Muehrcke lines are paired, white, transverse lines that signify an abnormality in the vascular bed of the nail. Muehrcke first described paired, narrow, white, transverse fingernail lines in a series of 65 patients with severe, chronic hypoalbuminemia.  He believed that the lines, as shown in the image below, were a specific sign of this single biochemical alteration. Others described patients receiving multiple cytostatic agents.  These nail alterations may also represent an example of chemotherapy-induced nail changes. Muehrcke lines represent a type of apparent rather than true leukonychia, as they involve abnormal nail bed vasculature, which alters nail plate translucency. The whiteness becomes unapparent with pressure and is not modified by nail growth.  Note the image below.
Transverse white lines of the nails occur with many different etiologies. They have been shown to occur in association with several conditions, including pellagra,  Hodgkin disease,  renal failure,  sickle cell anemia, or nail damage from paraquat.  See Pellagra, Hodgkin Disease, Acute Renal Failure, Chronic Renal Failure, and Sickle Cell Anemia for more information on these topics.
Transverse white bands appearing in the nail plate are often caused by trauma to the more proximal matrix in the area of the proximal nail fold; however, some bands, such as Mees lines and Muehrcke lines, are associated with systemic disease. The systemic disease–associated lines typically span the entire breadth of the nail bed or the nail plate, and they tend to be more homogeneous, to have smoother borders, and to occur on several nails at once.  Trauma-induced transverse white bands tend to be more linear, they resemble the contour of the proximal nail fold, and they usually do not spread across the entire breadth of the nail plate, whereas systemic disease–associated lines typically have a contour similar to the distal lunula and a rounded distal edge. Usually, a correlation between the onset of the bands and a systemic insult exists; a history of physical trauma to the cuticle area is not likely. 
The appearance of the paired, narrow, white bands on the fingernails is typically correlated with a serum albumin level below 2.2 g/100 mL. The lines have been shown to persist with chronic hypoalbuminemia and to disappear when the serum albumin level rises above 2.2 g/100 mL.  The exact pathogenesis has not been sufficiently articulated. A localized edematous state in the nail bed may exert pressure on the underlying vasculature, thereby decreasing the normal erythema typically seen through the nail plate. 
Muehrcke  observed the fingernails of 250 healthy adults, 500 patients with serum albumin levels in the reference range (values < 4 g/100 mL were taken as abnormal), 31 patients with nephrotic syndrome, and another 34 patients with hypoalbuminemia from other causes. The paired, white bands were not found in any healthy subjects or in patients with serum albumin levels of greater than 2.2 g/100 mL. Of the 31 patients with nephrotic syndrome, 23 (74%) had paired, transverse, white bands. Of the other 34 patients with hypoalbuminemia, 9 had serum albumin levels of less than 2.3 g/100 mL, 8 of whom developed the white bands. In 1 patient with a serum albumin level of 2 g/100 mL, the bands did not develop; however, her albumin level had decreased from 2.7/100 mL in the previous 2 weeks. The bands were more prominent in patients with albumin levels of less than 1.8 g/100 mL for at least 4 months.
Reporting on the reliability of Muehrcke lines as a sign of hypoalbuminemia, Conn and Smith  observed the development of transverse, white bands in 10 patients, all of whom had serum albumin values of less than 2.7 g/100 mL. Muehrcke lines were present in 10 (23%) of 44 patients with low albumin levels and with a variety of debilitating illnesses, but the lines were not observed in any patients with normal albumin levels.
More recently, Nabai  reported the appearance of white bands on the nails of a patient with hypoalbuminemia who had undergone a heart transplant. A month after the surgery, certain features of Muehrcke lines were noticed on both thumbnails and subsequently on other fingernails; the albumin level was reported at 3.6 g/mL. Even after the albumin levels returned to the reference range, the bands lasted for another 2 months.
Although these changes in the nails apparently resembled Muehrcke lines, features of the presentation differed from Muehrcke's original description. Muehrcke lines typically appear on the second, third, and fourth fingers; however, in this case, the bands appeared on all of the fingers in addition to the toenails (the bands were more pronounced on the fingernails). Additionally, the lines appeared and persisted at albumin levels of greater than 3.4 g/100 mL compared with levels of less than or equal to 2.2 g/100 mL as described by Muehrcke.  This case was the first report of transverse, white bands in a patient with a heart transplant.
Dermatologic manifestations of an obscure nature, such as nail dystrophy or nail pigmentation, have also been noted after cancer chemotherapy. Specifically, transverse, pigmented banding has been associated with certain chemotherapeutic agents. Doxorubicin is postulated to increase local melanogenesis at the nail matrix, nail plate, or nail bed. [11, 12, 13] Other agents inducing pigmented bands in nails include bleomycin, [14, 15] fluorouracil, [16, 17, 18, 19] cyclophosphamide, [20, 21] and daunorubicin hydrochloride. 
Physicians have reported nail changes with different combinations of drugs used in the treatment of a broad spectrum of cancers. Therefore, the nail changes do not seem to be associated with any one particular drug or drug combination. Reports also exist of patients receiving multiple chemotherapeutic agents  in whom the characteristic nail changes associated with Muehrcke lines were seen with slightly decreased serum albumin levels. 
In 1976, Nixon  described a patient with a glioblastoma who was receiving therapy with procarbazine hydrochloride, vincristine sulfate, and carmustine; this patient subsequently developed broad, white bands on the proximal half of the nails after 3 months. According to Nixon,  transverse pigmented bands are the more common nail alteration seen with cancer chemotherapy, and these changes are not associated with the changes in the nails seen with systemic disease.
Schwartz and Vickerman  reported the appearance of paired, white, transverse lines on the nails of a woman with squamous cell carcinoma of the left main-stem bronchus. She was treated with a combination of platinum, doxorubicin hydrochloride, vincristine sulfate, and methyl cyclohexylchloroethylnitrosurea (CCNU). Schwartz and Vickerman  referred to these bands as Muehrcke lines; however, as they pointed out, the woman did not have persistent hypoalbuminemia. Within 1 day of the patient noticing 2 transverse, narrow, paired, white bands on each fingernail, her serum albumin level was at 3.4 g/dL (reference range, 3.5-5 g/dL). Evidently, chemotherapy and possibly other conditions should be considered as potential causes of paired, transverse, pigmented bands on the fingernails.
In 1983, James and Odom  reported a case of a 75-year-old woman who developed horizontal bands on all 10 fingernails while being treated with a combination of cyclophosphamide, doxorubicin hydrochloride, and vincristine sulfate for histiocytic lymphoma. On each fingernail (toenails were not involved), 3 bands (1-2 mm wide) with regular borders traversed each nail. The bands were white, and a 1-mm strip of normal pink nail separated the bands from each other. The patient's serum albumin level was in the reference range, and renal function was normal. 
The frequency is unknown.