In a video, which was posted to Instagram on Tuesday evening, the Cruel Intentions star described how she was recently cast in a new show that was set to shoot in Canada but her health began to deteriorate.
“I was having trouble walking,” Blair said in the video.
“I couldn’t use my right hand at first and then my left hand, and I couldn’t talk well and I had a hard time getting dressed.”
It was then, the actor said, that she realised that she was “in a full-blown MS attack”.
In the caption for the video, the 47-year-old stated that despite regularly struggling with MS attacks, she is grateful for everything in her life.
“I have decided to share my story. I have MS. I struggle with MS attacks. I am in the thick of it, but I keep pushing forward,” Blair said.
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“I hope my story will inspire others with MS to do the same.”
In a bid to help other people with MS recognise when they experiencing an attack, Blair described what the term actually means, adding that it can also be referred to as a “relapse, flare or exacerbation”.
“MS attacks happen when old symptoms suddenly become worse or when new symptoms appear,” Blair added.
“MS attacks can be unpredictable and can have a big impact on your life.”
Several of the actor’s famous friends and fans have since flooded the video with comments of support and adoration.
While some sent their well-wishes to the actor, others thanked her for sharing her battle with the neurological condition.
“Thank you for continuing to share your story with the world,” one person wrote.
“It gives us who deal with MS courage and strength knowing we too can get through it no matter how dark our days may be.”
Another added: “You’re doing so much for so many people.”
Fellow actor Kate Beckinsale wrote: “So much love to you”.
According to the UK charity MS Trust, an MS attack or relapse is a "relatively sudden (over hours or days) episode of new symptoms or a worsening of existing MS symptoms". People call relapses by different names including an attack, a flare up, an episode, a blip or an exacerbation.
The video is the latest update from Blair who has posted many emotional photographs to social media since her diagnosis in October 2018.
Last month, the actor marked the end of a round of treatment for her MS with a celebratory photo of her newly shaved head.
In the photo, which the actress shared to Instagram, she smiled as she prepared to leave her hospital room with the help of an Alinker walking bike.
“Today is a banner day. I am being discharged from the care of an incredible team of nurses and techs and a visionary doctor who believes in my healing as much as I do,” Blair. captioned the image:
“This has been a process. And will continue to be one. I am immunocompromised for next three months at least. So no kisses please. I wanted to make sure any complications that might arise here were my private space. And we got through brilliantly.”
The NHS states that MS is a condition that can affect the brain and spinal cord, causing a wide range of potential symptoms, including problems with vision, arm or leg movement, sensation or balance.
It's a lifelong condition that can sometimes cause serious disability and is most commonly diagnosed in people in their 20s and 30s.