Xerocytosis (also called dessicocytosis) is defined by the presence in the peripheral smear of hyperchromic erythrocytes (picture 1). The hyperchromia (and elevated mean cell hemoglobin concentration) are indicative of marked dehydration of the red cells .
Most of these syndromes are rare hereditary disorders, which involve abnormalities in the mechanisms controlling cell water and cation content . However, a variable percentage of dehydrated cells may be present in more commonly seen hematologic disorders, such as sickle cell disease and hemoglobin C disease. These will be described here.
Since the red cell is freely permeable to water, dehydrated cells require the loss of cell solutes, which then induce osmotic water loss. (See "Control of red blood cell hydration".) Xerocytosis is characterized by a reduction of cell content of potassium, the main intracellular cation. This may be associated with a slight increase in cell sodium content, but the total cation content of the red cell is significantly lower than in normal controls.
The dehydrated cells are more resistant to lysis when incubated in hypotonic solutions. This decrease in osmotic fragility is due to the small baseline cell size. On the other hand, xerocytes exhibit a characteristic increase in cell rigidity and reduction in cell deformability. These changes are thought to play an important role in the hemolysis resulting from premature destruction of xerocytes. In vitro, these membrane deformability changes can be reversed by rehydrating xerocytes and decreasing the mean cell hemoglobin concentration (MCHC) to normal values .
Syndromes with hereditary xerocytosis cannot readily be distinguished from hereditary stomatocytosis. (See "Stomatocytosis".) Several intermediate forms of disease exist between overhydrated stomatocytosis (hydrocytosis) and classic xerocytosis.