Cystinosis is a rare genetic disorder in which an amino acid called cystine builds up inside cells because the body can’t transport it out properly. Cystine accumulates in tiny cell compartments called lysosomes, forming crystals that gradually damage the kidneys, eyes, muscles, and other organs. It affects roughly 1 in 100,000 to 200,000 newborns, and the most severe form accounts for about 95% of all cases.
How Cystinosis Works at the Cellular Level
Every cell in the body has lysosomes, small recycling centers that break down proteins and other molecules into reusable parts. One byproduct of that recycling is cystine, a compound made of two linked cysteine molecules. Normally, a transporter protein called cystinosin shuttles cystine out of the lysosome so it can be used elsewhere in the cell.
In cystinosis, mutations in the CTNS gene mean this transporter doesn’t work. Cystine has no way out, so it accumulates and eventually forms sharp, needle-like crystals inside the lysosomes. Over months and years, those crystals damage and destroy cells in whichever tissues they collect. The kidneys are hit first and hardest, but the crystals can deposit in the corneas, thyroid, pancreas, muscles, liver, spleen, and bone marrow.
The Three Types of Cystinosis
Infantile Nephropathic Cystinosis
This is the most common and most severe form, responsible for roughly 95% of cases. Babies appear healthy at birth, with normal weight and length, even though cystine accumulation has already begun in the womb. Symptoms typically surface between 6 and 12 months of age, starting with what’s known as renal Fanconi syndrome: the kidney’s filtering tubes malfunction and start leaking vital nutrients, electrolytes, and water into the urine. Parents often notice excessive thirst, frequent urination, unexplained dehydration, vomiting, constipation, and poor growth.
Without treatment, kidney function declines steadily. Most children with untreated infantile cystinosis reach end-stage kidney failure by ages 10 to 12. By ages 3 to 4, cystine crystals in the cornea typically cause significant light sensitivity.
Juvenile Nephropathic Cystinosis
This form makes up about 5% of cases and follows a slower course. Symptoms usually appear between ages 5 and 20, and the full pattern of Fanconi syndrome is less common. Many patients are first diagnosed in their teens, when light sensitivity prompts an eye exam that reveals corneal crystals. Kidney failure, when it occurs, typically develops between ages 15 and 30.
Adult (Ocular) Cystinosis
The rarest form causes only eye symptoms, mainly light sensitivity and discomfort from corneal crystal deposits. It shows up anywhere from young adulthood to middle age. The kidneys and other organs are spared, and an eye exam revealing cystine crystals is often the only clinical finding.
What Kidney Damage Looks Like
The earliest and most characteristic sign of infantile cystinosis is Fanconi syndrome. The kidney’s proximal tubules, the structures responsible for reclaiming glucose, amino acids, phosphate, bicarbonate, and water from urine, stop doing their job. The result is a cascade of problems: the body loses phosphate (weakening bones), bicarbonate (causing the blood to become too acidic), glucose, and other essential molecules through the urine.
For parents, the practical signs are a baby who drinks constantly, produces unusually large volumes of dilute urine, and fails to gain weight or grow as expected. Dehydration episodes can be severe and may require hospitalization. Over time, continued crystal accumulation destroys deeper kidney structures, and overall kidney filtration declines until transplantation or dialysis becomes necessary.
Eye and Corneal Crystal Deposits
Cystine crystals in the cornea are one of the most recognizable features of the disease, visible under a slit-lamp microscope as fine, evenly shaped needles scattered through the corneal tissue. They typically start at the outer edge of the cornea in infancy and gradually spread inward and deeper.
The clinical impact ranges from mild light sensitivity to serious vision problems. Early on, crystals scatter incoming light, reducing contrast sensitivity even when standard visual acuity tests still look normal. As deposits thicken, they can cause recurring corneal erosions, scarring, and in advanced cases, dense clouding of the cornea that severely limits vision. Uncontrolled blinking and squinting in bright light (blepharospasm) is common and can significantly affect daily life, especially for children.
Complications Beyond the Kidneys and Eyes
Because cystine accumulates in cells throughout the body, the disease eventually affects multiple organ systems, particularly in patients who weren’t treated early or consistently. The thyroid and pancreas are common targets: hypothyroidism and diabetes can develop as crystal deposits damage hormone-producing cells. Cystine also builds up in skeletal muscles, especially the small muscles of the hands and the muscles involved in breathing, which can lead to weakness, difficulty swallowing, and a restrictive pattern of breathing trouble. Swallowing difficulties arise from crystal deposits in the muscles of the mouth and throat.
Nearly every patient who hasn’t received early, consistent treatment will experience at least one major complication by age 30.
How Cystinosis Is Diagnosed
The gold standard for diagnosis is measuring cystine levels in white blood cells. Elevated levels confirm the disease. However, this test has limitations. In juvenile cystinosis, white blood cell cystine levels can be only slightly elevated, which may delay diagnosis. In at least one documented case, a patient with corneal crystals and kidney involvement had a white blood cell cystine level that was barely above normal, and the diagnosis was only confirmed by measuring cystine in skin and kidney cells grown in the lab.
Genetic testing for mutations in the CTNS gene provides a definitive answer and is increasingly used alongside or instead of the white blood cell test, especially in ambiguous cases. A slit-lamp eye exam showing characteristic corneal crystals also provides strong supporting evidence.
Treatment With Cysteamine
The cornerstone of cystinosis treatment is a medication called cysteamine, which works by entering the lysosomes and chemically converting trapped cystine into a different compound that can exit through an alternative transporter. Once in the cell’s main compartment, the compound is broken down, and the freed cysteamine can cycle back into the lysosome to remove more cystine. This recycling makes the drug efficient at lowering cystine stores throughout the body.
Oral cysteamine is taken every six hours to keep cystine levels consistently low. The dosing schedule is demanding, including a dose in the middle of the night for many patients, but the payoff is substantial. Children who start treatment early and maintain it consistently can preserve kidney function well into their twenties without needing a transplant, and their growth improves significantly compared to untreated children.
Oral cysteamine does not effectively reach the cornea, so a separate formulation of cysteamine eye drops is needed to dissolve corneal crystals. The standard regimen is one drop in each eye four times a day during waking hours. The drops must be refrigerated and replaced every seven days.
Side effects of oral cysteamine include a strong sulfurous body odor and breath, nausea, and gastrointestinal discomfort, which can make adherence difficult, especially for teenagers.
Kidney Transplantation
When kidney failure does occur, transplantation is the preferred treatment, and the outcomes are notably good. The transplanted kidney does not develop Fanconi syndrome because the donor organ has a functioning cystinosin transporter. Five-year graft survival in cystinosis patients is about 92%, and 10-year survival is around 87%, both of which are better than outcomes in matched control patients receiving transplants for other causes of kidney failure. Research has confirmed that cystinosis is actually an independent protective factor for graft survival.
In rare cases, cystine crystals have been found in transplanted kidneys on biopsy, but these come from the patient’s own immune cells that migrate into the new organ. They don’t appear to cause clinical problems or recreate the original kidney disease. Over half of transplanted cystinosis patients still have a functioning graft after 28 years of follow-up.
Life Expectancy and Long-Term Outlook
Before modern treatment, infantile cystinosis was fatal, usually from kidney failure in childhood. The combination of kidney transplantation and cysteamine therapy has transformed it into a manageable chronic condition. Life expectancy now extends past 50 years for patients who receive consistent treatment.
The key variable is how early treatment begins and how consistently it’s maintained. Children who start cysteamine in infancy often see their kidney capacity continue to grow normally up to age 3, rather than declining. Those who begin treatment later or take it inconsistently face a much higher risk of kidney failure and the cascade of complications that follow from crystal buildup in other organs. The treatment burden is high, with round-the-clock oral medication, frequent eye drops, and regular monitoring, but the difference in outcomes between treated and untreated patients is dramatic.
Gene Therapy in Clinical Trials
A phase 1-2 clinical trial published in the New England Journal of Medicine tested a gene therapy approach in six adults with cystinosis, ages 20 to 46. The treatment involved collecting each patient’s own blood stem cells, inserting a working copy of the CTNS gene using a viral vector, and infusing the corrected cells back after chemotherapy to clear space in the bone marrow. After treatment, oral cysteamine was stopped entirely and eye drops were discontinued one month later.
Over follow-up periods ranging from 29 to 63 months, five of the six patients showed decreased white blood cell cystine levels without needing cysteamine. The one patient who didn’t respond as well had received the lowest dose of corrected cells. Most side effects were related to the chemotherapy preparation rather than the gene therapy itself. While the trial is small and ongoing, it represents the first treatment approach that addresses the root genetic cause rather than managing cystine buildup after the fact.