Hereditary xerocytosis occurs more commonly in African populations, and it exhibits complex interactions with other hereditary alterations of red blood cells, including sickle cell disease and malaria resistance.Osmosis leads to the red blood cell having a constant tendency to swell and burst. This tendency is countered by manipulating the flow of sodium and potassium ions. A pump forces sodium out of the cell and potassium in, and this action is balanced by a process called the passive leak. In overhydrated hereditary stomatocytoses, the passive leak is increased and the erythrocyte becomes swamped with salt and water. The affected erythrocytes have increased osmotic fragility. Haemolytic anaemia results. For as yet unknown reasons, the cells take on the shape of a cup, with a mouth-shaped (stoma) area of central pallor. Overhydrated hereditary stomatocytosis is frequently linked to mutations in genes that encode components of the band 3 complex, such as RHAG. It is the altered band 3 protein complex which mediates the cation leaks that are characteristic of hydrocytotic hereditary stomatocytosis.Rare cases of hereditary spherocytosis can occur without cation leaks. These include cases of phytosterolemia nonleaky stomatocytosis, a disorder of lipid metabolism associated with mutations in ABCG5 and/or ABCG8, which encode sterol transporters. The resulting abnormal sterol composition of erythrocyte cell membranes causes them to appear as deformed stomatocytes on peripheral blood smear. Diagnosis
Ektacytometry may be helpful in distinguishing different subtypes of hereditary stomatocytosis. Variants
Haematologists have identified a number of variants. These can be classified as below.
Swan neck deformity is a deformed position of the finger, in which the joint closest to the fingertip is permanently bent toward the palm while the nearest joint to the palm is bent away from it (DIP flexion with PIP hyperextension). It is commonly caused by injury, hypermobility or inflammatory conditions like rheumatoid arthritis or sometimes familial (congenital, like Ehlers–Danlos syndrome). Pathophysiology
Swan neck deformity has many of possible causes arising from the DIP, PIP, or even the MCP joints. In all cases, there is a stretching of the volar plate at the PIP joint to allow hyperextension, plus some damage to the attachment of the extensor tendon to the base of the distal phalanx that produces a hyperflexed mallet finger. Duck bill deformity is a similar condition affecting the thumb (which cannot have true swan neck deformity because it does not have enough joints). Diagnosis
Diagnosis of swan neck deformity is mainly clinical. MRI of the hand may suggest volar plate attenuation of PIP and extensor tendon damage for DIP  Genetic screening tests such as for CMT disease may also be indicated. Treatment
Splinting for fingers. Passive stretching and clearing the deformity. References
External links
Swan neck deformity at Wheeless Textbook of Orthopedics, a technical review of the condition
Rheumatoid arthritis: Image showing deformities