Most white lines on nails are harmless and should be of no concern. However, if you have similar lines on many nails or a pair of lines that runs all the way across the nail, these could be an indication of a more serious underlying condition. If you have these types of white lines or if your lines don't go away as your nails grow, seek medical advice from your doctor.
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Random White Lines
White lines on nails most commonly appear as random white lines or streaks. They are caused by minor, often imperceptible, trauma to the nail. Random white lines can be found on any nail, are usually different shapes and sizes, and move along the nail, unchanged, as the nail grows. Eventually these lines will reach the end of the nail, where they can be clipped or filed off. When fungal infections of the nail first appear, they sometimes look like random white lines. With time, they will spread, become more irregular in shape and develop a yellowish tinge. Fungal infections may require treatment by a doctor.
In some instances, white lines on the nails appear as a pair of parallel lines that run horizontally across the nail, from one side to the other. These lines -- known as Muehrcke lines -- stay in the same place and do not move when the nail grows. When one squeezes the nail, Muehrcke lines temporarily disappear. They are usually most obvious on the second, third and fourth fingernails. Muehrcke lines are generally caused by low protein in the blood, which may be due to poor nutrition, liver disease or kidney disease. Improving protein levels will usually improve these lines as well. Seek advice from your doctor if you think you have these types of lines.
Mees lines are an uncommon cause of white lines on the nails. They are tiny breaks in the nail, which move up the nail as it grows. Mees lines also run horizontally across the nail, but they may or may not extend the whole way from one side to the other. Mees lines do not disappear when the nail is squeezed. They are typically found on several or even all of the nails. Mees lines are most often caused by arsenic poisoning. They can also be due to other toxins, such as carbon monoxide or lead, some medications and certain severe medical conditions like heart failure or a major infection. If you notice these types of lines, visit your doctor.
Lengthwise White Lines
Rarely, white lines will start at the base of the nail and run along the length of the nail toward the tip. Some of these are present from birth, whereas others develop later in life due to damage to the cells at the base of the nail. Regardless of when they develop, these types of lengthwise lines are harmless and do not need treatment.