Renal complications in Cystic Fibrosis
The precise effect(s) of dysfunctional CFTR in renal physiology is unknown, although CFTR is expressed in all nephron segments.
Background
The precise effect(s) of dysfunctional CFTR in renal physiology is unknown, although CFTR is expressed in all nephron segments. There is no significant pathology and the renal consequences of cystic fibrosis are thought to be secondary to:
treatment
the effects of cystic fibrosis on other organ systems
co-incidental co-morbidities
Events such as frequent drug exposure (e.g. nephrotoxic antibiotic use, NSAIDs, etc); immune complex damage; cystic fibrosis related diabetes; liver disease; malabsorption; and drugs post organ-transplant may all add to the overall injurious effect of cystic fibrosis on the kidney.
It appears that there is no particular renal pathology associated with cystic fibrosis - a study in 2009 described 13 adult cystic fibrosis patients (out of a total of 521) who had a range of abnormalities including diabetic nephropathy, amyloid and primary renal disease¹.
Specific diseases
Nephrolithiasis
Cystic Fibrosis patients have reported rates of nephrolithiasis of 3-6% compared with 1-2% in age matched controls. Calcinosis can also occur with vitamin D excess. Renal stones can be caused by:
hypercalciuria
hypocitraturia
hyperoxaluria
hyperuricosuria
low urine volumes with increased urinary saturation of calcium oxalate (this is usually secondary to malabsorption with high gastrointestinal losses).
A child with proven renal stones should have their management discussed with the regional renal service for their location.
Antibiotic induced renal impairment
The frequent use of systemic aminoglycosides is a significant risk factor for proximal tubule damage. Associated risk factors for nephrotoxicity include CFRD, dehydration during intravenous antibiotic use e.g. from diarrhoea and vomiting, pre-existing renal impairment.
Monitoring renal function
Monitoring renal function is part of the Annual Review testing.
Electrolyte disorders
Metabolic alkalosis
Cystic Fibrosis can present as acute or chronic metabolic alkalosis. High loss of NaCl (from sweat, gut, bronchial mucosa, saliva and tears), along with low chloride intake leads to chloride depletion. This causes excessive bicarbonate resorption which leads to alkalosis.
Low urinary sodium
Urinary sodium concentration can be depleted due to everyday losses in Cystic Fibrosis. In younger children this can manifest itself as poor weight gain. If a patient is not growing adequately urinary sodium concentration should be checked. Normal value is >20mmol/L. A low result will need nutritional review.
Pseudo-Bartter's syndrome
(leads to hypochloraemic, hypokalaemic metabolic alkalosis)
Symptoms:
In younger children Pseudo-Bartter's syndrome is often preceded by anorexia, vomiting, fever and weight loss. In older children it can be induced by extreme exercise with sweating. Severe respiratory disease, poor nutritional status and high enzyme intake are also associated with chronic metabolic alkalosis.
Investigations:
blood gas
serum electrolytes
urinary sodium and potassium, chloride.
Blood pressure
Treatment:
Fluid and electrolyte replacement. This may have to be intravenous if severe electrolyte disturbance