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Involuntary tics are very common, with as many as one in five children experiencing them at some point. They may be temporary or long-lasting. Tic disorders like Tourette syndrome usually are diagnosed during childhood, though sometimes this happens later.

A rapid heart rate—tachycardia—is a key feature of postural orthostatic tachycardia syndrome, or POTS, a mysterious condition that involves the nervous system and can cause a constellation of symptoms, including fatigue, shortness of breath, headaches, and brain fog, as well as pain in the chest, extremities, or elsewhere in the body. At least half of all cases of POTS are thought to be triggered by an infection (such as mononucleosis), which then spurs inflammation or an autoimmune response. Treatment includes drinking lots of water, eating lots of salt, and graded exercise training.

Long-established treatments for essential tremor, such as medication and deep brain stimulation surgery, have been joined by a leading-edge technique called focused ultrasound. This incision-free procedure aims sound waves at a spot deep in the brain responsible for tremors. While scientists study ultrasound's long-term effects, the procedure is currently used to treat shakiness on only one side of the body. Other common causes of tremor include Parkinson's disease, caffeine, alcohol withdrawal, anxiety, and fatigue.

People sometimes experience a sudden tingling or numbness in the feet or hands that goes away. But if the problem keeps occurring, followed by painful or burning sensations, this could be a sign of small-fiber neuropathy. While not life-threatening, small-fiber neuropathy should always be checked out, as it could be a symptom of something more serious.

Though decades of research have suggested risk factors for amyotrophic lateral sclerosis, a specific cause remains unknown. New research studied professional football players and found that they had a much higher risk of developing ALS than men in the general population, though the study was only observational.

Millions of people have tinnitus, a condition where a person hears a sound inside the head that does not come from any external source. There are many possible causes and no cure, but there are ways to ease the symptoms, one of which is sound therapy, which uses external sound to alter a person's perception of or reaction to tinnitus.

Essential tremor causes involuntary shaking, most often in the hands, head and voice. It may be an inherited condition and is different from another well-known tremor disorder called Parkinson's disease. There is no cure, but symptoms can be managed through medication and lifestyle changes.

Ask the doctors

Q. My friend was recently diagnosed with fibromyalgia, but it seems like she might be imagining her symptoms. Is fibromyalgia a real condition?

A. The short answer to your question is yes. Fibromyalgia is a real condition that affects some four million Americans. It's a chronic pain syndrome that experts believe may be caused by a malfunctioning nervous system. Researchers using magnetic resonance imaging to examine the brains of people with fibromyalgia have found abnormalities in the part of the brain that processes pain signals from the body. It appears that this part of the brain is essentially boosting the intensity of normal pain signals, potentially causing the body to feel pain without a physical cause.