M is capitalized only to give a poetic effect.