She looked too healthy. In her early 40s, showing zero signs of distress, my newest patient, Lucy, seemed out of place in a bustling emergency room. Shaking her hand as she sat on the gurney, I asked her what brought her in.

“Well,” she began, “I couldn’t move my right arm.” She glanced at her husband as if to confirm: You saw it too, right?

“Today?” I blurted, having just shaken her right hand.

“For about 45 minutes.”

Maybe she had simply slept wrong and her arm had fallen asleep. But although Lucy couldn’t remember whether her arm had been all right when she woke up, she was sure she hadn’t felt any “pins and needles” upon awakening. She had gone to bed with a sinus infection, which had begun the day before. Showing me where her sinus hurt, she placed two fingers below her left eye, over the maxillary sinus, the largest of three pairs of sinus cavities. “Our family doctor said it was sinusitis and prescribed Zithromax,” Lucy said. 

These days, it seems you can’t depart a doctor’s office without everyone’s reflex antibiotic, Zithromax. But strictly speaking, a diagnosis of sinusitis requires fever and deep-seated tenderness.

“Does it still hurt?” I asked. 

She shook her head.

Maybe the Zithromax, which can sometimes cause insomnia, had disturbed Lucy’s sleep and made her doze in a position that pinned her arm. But there was no way to prove that.

My detailed neurological exam — checking eye movements, strength in all extremities, sensation, reflexes and coordination — found nothing amiss. Without a benign explanation for the arm weakness, I had to consider the worst.