Cystinosis or neuropathic cystinosis is an inherited (autosomal recessive) lysosomal storage disorder which results in the accumulation of the amino acid cysteine in various organs of the body due to a defect in their transport out of the lysosomes. This stored cysteine then crystallises within the lysosomes of many cell types, leading to widespread tissue and organ damage.
Cystinosis is acquired by inheriting the gene from the parents as it is a recessive genetic disease. Being recessive implies that the parents carry a recessive gene each which may cause cystinosis in their children but they themselves do not show any symptoms of the disease. However it is difficult to predict, as the recessive gene may remain dormant for many generations until two people with the recessive gene have children and they show symptoms.
Cystinosis can be classified into three types on the basis of age at diagnosis and the symptoms in each of these vary:
- Infantile onset cystinosis: This condition involves multiple organs with symptoms ranging from mild to severe. The most commonly experienced symptoms include polyuria, polydipsia, dehydration, vomiting, metabolic acidosis, hypophosphatemic rickets, constipation, loss of appetite, craving for salty and spicy foods, recurrent bouts of fever and manifestations of heat intolerance. If left untreated, it may lead to renal failure by the age of 7-10 years.
- Adolescent onset cystinosis: It manifests most commonly in early adolescence and is diagnosed by age of 12 years. Its symptoms are usually restricted to kidneys and eyes and it involves the deterioration of renal function which may lead to end stage renal disease by the age of 15 years.
- Adult onset cystinosis: It develops after the age of 20 years and is manifested with legal blindness, distal vacuolar myopathy, cerebral calcifications or atrophy, swallowing dysfunction, diabetes mellitus, and liver disease
Cystinosis is diagnosed through various laboratory tests as well as imagining techniques:
- The laboratory tests include an assessment of serum electrolyte levels, blood gases, urine electrolyte levels and levels of cysteine in polymorphonuclear leukocytes or cultured fibroblasts.
- The imaging studies include the use of techniques like renal ultrasonography, radiography of kidneys, ureters, and bladder, computed tomography scanning and magnetic resonance imaging.
- Other evaluation techniques include slit-lamp examination of the eyes for detection of corneal and conjunctival crystals, funduscopic examination and biopsy of the kidney with histologic examination.
Although there are many therapies for the symptomatic relief of cystinosis, but the outlook for patients was drastically improved with the approval of Cysteamine (Cystagon) by the Food and Drug Administration (FDA) for standard treatment of Cystinosis. Cysteamine is a cysteine-depleting agent that lowers the levels of cysteine within the cells and has been proven to be effective in delaying or preventing renal failure and improving the growth of children with the disorder. Other therapeutic strategies include:
- Replacement of urinary losses through fluid and electrolyte therapy
- Management of volume depletion and dehydration states
- Indomethacin therapy to limit water loss in patients with nephropathic cystinosis
- Growth hormone therapy
- Thyroid replacement
- Kidney transplantation