Cystinosis

Cystinosis is a rare genetic disorder characterized by the accumulation of cystine within cells, leading to widespread organ and tissue damage. Understanding this condition involves delving into its causes, symptoms, and available treatments.

What is Cystinosis?

Cystinosis is a lysosomal storage disorder caused by mutations in the CTNS gene, responsible for encoding the cystinosin protein. This protein plays a crucial role in transporting cystine out of lysosomes. When cystinosin is deficient or dysfunctional, cystine accumulates within lysosomes, forming crystals that can damage various tissues and organs.

Types of Cystinosis

There are three main types of cystinosis, each with varying degrees of severity and onset:

Nephropathic (Infantile) Cystinosis: The most severe and common form, typically diagnosed in infancy. It primarily affects the kidneys but can also impact other organs.
Intermediate (Juvenile) Cystinosis: Diagnosed in childhood or adolescence, this type is less severe than nephropathic cystinosis but still requires medical intervention.
Ocular (Adult) Cystinosis: The mildest form, often detected in adulthood, primarily affecting the eyes without significant kidney involvement.

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Causes of Cystinosis

Genetic Mutations

Cystinosis is an autosomal recessive disorder, meaning an individual must inherit two copies of the mutated CTNS gene—one from each parent—to develop the disease. The CTNS gene is located on chromosome 17p13. The mutations lead to a loss or reduction of cystinosin function, resulting in the pathological accumulation of cystine.

Inheritance Patterns

Parents who are carriers of a single CTNS mutation do not exhibit symptoms but have a 25% chance of passing the disorder to their offspring if both parents carry the mutation. Genetic counseling is recommended for families with a history of cystinosis to understand the risks and implications.

Symptoms of Cystinosis

Early Symptoms

The symptoms of cystinosis can vary depending on the type and severity of the disease. In nephropathic cystinosis, early signs often appear within the first year of life and may include:

Fanconi Syndrome: A disorder of the kidney's proximal tubules leading to the loss of essential nutrients and electrolytes in the urine. Symptoms include excessive thirst (polydipsia), excessive urination (polyuria), and rickets due to phosphate wasting.
Failure to Thrive: Poor growth and weight gain despite adequate nutrition.
Dehydration: Persistent dehydration due to impaired renal function.

Progressive Symptoms

As cystinosis progresses, additional symptoms may develop:

Kidney Dysfunction: Progression to end-stage renal disease (ESRD) typically occurs by the age of 10 if untreated.
Photophobia: Extreme sensitivity to light due to cystine crystal accumulation in the cornea.
Hypothyroidism: Decreased thyroid function.
Muscle Weakness: Generalized muscle weakness and wasting.
Diabetes: Pancreatic involvement may lead to diabetes mellitus.
Neurological Complications: Including difficulty swallowing, cognitive impairment, and myopathy.

Ocular Symptoms

In ocular cystinosis, the primary symptoms are related to the eyes:

Corneal Crystals: Accumulation of cystine crystals in the cornea, causing photophobia and potential vision impairment.
Retinal Damage: In rare cases, cystine crystals can affect the retina, leading to vision loss.

Diagnosis of Cystinosis

Clinical Evaluation

Diagnosis often begins with a thorough clinical evaluation, including a detailed medical history and physical examination. Symptoms such as growth retardation, Fanconi syndrome, and photophobia may prompt further investigation.

Laboratory Tests

Several laboratory tests are crucial for diagnosing cystinosis:

Cystine Levels: Measurement of cystine levels in white blood cells (leukocytes) is a definitive diagnostic test. Elevated cystine levels confirm the diagnosis.
Genetic Testing: Identifying mutations in the CTNS gene through genetic testing can confirm the diagnosis and help with family planning.
Renal Function Tests: Assessing kidney function through blood and urine tests to determine the extent of renal involvement.

Imaging Studies

Imaging studies such as kidney ultrasound and bone density scans can help assess the extent of organ damage and complications associated with cystinosis.

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Complications of Cystinosis

Renal Complications

The most significant complication of cystinosis is kidney damage, leading to chronic kidney disease (CKD) and ultimately ESRD. Renal complications necessitate renal replacement therapies, including dialysis and kidney transplantation.

Endocrine Complications

Cystinosis can affect various endocrine glands, leading to hypothyroidism, diabetes mellitus, and hypogonadism. Regular monitoring and hormone replacement therapies are often required.

Neurological Complications

Neurological complications, including cognitive impairment, swallowing difficulties, and muscle weakness, can significantly impact the quality of life. Physical therapy and supportive care are essential components of management.

Treatment Options for Cystinosis

Cysteamine Therapy

Cysteamine is the cornerstone of cystinosis treatment. It reduces cystine accumulation by converting it into cysteine and a cysteine-cysteamine mixed disulfide, which can exit lysosomes. Cysteamine therapy includes:

Oral Cysteamine: Taken several times a day to maintain low cystine levels.
Cysteamine Eye Drops: Used to reduce corneal cystine crystals and alleviate photophobia.

Renal Replacement Therapy

In advanced stages of kidney disease, renal replacement therapies become necessary:

Dialysis: Hemodialysis or peritoneal dialysis to manage ESRD.
Kidney Transplantation: Offers the best long-term outcome for patients with ESRD. Post-transplant care includes continued cysteamine therapy to prevent cystine accumulation in the transplanted kidney.

Supportive Care

Supportive care measures are vital to managing the multisystemic effects of cystinosis:

Nutritional Support: Addressing growth retardation and maintaining adequate nutrition.
Hormone Replacement: Managing endocrine dysfunctions like hypothyroidism and diabetes.
Physical Therapy: Improving muscle strength and mobility.
Regular Monitoring: Frequent follow-ups with a multidisciplinary team to monitor disease progression and manage complications.