What Are Cystine Stones

Introduction

The prevalence of health-related kidney issues is on the increase and while most have been accredited to lifestyle choices, some are caused as a result of genetic predisposition. Cystine stones are types of kidney stones, making up 1% of stones. Contrary to the sole belief of a single type of kidney stone, there are in fact, different forms of kidney stones. Other common forms include uric acid and struvite forms of kidney stones as well.¹

What are cystine stones?

Cystine stones are formed by the excess build-up of cystine, an amino acid, in the kidney. Unlike other kidney stones, they are relatively uncommon. They account for about 1-2% of kidney stones but 6-8% of all pediatric calculi. Cystine stones have a higher recurrence rate and are larger than other kidney stones.² 

Significance of understanding cystine stones

According to the UK Health Economics of Kidney Disease June 2023 report, kidney disease is projected to be the fifth highest cause of life years lost by 2040.³ With the high prevalence of health-related kidney issues, it is paramount for us to know the causes, differences, symptoms, and possible treatment options for the various types of kidney disease.

In this article, we will take you through an understanding of cystine stones, how they are formed, symptoms, diagnosis, treatment options, and how they are different from other types of kidney stones.

Formation of cystine stones

The human systems are intercalated. After every meal is taken, the systems work together in an ordered sequence, from breaking down food particles and absorbing nutrients to eliminating waste for optimal body operations. The human kidney(s)is responsible for filtering wastes and reabsorbing nutrients.⁴ The inability of the kidney tubules to reabsorb cystine coupled with the insoluble characteristic of cystine leads to cystine stone formation.

Cystine stones are caused as a result of cystinuria. Cystinuria is an inevitable genetic defect that affects cysteine's proximal renal tubular reabsorption. This is a result of mutations in the two prominent coding genes. Due to this mutation, cysteine is present in the urine, which causes the formation of cystine stones.⁵

Amino acids are the building blocks of proteins. The excretion of amino acids is a vital body process that helps regulate amino acid homeostasis in the body. Cystine is an essential amino acid and the least insoluble. Although cystine is primarily responsible for cystinuria, there are, in fact, three other amino acids present due to the genetic mutation of cystinuria. Lysine, ornithine, and arginine are, in fact, present, but due to the insolubleness of cystine in urine at pH levels and normal body temperature, stones form.²

Symptoms 

Kidney stones have varying sizes, from the size of a bean to as large as a pebble. Smaller cystine stones often go unnoticed as they eventually find their way out and are excreted, but bigger ones can cause discomfort and other complications. 

Cystine stones show similar symptoms to other kidney stones. The symptoms of cystine stones include:⁶

1. Feeling pain while urinating
2. Pain in the abdomen, groin, or pelvis
3. Sharp pain on one side of the body
4. Blood in the urine
5. Nausea and vomiting.

Diagnosis

There are a range of tests and scans that can be performed in order to help diagnose the presence of cystine stones in your kidneys. Some of these diagnosis methods include:²

• Cystine supersaturation - this gives an insight into the risk potential of an individual having a cystine stone. A portion of the patient's urine is incubated with cystine crystals for 48 hours at 37 C. After incubation, the sample is saturated; this is divided by the original patient's sample, giving a ratio that suggests a ratio to prevent cystine stone formation. This test is not reliable for thiol-based medications e.g. D-penicillamine and tiopronin.
• Cystine capacity test -The patient's urine sample is incubated for 48 hours at 37 C, after which all solidified cystine is removed. If additional cysteine is gained after incubation, the test is positive. If the solidified cystine is partially dissolved, the test is negative. It is suitable for cystinuria patients using thiol-based medications.
• Ultrasound - Large cystine kidney stones can be identified by an ultrasound, but an ultrasound result can't differentiate cystine kidney stones from other kidney stones. Large kidney stones are capable of causing obstruction in the UTI, and ultrasound can measure the renal resistive index. It is the most preferred imaging type for stones in children and in pregnancy.
• CT Scans- they pick up more kidney stones with lesser densities. They are capable of identifying smaller stones and will show cystine stones clearly. It is regarded as “the gold standard” for diagnosing kidney stones, although it doesn't clarify the chemical composition of these stones due to overlapping densities. Some dual-energy CT scans are distinguished based on chemical composition beams, but they are expensive.

Treatment and management

Hydration

One of the most crucial steps in treating cystine stones is hydration. Increasing fluid intake per day will generate an increased urine rate, which will dissolve the formed cystine stones, allowing for the dilution of the solidified cystine composition.

It is crucial to take intentional and helpful hydration steps to prevent the escalation of your condition.

Use these helpful tips to keep track:

• Set timely water reminders per day.
• Drink 240ml every hour during the day, 480ml before bed, and at least once overnight, which will help with your hydration therapy.² 

Researchers have suggested keeping cystine levels between a range of 90 mg/L- 250 mg/L. A range of 90 mg/L is urged as optimal.²

Dietary measures

Cystinuria is a genetic disease that can be well-managed if certain dietary measures are taken. For patients with cystinuria, consuming aiding foods or supplements can further aggravate their cystine level. 

Dietary sodium should be limited to 2 grams/day, and a low-protein food intake will greatly assist in lowering urinary cystine levels. High-protein foods like turkey, beef, fish, beans, milk, cheese, etc., should be decreased so as not to aid in the excessive buildup of cysteine in urine.

Predisposed children actively growing shouldn't be cut out of protein as it is required for their optimal growth, but foods rich in methionine can be cut down. 

Medications

Cystine molecules are held together by disulfide bonds, and thiol-based drugs have sulfhydryl groups that reduce cystine disulfide bonds. Patients who have >500mg of urinary cystine in a day may require thiol medication alongside hydration therapy.

Thiol-based drugs like penicillamine, tiopronin, captopril, and bucillamine are medications used in the treatment of cystine stones. 

Surgical management

Compared with other kidney stones, surgery for cystine stones is recommended earlier to prevent further stone formation. This is particularly important in children patients to prevent future surgeries.

Due to resistance to shock, ureteroscopy with laser lithotripsy is also often preferred for cystine stones.

Complications and recurrence

The treatment of cystine stones can be painful and unpleasant. Patients with cystine stones require more surgeries compared to other kidney stone patients and are at high risk of chronic kidney disease.

Potential complications include:

• Renal and ureteral damage
• Scarring
• Pain
• Repeated surgeries
• Renal failure

To prevent these, an aggressive approach is focused on increasing fluid intake and optimizing urine pH.

Summary

Cystine stones are caused by the accumulation of cystine in the urine. This is due to a condition called cystinuria. This condition cannot be prevented as it is inherited. However, precautionary steps to minimize the amount of cystine present in the urine can be taken. It is important to seek professional healthcare advice to ensure a proper diagnosis to ascertain if it is a cysteine stone. Ensure you seek your GP to determine the extent and severity of your condition so that adequate treatment can start. As humans, the vitality of our body is essential for a hitch-free life, maintaining a healthy diet, and staying hydrated to prevent the development of cystine stones.

FAQ’s

1. What is the difference between cystine and cystine stones?

Cystine is an essential amino acid required by the body. However, due to genetic defects, cystine accumulates in the kidneys, ureter, and bladder, leading to the solidification of cystine into stones.

2. Are cystine stones acidic or basic?

Cystine stones form in an acidic urine pH level. That is, while hydration dietary choices help alkalize the urine pH, aiding cystine solubility.

3. My symptoms are similar to what I read in the article. Does it mean I have cystine stones?

Cystine stones have similar symptoms to other kidney stones, and in medicine, there are differential diagnoses- a list of medical conditions with similar symptoms. To ensure a proper and adequate diagnosis, kindly visit a doctor.