There was nothing unusual about M.L.'s headache -- until her left side suddenly became paralyzed and she fell out of her chair.
''Even as I crawled to the phone to call a friend, I still thought it might be just a sinus headache," said M.L., who asked to be identified only by her initials.
Before she fell, the 58-year-old had spent days applying warm compresses to relieve the pressure in her face and taking antibiotics for presumed sinusitis. But nothing had helped and her left arm became progressively weaker.
''The pain wasn't anything I hadn't experienced before -- pain behind my eyes, over my nose, into the upper parts of my cheek," said the grandmother who had suffered from sinus headaches before this incident last year.
But now with her left side partially paralyzed and a friend driving her to the emergency room, thoughts of something much more serious began to sink in.
''My friend wheeled me into the emergency room yelling, 'I think she is having a stroke!' " M.L. remembered, ''and, of course, everyone came running."
Once inside Massachusetts General Hospital, she was found to have no evidence of sinusitis. The doctors did find muscle weakness and paralysis from her left facial muscles to her left arm and leg. She couldn't squeeze the doctors' hands. But her reflexes and her speech were normal. Her pupils were equal and reacted appropriately to light.
''The clinical exam indicated that there was something clearly wrong in the right side of her brain," which controls the left side of her body, said Dr. Robert Friedlander, a neurosurgeon at Brigham and Women's Hospital, who wrote about M.L.'s unusual case in the New England Journal of Medicine.
The doctors considered a variety of conditions, including a brain tumor, cancer spreading from elsewhere in her body, or possibly an infection. Because the symptoms had progressed over days, a stroke was less probable.
The CT and MRI resolved their questions: There in black and white was a large infection deep inside M.L.'s right brain -- a sphere of bacteria pushing on her motor cortex and increasing the pressure inside the ventricles in her brain.
''This was a big abscess," Friedlander said. ''Without immediate and appropriate surgical and medical treatment, it could have been fatal."
M.L. remembers most of the first 24 hours in the hospital as a blur. But some images stand out. ''To have your adult children standing by the side of your bed weeping was quite an experience," she said.
By the next morning, M.L. was undergoing neurosurgery. She was awake and remembers Dr. Michael Medlock of Mass. General screwing a halo into her skull. ''I drilled a hole the size of a quarter into her skull," said Medlock, a neurosurgeon who sees roughly one brain abscess per year.
He drained some fluid from the infection to test it and relieve her pain. Eventually the lab identified two types of bacteria in the fluid; one had likely originated in her mouth, an organism called Haemophilus aphrophilus.
But now that her doctors had explained the cause of M.L.'s symptoms, they were presented with a new mystery: How did bacteria get from M.L.'s mouth to her brain? How did it cause a brain abscess?
''We see brain abscesses in immune-compromised individuals but, for other patients, the most common way to get a brain abscess is from infections in the ear, sinus, heart, or lungs," said Friedlander. ''In her case, there was no sign of infection in any of these places, and we can assume it most likely came from her blood."
Two weeks before the event, M.L. had been to the dentist for teeth cleaning and to have four cavities filled.
''I remember them using . . . really high-pressure water [for cleaning]," she said. ''It seemed pretty violent, and the theory is that this is how the bacteria got in my blood and traveled to my brain."
According to Friedlander, the infection could not have been prevented. ''We don't know why exactly it happened; we suspected dental work but really we will never know for certain."
''These are very very rare; the risk of not getting routine dental care is much greater than the chance of getting a brain abscess," Friedlander said. ''I still go to see the dentist regularly."
Another of her doctors, Dr. John Beigel, theorized that a small hole found in the wall that divides the right and left sides of M.L.'s heart might make her more susceptible to abscesses. The hole, a rare condition, allows some of her blood to bypass her lungs, which otherwise might have filtered out the bacteria that triggered the infection.
M.L. was given antiseizure medication and steroids to reduce the swelling in her brain. After surgery and two months of intravenous antibiotics and physical therapy, M.L. fully recovered. She now has to take antibiotics before any dental procedures.
Once the impenetrable wall of M.L.'s abscess was broken and the bacteria drained, the same inflammatory cells that helped create the problem dissolved it away, but not without changing M.L.'s life. ''I am doing stuff now I want to do, not working so hard, and really enjoying my friends and family."
A version of this case first appeared as part of the Massachusetts General Hospital Case Records in the New England Journal of Medicine.
Dr. Terry Schraeder can be reached at firstname.lastname@example.org.