diff --git a/after/syntax/c.vim b/after/syntax/c.vim
index 5996c7a..12b7f7c 100644
--- a/after/syntax/c.vim
+++ b/after/syntax/c.vim
@@ -45,7 +45,7 @@ if exists('g:c_doxygen') && g:c_doxygen
 
   " Match: @param name description.   @retval name description.
   "               ^^^^                        ^^^^
-  syn region cDoxygenSpecial matchgroup=cDoxygenComment start='@\(param\(\[\(\|in\|out\|in,out\)\]\)\|retval\)\=\s\+' end='\(\s\|$\)' contained display
+  syn region cDoxygenSpecial matchgroup=cDoxygenComment start='@\(param\(\[\(\|in\|out\|in,out\)\]\)\?\|retval\)\=\s\+' end='\(\s\|$\)' contained display
 
   " Match: @tparam name description.
   "                ^^^^