Cystinosis

Cystinosis

Cystinosis is a rare, inherited metabolic disorder characterized by the accumulation of the amino acid cystine within lysosomes, leading to cellular damage. This condition primarily affects the kidneys and eyes, but can also impact other organs and tissues.

Pathophysiology

Cystinosis is caused by mutations in the CTNS gene, which encodes the protein cystinosin. Cystinosin is responsible for transporting cystine out of lysosomes. When this transport is impaired, cystine accumulates, forming crystals that can cause cellular dysfunction and damage.

Types of Cystinosis

Cystinosis is classified into three main types based on the age of onset and severity of symptoms:

Nephropathic Cystinosis

This is the most common and severe form, typically presenting in infancy. It is characterized by renal tubular Fanconi syndrome, leading to excessive loss of water, sodium, potassium, and other substances in the urine.

Intermediate Cystinosis

Also known as juvenile or adolescent cystinosis, this form presents later in childhood or adolescence. It is less severe than nephropathic cystinosis but can still lead to significant kidney damage over time.

Ocular Cystinosis

This form primarily affects the eyes, causing photophobia and corneal crystal deposits. It usually presents in adulthood and does not typically involve kidney dysfunction.

Clinical Manifestations

Renal Symptoms

- Fanconi Syndrome: Characterized by polyuria, polydipsia, and growth retardation due to renal tubular dysfunction. - Chronic Kidney Disease: Progressive renal impairment leading to end-stage renal disease if untreated.

Ocular Symptoms

- Photophobia: Sensitivity to light due to corneal cystine crystal deposits. - Corneal Crystals: Visible deposits in the cornea that can be detected by slit-lamp examination.

Other Symptoms

- Hypothyroidism: Due to cystine accumulation in the thyroid gland. - Diabetes Mellitus: Resulting from pancreatic involvement. - Muscle Weakness: Due to cystine accumulation in muscle tissue.

Diagnosis

Diagnosis of cystinosis is based on clinical presentation, family history, and laboratory tests. Measurement of cystine levels in leukocytes is a key diagnostic test. Genetic testing can confirm mutations in the CTNS gene.

Treatment

Cysteamine Therapy

Cysteamine is the primary treatment for cystinosis. It reduces cystine accumulation by converting cystine into cysteine and cysteine-cysteamine mixed disulfide, which can exit the lysosome.

Supportive Care

- Renal Replacement Therapy: Dialysis or kidney transplantation for end-stage renal disease. - Electrolyte Replacement: To manage Fanconi syndrome. - Ocular Drops: Cysteamine eye drops to reduce corneal crystal deposits.

Prognosis

With early diagnosis and treatment, individuals with cystinosis can have improved outcomes. However, untreated cystinosis can lead to significant morbidity and mortality due to renal failure and other complications.

See Also

- Lysosomal storage disease - Fanconi syndrome - End-stage renal disease