This page aims to give advice to healthcare professionals (usually non-nephrologists) who need to respond to the finding of renal cysts on a scan. (We have separate pages to guide the management of patients with confirmed diagnoses of polycystic kidney disease, ADPKD and to provide further information to patients with ADPKD).
Contents
What are renal cysts?
Renal cysts are common, and one could argue that they are sufficiently common as to be a normal variant. About one-third of people aged over 70 years of age have at least one simple renal cyst, and these increase in number with age, and can be bilateral. The vast majority are simple, with no worrisome consequences, and require no special investigation, management or follow-up; many are asymptomatic, and are found coincidentally at unrelated scans.
A small minority of renal cysts require more in the way of thought and attention. The questions in such cases are…
Is this cyst malignant or premalignant ?
By and large, any reporting radiologist or sonographer will make a valued judgement with respect to this question in their report. If the latter merely mentions “simple” cyst/s, then this can be taken at face value and no further investigation is required. Cysts that are more complex will often require further evaluation with other imaging modalities and the preferred investigation will often be noted if not booked. If unclear from the report, the following terms would usually imply a degree of complexity that warrants further discussion:
- Complex cyst
- Septation
- Calcification
- Solid components
- Haemorrhage
- Abnormal enhancement
- Thickened or irregular walls
Reporting radiologists will sometimes use the Bosniak scheme of classifying renal cysts. This is a 4-point scale ranging from class I, which is a truly simple cyst, to class IV which is frankly malignant. Anything other than class I requires expert evaluation.
The worry in such cases is renal malignancy, and as such is the province of the urologists. They can arrange for difficult cases to be reviewed by their MDT, or they may arrange further imaging as required.
Radiological reports may also use a variety of other terms to further describe renal cysts; in the majority of cases, these are still simple. Such terms include:
- Exophytic: protrudes from the surface of the kidney
- Anechoic: is black on scan as its contents are uniform and not dense
- Parapelvic: is usually a simple cyst that protrudes into the renal sinus
Multiple simple cysts are not uncommon; the number itself is of no concern as long as the cysts are thought to be simple. This does however, bring us to our next question…
Do multiple renal cysts represents an inherited disease that will progress to chronic kidney disease ?
Autosomal dominant polycystic kidney disease ( PKD ) is one of the most common inherited diseases in the country (prevalence c. 1:1000), but is still uncommon in comparison to the number of people with simple renal cysts. Multiple simple renal cysts do not necessarily imply polycystic disease, and the term “multicystic” may be used to differentiate between multiple simple cysts and true PKD. The latter is usually suggested by the presence of many, many cysts (there is no exact definitive number, but terms such as “innumerable” are often used) with resulting enlargement of the kidneys.
The presence of multiple simple cysts in a small kidney is often degenerative and is not inherited. Other features that would support a diagnosis of inherited PKD would include the presence of liver cysts, as the latter are not uncommon accompaniments to renal cysts, and indeed, cases exist where the number of hepatic cysts outweighs those in the kidneys; interestingly, they rarely cause any significant liver failure, though they can cause hepatomegaly. Further evidence that supports a diagnosis of PKD are a positive family history, haematuria, the presence of CKD and hypertension. Also enlarged kidneys with cysts are almost always in the ‘PKD’ spectrum. Genetic screening, although possible, is rarely required in most cases of PKD.
Much of our knowledge about PKD comes from the majority of people who have autosomal dominant disease in which the clinical picture tends to run in a similar way to previously affected family members. With time however, we are identifying cases which do not fit the classical type, and such cases may require further consideration and maybe genetic testing. The following may highlight an unusual case:
- Radiological PKD in the absence of a family history ( though there may be good reasons why this is so )
- PKD where the number of liver cysts far outweighs the number of renal cysts
- Atypical features of the cysts e.g. unilateral or asymmetric, few but very large cysts
- PKD in association with diabetes or other systemic or syndromal features
Other diseases exist where the kidneys contain many small cysts; these include medullary sponge kidney and nephronophthisis. These are uncommon, but should be highlighted in a report. Multiple small renal cysts may also be seen as a complication of lithium therapy and do not in themselves represent a problem.
Polycystic and multicystic diseases of the kidneys are the province of the nephrologists and queries regarding these should be directed accordingly.
Acknowledgements
This page was written in March 2024 by Paddy Gibson, with a contribution from Paul Phelan.