Actress Kristen Johnston has been diagnosed with a rare autoimmune disorder which has left her struggling to walk.

The 3rd Rock From the Sun star has spent months trying to discover the cause of severe muscle weakness which has affected her mobility, and after visiting a number of different doctors, she was finally diagnosed with "lupus myelitis".

Johnston took to her page on Tuesday (17Dec13) to tell her followers about her health crisis, writing, "I've put this off as long as possible, but it's time for me to be upfront with you about something that's been going on with me...

"I've been really, really sick since early September. Full body muscle weakness to the extent that just walking difficult, stairs impossible and I even need a neck brace to hold my head up.

"Lying down I'm fine, which is great for writing or killing time on social media, but it's kind of disastrous when you're in a sitcom that requires tons of physical comedy."

She adds, "My immune system has decided to go rogue & attack my muscles. The good news: I'm finally on the right treatment, feeling better & back at work!"

Johnston is currently working on U.S. sitcom The Exes, but she had to miss two episodes due to her health issues, and credits former King of Queens star Leah Remini for stepping in to help cover her.