Autosomal dominant polycystic kidney disease
Autosomal dominant polycystic kidney disease (ADPKD) is relatively a common genetic disorder, occurring in approximately 1 in every 400 – 1000 live births. It is generally an adult-onset, multisystem disorder characterized by gradually growing renal cysts that can originate from all areas of the kidneys, though they more commonly emerge form distal regions of the nephron and the collecting duct. Mutations in PKD1 or PKD2, which encode polycystin 1 and 2, respectively, are the most common cause of ADPKD. Patients with PKD2 have a less severe phenotype than those with PKD1, though not benign. Cysts occur later in PKD2 disease, as does end-stage renal disease (mean age of ESRD: 74.0 vs 54.3 years in PKD1).
On a renal sonogram, kidneys are usually large with multiple cysts appearing as bunch of grapes. The number of cysts required for diagnosis vary depending on the age of the patient. Simple renal cysts will appear anechoic (black) with well-defined margins and posterior acoustic enhancement (brightness or white area past the cyst). Hemorrhagic or infected cysts will demonstrate echogenic material within the cyst, without internal blood flow. Calcification may be seen in some cases. Presence of liver cysts in addition to renal cysts is a clue to the presence of ADPKD. Polycystic liver disease is characterized by presence of multiple cysts scattered throughout the liver parenchyma, which form owing to overgrowth of the biliary epithelium.
In terms of risk stratification, Magnetic resonance-based, height-adjusted total kidney volume (htTKV) over 600 ml/m predicted the development of CKD stage 3 within 8 years in the Consortium for Radiologic Imaging in Polycystic Kidney Disease (CRISP) cohort. This was a prospective, observational, longitudinal, multicenter study included 241 adults with ADPKD and preserved renal function. In the same cohort, an ultrasound kidney length over 16.5 cm and htTKV over 650 ml/m had the best cut point for predicting the development of CKD stage 3. When MRI is not available, kidney length on ultrasound can be used for risk stratification in these patients.
Patients with ADPKD are at increased risk of nephrolithiasis. Associated cyst wall calcifications and parenchymal characteristics make it difficult to detect stones on ultrasound in these patients. Shadowing might not be always seen and twinkling artefact on color Doppler helps in such cases. Following is a nice image of a small stone with both shadowing and twinkling. Stones of this size are frequently missed when using handheld ultrasound devices.