Excruciating leg pain hampered her for weeks. What was wrong?

An MRI. ruled out the most common cause of sciatica. Then her rheumatologist asked about her asthma.

The 56-year-old settled into her office chair. Sitting was extremely uncomfortable these days. She clicked on the link to her Facebook page and began her post: "For the past few weeks I've suffered a lot from sciatica, which is excruciating pain in my leg caused by a pinched or irritated nerve." It hurt to stand; it hurt even more to sit down - the weight of his body was torture on his thighs. The only times she barely felt any pain were when she was lying down and walking. She was a journalist, and for the past few weeks she's had to do all of her reporting and writing from her couch. She had seen her doctor, who gave her a muscle relaxer, and her chiropractor, who adjusted her back. Neither provided relief.

The patient has been in good health all her life. Then, at age 50, she battled a horrific bout of colon cancer. She fully recovered with lots of help from surgery and chemotherapy. Four years later, she unexpectedly developed severe asthma. A caring pulmonologist helped her get it under control, but she hadn't felt well since. It was as if she had crossed a threshold. She had these weird bouts of aches and pains, occasional numbness and weakness that lasted for days. The lymph nodes exploded to a painful size, then disappeared. Now this terrible leg pain. Strangely, her back didn't hurt at all, but her legs, especially the left, were killing her.

It was her husband who named her pain: sciatica. But he didn't know how to fix it. Finally, she decided to ask the crowd – or at least her crowd of Facebook friends. "Any suggestions from people who've been there?" Within hours, she got more than 60 responses. Most offered heartfelt sympathy and welcome, but not helpful. Finally, a Facebook friend who was married to a physical therapist sent him a message: See my husband. He can tell you right away if you need an M.R.I. or if P.T. is likely to be sufficient.

Nothing about the MRI

She saw her friend's husband, who gave her some exercises and told her that if they helped her, great, but if they didn't, she probably needed an MRI. She adhered religiously to the prescribed regimen, but the pain persisted. The sciatic nerve is the largest nerve in the body, and the irritation of this nerve that causes the pain better known as sciatica is usually the result of pressure from the bony spine. These dots can often be seen on an M.R.I. But not, as it turned out, on his M.R.I.

And that's why, three days later, she went to the office of Dr. Dustin Nowacek, a neurologist at Bronson Methodist Hospital in Kalamazoo, Michigan. The patient detailed her weeks of pain and weakness. The incline was OK; the walk was mostly painless, although she got tired far too quickly. But just about everything else — especially sitting or standing still — was excruciating.

Nowacek noted that the patient limped when moving from the chair to the examination table. On examination, his left foot was noticeably weaker than the right. Indeed, she was unable to keep her left foot pointed when he applied downward pressure. And on the right, the foot was strong, but his big toe was not. She couldn't keep it pointed when he pressed it with his thumb. When the doctor tapped the Achilles tendon on his right leg, the foot shook in the expected response. But on the left side, nothing: no reflex movement at all. She was a bit alarmed when the doctor pulled out a safety pin and told her he was going to prick her with it to test her ability to feel pain. Oh, she could feel pain, she assured him. And she could feel the sharp tip of the spike everywhere – until it pricked the top of her left foot. There, she felt nothing.

This type of nerve damage, which affects both the ability to feel and the ability to move, can have many causes. It was uneven rather than symmetrical, which narrowed the possibilities. Could it be caused by his cancer, back after six years? It seemed unlikely. Thyroid disease was possible, but her symptoms didn't quite match. It was more likely a type of infection: Lyme disease or H.I.V. or hepatitis. Autoimmune diseases could also do this. Nowacek sent a series of blood tests to look for all of this.