Tag Archives: Kidney

Medullary nephrocalcinosis

Nephrocalcinosis: by default, we apply this term to ‘medullary’ calcification (= medullary nephrocalcinosis), though it can occur in renal cortex (e.g. in renal cortical necrosis of pregnancy or chronic active glomerulonephritis). Nephrocalcinosis is associated with conditions that cause hypercalcemia, hyperphosphatemia, and increased excretion of calcium, phosphate, and/or oxalate in the urine. Hypocitraturia also may contribute, especially in those with distal (type 1) renal tubular acidosis. Citrate normally inhibits crystal formation by forming a complex with calcium.

Important conditions that cause Medullary nephrocalcinosis: With hypercalcemia + hypercalciuria: primary hyperparathyroidism, sarcoidosis, too much vitamin D. Conditions with hypercalciuria but no hypercalcemia: Distal renal tubular acidosis, medullary sponge kidney, too much furosemide.

On a renal sonogram, the medullary pyramids appear bright or white (hyperechoic) instead of anechoic or hypoechoic. Kidney appears like a stretched chrysanthemum flower. Acoustic shadowing may or may not be seen. Sometimes, the whole pyramid becomes hyperechoic while sometimes it’s only the rim (especially in early stages).

Here is another example with CT scan 3D reconstruction (bone window)

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.

Parapelvic cyst mimicking hydronephrosis

Parapelvic cysts can mimic hydronephrosis because of their anechoic nature (like urine, clear fluid in the cysts is also black on ultrasound) and close proximity to the collecting system. Hydronephrosis appears as branching, ‘interconnected’ anechoic area, while parapelvic cysts are seen as ‘noncommunicating’ renal sinus cystic masses. In addition, a parapelvic cyst is more spherical as opposed to irregular/cauliflower contour of hydronephrosis and is not connected to the ureter distally. When the sonogram is not clear enough, a CT scan with contrast should be considered to differentiate between these two conditions. On CT, the contrast will not enter the cyst while the collecting system lights up. See one of my previously published case reports for a better idea in addition to below infographic.

Increased renal cortical echogenicity does not always indicate chronic kidney disease

Echogenicity of the renal cortex relative to liver or spleen can be evaluated both qualitatively and quantitatively, though qualitative method is commonly used. Normal renal cortex is usually hypoechoic (less bright) or sometimes isoechoic (similar brightness) to that of liver or spleen. Increased cortical echogenicity is commonly attributed to chronic kidney disease (CKD) and has been correlated with interstitial fibrosis, tubular atrophy, and glomerulosclerosis in histologic studies. However, increased echogenicity can also be seen in acute kidney injury (AKI) where inflammatory infiltrates and proteinaceous casts reflect sound waves (e.g. acute glomerulonephritis, acute tubular necrosis). 

Note that CKD is usually associated with decreased kidney length and cortical thickness in addition to increased echogenicity (except diabetic nephropathy and infiltrative diseases). If a patient with no significant co-morbidities is found to have elevated serum creatinine and bright renal cortex, think of AKI/glomerulonephritis especially if the length and thickness are preserved. The normal pole-to-pole kidney length in adults is 10-12 cm, and varies with body size. Cortical thickness is measured from the base of the medullary pyramid to the outer margin of the kidney. It is generally around 7-10 mm, being thicker at the poles. Following is an infographic illustrating hyperechoic cortex in a patient with lupus nephritis.

Here is another example from a patient with AKI stage III secondary to ethylene glycol poisoning. Note normal sized kidneys with bright cortex, likely secondary to calcium oxalate deposition.

Bilateral hydronephrosis with a stone in the urinary bladder

In the first image, you can see bilateral severe hydronephrosis as evidenced by the dilated collecting system filled with anechoic (black) urine exerting pressure on the renal parenchyma. Second image shows bladder ultrasound with echogenic debris and a stone appearing as hyperechoic curved structure with a shadow. The echogenic material could be a blood clot also. Usually but not always, debris moves with moving the patient while clot remains adherent to the bladder wall. On color Doppler, Stones demonstrate twinkling, which is a rapidly alternating focus of intense color signals resembling turbulent flow that is more pronounced with rougher stones. Don’t forget to scan urinary bladder in any patient with hydronephrosis, even if they have a Foley catheter.

Hydronephrosis

On the sonogram, hydronephrosis appears as branching, interconnected areas of decreased echogenicity (anechoic or black in general, indicating the presence of fluid) in the renal collecting system. The source of obstruction is usually located distal to the kidney, for example, a stone in the pelviureteric junction, ureter or ureterovesical junction or bladder outlet obstruction from enlarged prostate, stone or a mass.

On the other hand, the collecting system of a normal kidney is not well-visualized unless distended and is embedded in the surrounding echogenic sinus fat. The renal pelvis area is hypoechoic but not ‘black’ unless there is hydronephrosis.

As the hydronephrosis increases in severity, the urine moves proximally into the kidney exerting pressure on the parenchyma. While there is no universally accepted grading system, hydronephrosis is often classified as mild, moderate or severe in routine clinical practice.

In mild hydronephrosis, there is dilatation of the renal pelvis and calyces but the pelvicalyceal pattern is retained and the cortex remains unaffected. Distinct medullary pyramids may be seen in some cases though not necessary to make a diagnosis as pyramids are not always appreciable even in a normal kidney.

In moderate hydronephrosis, medullary pyramids start to flatten due to back pressure in addition to dilatation of pelvicalyceal system and outpouching of the calyces, which is sometimes referred to as ‘cauliflower appearance’. Cortical thickness is preserved.

In severe cases, renal pelvis and calyces appear ballooned and cortico-medullary differentiation is lost making the cortex thin.

Here is the video form: