polycystic kidney disease ( pkd ) is one of the most common inherited diseases in adults affecting 1 in 4001000 live births .
it is the third most common cause of end - stage renal disease ( esrd ) in the united states and accounts for 10% of all cases of esrd .
pkd is a member of a family of inherited renal cystic disorders and is characterized by simple cysts distributed throughout the kidney , although the disease is not limited to the kidneys .
it is better thought of as a systemic disease with cystic abnormalities found in the kidneys , liver , cerebral vasculature , colon and a higher propensity for inguinal hernias and nephrolithiasis .
uric acid stones occur at a much greater frequency than the general population , while struvite and calcium phosphate stones occur at similar frequencies .
calcium - oxalate - containing stones seem to occur with decreased frequency in patients with pkd .
interestingly , cystinuria or cystine stones have not been described in patients with pkd , despite the high risk of stone disease in this patient population .
cystinuria is a complex genetic metabolic disorder that results in high urinary excretion of cystine and other di - basic amino acids .
affected patients may have recurrent renal colic , urinary tract obstruction , cystine crystalluria or precipitate cystine calculi .
we report a case of a patient with pkd who was incidentally found to have cystine crystalluria , an interesting clinical observation not previously reported in the medical literature .
a 37-year - old male presented to renal clinic for routine follow - up for pkd . his initial diagnosis of pkd was made after an evaluation of an episode of painless gross haematuria at age 27 .
physical examination at that time was normal except for the blood pressure that was 148/86 mmhg .
his past medical history is unremarkable , specifically for no history of flank pain , renal colic or symptomatic kidney stones .
computed tomography scanning at initial presentation revealed marked renal volume enlargement , greater than 50 cystic lesions in each kidney bilaterally all consistent with simple cysts ( figure 1 ) . additionally , hepatic cysts and colonic diverticuli were noted .
finally , several renal parenchymal calcifications were noted , thought to be consistent with retained bilateral nephrolithiasis .
urinalysis at initial presentation was notable for specific gravity of 1.005 , ph 7.5 , trace blood and no proteinuria .
he has done well over time with only an occasional episode of painless gross haematuria , always associated with physical sports activity .
his hypertension has worsened prompting treatment with an angiotensin receptor blocker , irbesartan 75 mg orally per day . on recent routine follow - up evaluation ,
urinalysis revealed a specific gravity of 1.005 , ph 7.5 , no protein , trace blood and numerous hexagonal - shaped crystals ( figure 2 ) .
the patient was further evaluated with a 24-h urine collection that revealed 1645 mg / day cystine , creatinine 2100 mg and volume 3010 ml .
he was counseled about the increased risks of nephrolithiasis , and hydration strategies were discussed .
the patient remains in good health and has yet to develop a symptomatic kidney stone over a 3-year follow - up period .
kidney stones occur at an increased frequency in patients with pkd . in a study by torres et al . , 20% of patients with pkd had an episode of nephrolithiasis .
interestingly , uric acid stones occurred with a much greater frequency than that in a cohort of patients without pkd .
the aetiology of stone disease in pkd has not been completely elucidated , but recent reports suggest that it is multifactorial in nature , including both anatomic and metabolic factors .
anatomic abnormalities include parenchymal distortion from cystic displacement leading to urinary stasis , which is thought to contribute to the pathogenesis of kidney stones formation in patients with pkd .
urinary concentrating defects that occur early in the development of pkd seem to correlate with degree of renal anatomy distortion and the subsequent development of kidney stones .
metabolic factors suggested to contribute to nephrolithiasis include hypocitraturia , and perhaps disordered uric acid tubular physiology .
it would appear that disruption of normal tubular transport mechanisms by cystic dilation is central to these metabolic issues .
hypocitraturia was found to be significantly decreased in 10 of 15 patients described in a study by torres et al . .
while hypocitraturia is associated with increased stone formation risk in the general population , it is unclear if this defect is specific to pkd .
defective renal ammonigenesis likely leads to a reduction in renal buffering capacity and is proposed to contribute to the increased frequency of uric acid nephrolithiasis in patients with pkd .
cystinuria is a rare autosomal recessive metabolic disorder associated with urinary supersaturation of di - basic amino acids . in the general population ,
cystine stones represent 12% of all urinary calculi ; cystine stones or cystine crystalluria have not been described in patients with pkd despite the multiple risk factors these patients have for development of nephrolithiasis .
cystinuria is transmitted via an autosomal recessive trait that results in decreased proximal tubular cystine reabsorption and increased urinary cystine excretion .
the defect is located on chromosome 2 on gene scl3a1 or on chromosome 19 on gene scl7a9 .
this gene product normally produces a protein involved with cystine transport in both the kidney and small intestine .
the severity of the defect depends on the chromosome involved and the amount of cystine excreted .
identification of the pathognomonic hexagonal - shaped crystals on urine sediment analysis and measurement of daily urinary cystine excretion are accepted diagnostic methods .
. however , not all patients with cystinuria will excrete observable cystine crystals ; thus , there appears to be a certain level of excreted cystine required to precipitate crystals in addition to other stone formation factors .
other urolithiasis risk factors observed in patients with cystinuria include hypercalciuria , hyperuricosuria and hypocitraturia . in patients with homozygous cystinuria
, mixed calcium  cystine stones can account for up to half of these stones .
while patients with pkd are at increased risk for kidney stones in general , there does not seem to be increased risk for cystine crystalluria or cystine stones
. this may simply be due to the fact that cystinuria and pkd are completely unrelated genetic diseases .
clearly , both diseases produce stones by altered tubular transport mechanisms . in both diseases ,
factors that favor stone formation include similar risk factors that are observed in the general population , including hypercalciuria , relative urinary stasis and perhaps hyperuricosuria .
interestingly , a promoter of stone formation in pkd is low urinary ph , a finding consistently not present in our patient .
we postulate that our patient likely has either a genetic or acquired renal acidification defect .
the nature of this defect is unknown in our case , as in the work by torres et al . ; renal ammonia trapping and transport seems to be impaired in patients with advanced pkd .
another possible mechanism for reduced cystine stone formation in patients with pkd and cystinuria may be due to the relative frequency that these patients are prescribed ace - inhibitors .
ace - inhibitors , specifically captopril , due to their thiol group bind to cystine and increase their solubility almost 200-fold .
however , the efficacy of captopril in cystinuria crystalluria has been challenged recently . of note
, our patient was not taking captopril , nor was he genetically tested for either pkd or cystinuria . in conclusion , pkd is a relatively common genetic disease characterized by cysts in the kidney and other organs in the body .
in addition to cysts , patients with pkd are at high risk for nephrolithiasis , nearly twice that of the general population .
patients with pkd tend to form calcium and uric acid and based stones at a higher rate than patients without pkd .
cystine crystalluria and stones are rare in the general population , and there is no description of their formation to date in a patient with pkd . we have presented an interesting unique circumstance of cystine crystalluria in a patient with pkd .
we suspect that this astute clinical observation has occurred by chance , though it is compelling that these two genetic diseases have occurred together in the same patient , and perhaps could represent digenetic inheritance outside the scope of mendelian inheritance .
while it is commonly accepted that other genetic associations may occur in patients with cystic renal diseases , no instances of these two genetic defects have been described together , nor have been systematically studied .
this observation should serve as a notice that further study may reveal a potential relationship between these two genetic renal diseases .
finally , this patient has yet to experience a single symptomatic nephrolithiasis , which is remarkable , considering the cumulative risk for nephrolithiasis in patients with pkd in general and this patient in particular .
the views expressed in this report are those of the author and do not necessarily reflect the official policy or position of the department of the navy , department of defense , nor the u.s . government .
 cystinuria is a rare autosomal recessive metabolic disorder of renal and intestinal cystine transport . 
 cystine stones are found in only 12% of all stone formers . 
 patients with cystinuria are at high risk for nephrolithiasis and subsequent morbidity . 
 our patient is a 37-year - old male who presented for routine follow - up for polycystic kidney disease ( pkd ) . 
 he denied any history of passing nephroliths . 
 he had no family history of pkd or personal history of kidney stones . 
 serum creatinine was 1.2 mg% . 
 on routine urine microscopy , he was found to have multiple hexagonal cystine crystals . 
 urine ph was 7.5 . 
 renal ct scan revealed enlarged polycystic kidneys and scattered bilateral intra - renal calculi . 
 urinary quantification of cystine was 1645 mg / day ( normal excretion rate 30 mg / day ) . 
 patients with pkd are at increased risk for nephrolithiasis for a number of reasons including urinary acidification , concentrating defects and hypocitraturia . 
 the molecular , cellular and genetic basis for cystinuria is distinctly different and presumably unrelated to the genetic defects in pkd . 
 we suspect that the occurrence of these two unrelated genetic diseases in the same patient is a coincidental finding . 
 even after a thorough review of the published literature , we were unable to find a genetic relationship between cystinuria and cystic renal diseases . 
 to our knowledge , this is the first report of a finding of cystinuria in an adult with pkd . 
Introduction
Case report
Discussion