Nephropathic Cystinosis - Types, Symptoms, Causes, Diagnosis, and Treatment

Introduction

Nephropathic cystinosis is a rare autosomal recessive lysosomal disorder that leads to end-stage kidney disease and other complications, such as crystal deposition in the conjunctiva and cornea. Nephropathic cystinosis is caused due to mutations in the 17p13 chromosome in the CTNS gene. Chromosome 17p13 encodes for lysosomal cystine transporter cystinosis.

What Are the Types of Nephropathic Cystinosis?

There are three types of nephropathic cystinosis:

• Infantile Nephropathic Cystinosis: Infants with Nephropathic cystinosis are usually asymptomatic with normal birth weight and length, although cystine accumulation begins in utero. The symptoms are evident or appear in the first year of life. The neonates present with Fanconi syndrome, a dysfunction of the proximal tubule causing polydipsia, dehydration, polyuria, proximal renal tubular acidosis, urinary loss of electrolytes, and growth retardation. Glucosuria and aminoaciduria may be detected in urine. One should suspect renal glycosuria or Fanconi syndrome in the case of normal serum glucose levels and glycosuria. Fanconi syndrome is the first symptom of cystinosis due to high protein turnover. If left untreated infantile nephropathic cystinosis may lead to end-stage renal disease. Around 10. 95 % of nephropathic cystinosis cases are infantile type.

• Juvenile Nephropathic Cystinosis: The symptoms of juvenile nephropathic cystinosis present at an older age. The individuals may have end-stage renal disease and be diagnosed in the second decade of life. There may be the onset of photophobia, and further evaluation might lead to the detection of cystine crystals in the cornea. 5 % of the nephropathic cystinosis cases are juvenile.

• Non-nephropathic Cystinosis: This is an adult form of cystinosis and presents only with ocular cystinosis. The deposits are limited to only the conjunctiva and cornea. This type of nephropathic cystinosis may show a continuum, with the other milder forms form a juvenile period. Hence the renal function of individuals with non-nephropathic forms show a familial basis with non-nephropathic cystinosis and should be monitored.

What Are the Signs and Symptoms of Nephropathic Cystinosis?

The possible signs and symptoms include:

Eyes:

• Cataract.

• Retinopathy (retinal blindness, visual field constriction, impaired color vision).

• Band keratopathy.

• Corneal erosion.

• Iris thickening.

• Filamentary keratopathy.

• Corneal cystine crystals (photophobia, blepharospasm).

Neurologically:

• Neurocognitive impairment.

• Nonverbal learning difficulties.

• Visual motor function impairment.

• Cystine encephalopathy (mental deterioration, cerebellar and pyramidal signs, pseudobulbar palsy, distal myopathy, weakness of the extremities, dysphagia, pulmonary insufficiency.

• Behavioral problems.

Endocrine and Reproductive:

• Hypothyroidism.

• Primary hypogonadism.

• Azoospermia.

• Insulin-dependent diabetes mellitus.

• Delayed puberty.

• Growth retardation.

Gastrointestinal:

• Hepatomegaly.

• Exocrine pancreatic insufficiency.

• Splenomegaly.

Kidney:

• Deterioration of renal function.

• Renal Fanconi syndrome (polyuria, polydipsia, proximal renal tubular acidosis, dehydration, urinary loss of electrolytes, glycosuria, aminoaciduria).

• Glomerular proteinuria.

Hematology:

• Pancytopenia (accumulation of cystine crystals in bone marrow).

Skin:

• Hypohydrosis.

• Heat intolerance.

• Hypopigmentation of skin and hair.

Bone and Muscle:

• Hypophosphatemic rickets (frontal bossing, genua vara, rachitic rosary, metaphyseal widening).

What Is the Cause of Nephropathic Cystinosis?

All types of nephropathic cystinosis have an autosomal recessive pattern of inheritance. Nephropathic cystinosis is caused by the defective transport of cystine through the lysosomal membrane. This was caused due to an impairment in the cystinosis gene (CTNS). CTNS is located on chromosome 17p13. CTNS gene has 12 exons, of which 10 code for cystinosis. In individuals with nephropathic cystinosis, the gene mutations cause loss of protein function leading to the prevention of cystine transport through the lysosomal membrane. As a result, the parents of the individuals are heterozygotes and carry the single gene for the disease. Heterozygous individuals have not been reported with cystine crystals in any cell or tissue.

How Is Nephropathic Cystinosis Diagnosed?

Serum electrolyte measurements are used to check for the severity of hypokalemia, acidosis (normal anion gap and hypochloremic), hyponatremia, hypophosphatemia, and low bicarbonate concentration in individuals with nephropathic cystinosis. Blood gasses check for metabolic acidosis and the degree of respiratory compensation. Urine analysis shows glucosuria, osmolality, and tubular proteinuria, which includes generalized aminoaciduria. In addition, the measurements of urine electrolytes detect the loss of bicarbonates and phosphaturia.

Imaging Studies:

Renal ultrasonography is indicated in individuals with elevated urine calcium excretion to rule out nephrocalcinosis. X Rays for kidneys, ureters, and bladder to evaluate urinary tract calcifications in individuals with hypercalciuria or for severe abdominal pain. Computed tomography and magnetic resonance imaging are used to examine individuals who have had infantile nephropathic cystinosis without central nervous system symptoms.

Other Tests:

Slit lamp examination of the eyes detects corneal and conjunctival cystine crystals at the age of one. However, photophobia is not apparent till the age of three to six. Evaluation of the eye fundus may show peripheral retinopathy that is severe. In some cases, retinopathy may cause blindness.

What Is the Treatment for Nephropathic Cystinosis?

Treatment and management for nephropathic cystinosis involve:

Medical Approach:

Past treatment approaches involved only treating metabolic acidosis and replacing electrolytes lost in the urine, and later chronic kidney disease was treated. However, advanced approaches like the availability of phosphocystamine drugs and kidney replacement therapy have improved the prognosis of the disease.

1. Replacement of Urinary Losses: The affected individual should be well hydrated and administered supplements of bicarbonates and potassium. Rickets may be treated with vitamin D and phosphate supplementation. It is a supplement if carnitine levels are high in the urine. If the individual experiences fever, vomiting, and diarrhea, fluid and electrolyte therapy may be offered.

2. Volume Depletion or Dehydration States: Large amounts of fluids and electrolytes should be administered, which may exceed conventional limits. Urine loss should be monitored and replaced during hospital admission. The rehydration fluid must contain sodium, potassium, glucose, and bicarbonate. The concentration is adjusted based on the serum laboratory values. Even after rehydration, individuals should have access to water and fluids as they are prone to dehydration.

3. Cystamine Therapy: Delayed release cysteamine bitartrate is used to manage nephropathic cystinosis in children two or older. The delayed release of cysteamine bitartrate helps in controlling cystine levels.

4. Indomethacin Therapy: This helps limit water loss in individuals with nephropathic cystinosis by reducing the glomerular filtration rate.

5. Growth Hormone Therapy: Individuals may be treated with recombinant growth hormone. The treatment is safe and efficient for children with nephropathic cystinosis before renal replacement therapy.

6. Thyroid Replacement: Indicated in cases with hypothyroidism.

7. Kidney Transplantation: Usually performed in individuals with infantile nephropathic cystinosis.

Conclusion

Nephropathic cystinosis is a rare autosomal recessive lysosomal disorder that leads to end-stage kidney disease and other complications, such as crystal deposition in the conjunctiva and cornea. Nephropathic cystinosis does not have a curative therapy. However, advanced treatment approaches provide the affected individual with a better quality of life.