Some conditions cause deep horizontal ridges rather than shallow vertical ridges. These Beau's lines appear because an illness or disorder interrupts nail growth under the cuticle. Diabetes is the most common disorder that causes Beau's lines. Thyroid disease, syphilis, and mumps are also capable of stunting nail growth and causing the horizontal ridges. Trauma to the nail and cancer chemotherapy drugs can also cause Beau's lines to form, or they may signify malnutrition rather than an illness.
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