LPI is an autosomally recessively inherited amino acid disorder due to defective transport of cationic amino acids lysine, arginine and ornithine in the intestine and kidney tubules. The absence or dysfunction of the transport process leads to low plasma and high urine concentration of the cationic (dibasic) amino acids. Clinical presentation of the disease usually takes place during the weaning period when breast feeding is replaced by cows milk and other high protein diets. Nausea, vomiting and mild diarrhea usually are the first symptoms followed by failure to thrive and growth retardation. Later liver and spleen become enlarged, muscles are hypotonic and osteoporosis can cause bone fractures. High protein intake can lead to hyperammonemia and even to coma, possibly accounting for the mild intellectual deficit found in few cases of LPI. Many patients have spontaneously developed aversion to protein rich food. A pulmonary complication of unknown mechanism, alveolar proteinosis has occurred in few patients during adulthood with fatal outcome.
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