Defects of Amino Acid Reabsorption
A. Congenital Cystinuria
Increased excretion of cystine results in the formation of cystine calculi in the urinary tract. Ornithine, arginine, and lysine are also excreted in abnormally large quantities.
There is also a defect in absorption of these amino acids in the jejunum. Nonopaque stones should be examined chemically to provide a specific diagnosis.
Treatment goals include a large fluid intake and keeping the urine pH above 7 by giving sodium bicarbonate and sodium citrate plus acetazolamide at bedtime to ensure an alkaline night urine. In refractory cases, a low-methionine (cystine precursor) diet may be necessary. Penicillamine has proved useful in some cases.
B. Aminoaciduria
Many amino acids may be poorly absorbed, resulting in unusual losses. Failure to thrive and the presence of other tubular deficits suggest the diagnosis. There is no treatment.
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C. Hepatolenticular Degeneration (Wilson’s Disease)
In this congenital familial disease, aminoaciduria and renal tubular acidosis (RTA) are associated with cirrhosis of the liver and neurologic manifestations. Hepatomegaly, evidence of impaired liver function, spasticity, athetosis, emotional disturbances, and Kayser-Fleischer rings around the cornea constitute a unique syndrome. There is a decrease in synthesis of ceruloplasmin, with a deficit of plasma ceruloplasmin and an increase in free copper that may be etiologically specific.
Penicillamine is given to chelate and remove excess copper. Edathamil (EDTA) may also be used to remove copper.
D. Multiple Defects of Tubular Function (De Toni-Fanconi-Debré Syndrome)
Aminoaciduria, phosphaturia, glycosuria, and a variable degree of RTA characterize this syndrome. Osteomalacia is a prominent clinical feature; other clinical and laboratory manifestations are associated with specific tubular defects described previously.
Treatment consists of replacing cation deficits (especially potassium), correcting acidosis with bicarbonate or citrate, replacing phosphate loss with isoionic neutral phosphate (mono- and disodium salts) solution, and ensuring a liberal calcium intake. Vitamin D is useful, but the dose must be controlled by monitoring levels of serum calcium and phosphate.
E. Defects of Phosphorus & Calcium Reabsorption
Several sporadic, genetically transmitted, and acquired disorders are grouped under this category and are characterized by persisting hypophosphatemia because of excessive phosphaturia and an associated metabolic bone disorder, rickets in childhood, and osteomalacia in adulthood.
Response to vitamin D therapy (1,25,-dihydroxycholecalciferol, the active analog of vitamin D) is variable.
F. Defects of Glucose Assorption (Renal Glycosuria)
Renal glycosuria results from an abnormally poor ability to reabsorb glucose and is present when blood glucose levels are normal. Ketosis is not present. The glucose tolerance response is normal. There is no treatment for renal glycosuria, just reassurance.
G. Defects of Bicarbonate Reabsorption
Proximal RTA, type II, is due to reduced bicarbonate reclamation in the proximal tubule, with resultant loss of bicarbonate in the urine and decreased bicarbonate concentration in extracellular fluid. There are increased K+ losses into the urine and retrieval of Cl– instead of HCO3. The acidosis is therefore associated with hypokalemia and hyperchloremia. Transport of glucose, amino acids, phosphate, and urate may be deficient as well (Fanconi syndrome).
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