Simple Renal Cyst

A simple renal cyst is a benign, fluid-filled cavity in the kidney, thin-walled, unilocular and without solid components, septa or calcification.

• It is the commonest renal mass: present in about 25 percent of adults over 50 years and 40 to 50 percent over 70 years, men more than women.
• Cysts originate from tubular diverticula; they are usually solitary but can be multiple and bilateral, unlike polycystic disease.
• Risk factors: age, male sex, hypertension, chronic kidney disease.

Almost always asymptomatic and incidental.

• Large cysts (above 5 to 10 cm) may cause flank pain or fullness, palpable mass, hypertension (renin mediated), haematuria after rupture or bleeding, hydronephrosis, or infection (fever, loin pain).
• Differential diagnosis: complex cyst, cystic renal tumour, autosomal dominant polycystic kidney disease, hydatid cyst (endemic in Turkana and pastoralist areas), hydronephrosis, calyceal diverticulum, renal abscess, parapelvic cysts, multicystic dysplastic kidney.
• Complications: haemorrhage, infection, rupture, obstruction (parapelvic), hypertension, compression of vessels, rarely malignant change (not for true simple cysts).

Ultrasound is diagnostic: round, sharply marginated, anechoic, thin smooth wall, posterior acoustic enhancement, no septa, calcification or solid component, with through-transmission of sound.

• CT with and without contrast for cysts that do not show the ultrasound features of a simple cyst: homogeneous water density (0 to 20 Hounsfield units), thin imperceptible wall, no enhancement (category I); magnetic resonance imaging as an alternative.
• Urinalysis and creatinine; blood pressure.
• Family history and bilateral cysts should prompt consideration of polycystic kidney disease.

Supportive: reassurance and no treatment for asymptomatic cysts; no follow-up imaging is needed for a typical simple cyst on ultrasound or category I on CT.

Pharmacological

• Pain: paracetamol 1 g every 6 hours; avoid NSAIDs if renal impairment.
• Infection: ciprofloxacin 500 mg twice daily or trimethoprim-sulfamethoxazole 960 mg twice daily for 4 to 6 weeks (lipophilic drugs penetrate cysts); ceftriaxone 2 g IV daily if septic, then drainage.
• Hypertension: ACE inhibitor or amlodipine as needed.

Procedures (symptomatic or compressive cysts, 5 cm or larger):

• Percutaneous aspiration with sclerotherapy (absolute ethanol 95 percent, 5 to 10 percent of aspirated volume, retained 10 to 20 minutes; alternatives: polidocanol, bleomycin) has a success rate of 80 to 90 percent; aspiration alone has a high recurrence.
• Laparoscopic cyst decortication or marsupialisation for recurrence, parapelvic cysts, or proximity to the collecting system.
• Drainage for infection.

Special groups: kidney transplant donors (assess cysts), children (rare; further evaluation), pregnancy (conservative).

Red flags: enhancing component, septa, wall thickening, growth, haematuria, weight loss (reclassify as complex).

Follow-up: none for asymptomatic typical cysts; follow-up imaging at 6 to 12 months if the cyst is atypical or symptomatic; annual review for patients with multiple cysts and hypertension.