Unilateral Left Renal Atrophy / Non-visualization on Ultrasound

Why Can't the Kidney Be Seen on USG?

1. The kidney is truly absent (agenesis, prior nephrectomy)
2. The kidney is present but severely atrophic - too small and echogenically similar to surrounding retroperitoneal fat to be distinguishable on ultrasound

Causes

1. Congenital / Developmental

• Complete failure of nephrogenesis - the ureteric bud or metanephric blastema simply never formed
• Occurs in ~1 in 1,000-1,500 individuals; slightly more common in males and on the left side
• About 50% have an absent ipsilateral ureter; the bladder trigone lacks the ureteral ridge on cystoscopy
• Associated with genital anomalies (absent vas deferens in males, uterine/vaginal anomalies in females), VACTERL syndrome (vertebral, anal, cardiac, tracheo-esophageal, renal, limb defects)
• The contralateral kidney is usually enlarged by compensatory hypertrophy
• Smith and Tanagho's General Urology, 19th Ed., p. 535

• Small kidney with normally differentiated but reduced number of nephrons
• Hard to distinguish from acquired atrophy on imaging
• One subtype (oligomeganephronia) has markedly reduced nephron number with glomerular hypertrophy
• Goldman-Cecil Medicine, p. 1263

• Abnormal metanephric differentiation with aplastic or cystic tissue
• Unilateral MCDK involutes over time and can disappear entirely on imaging - a perfectly normal-appearing solitary right kidney with no left kidney on USG may represent an involuted MCDK
• The ipsilateral ureter is typically atretic
• Contralateral VUR/obstruction is common
• Goldman-Cecil Medicine, p. 1268-1269

2. Vascular / Ischemic (Acquired)

• Atherosclerotic (most common in elderly) or fibromuscular dysplasia (young women)
• Chronic ischemia leads to progressive nephron loss and cortical atrophy
• A kidney size discrepancy of >2 cm on USG suggests chronic unilateral disease, such as seen in RAS
• Often associated with hypertension (which may be the only clinical clue)
• The Washington Manual of Medical Therapeutics; Brenner & Rector's The Kidney

• Embolic occlusion (e.g., from atrial fibrillation, paradoxical embolism) or thrombosis of the renal artery causes acute ischemia → infarction → eventual atrophy
• Frequently asymptomatic if it occurs gradually or if the episode was silent
• Symptom to Diagnosis, 4th Ed., p. 3384-3386

• Cholesterol crystal emboli from aortic atherosclerotic plaques
• Can cause patchy or global renal atrophy

3. Obstructive / Reflux-Related

• Particularly in infancy and childhood, chronic reflux causes reflux nephropathy (formerly called "chronic atrophic pyelonephritis")
• Results in a dwarfed, scarred kidney, sometimes with calyceal distortion
• The kidney may become so small it is not identifiable on USG
• Smith and Tanagho's General Urology, p. 1586

• Prolonged partial or complete obstruction → hydroureteronephrosis → interstitial fibrosis, tubular atrophy → end-stage small kidney
• Causes include ureteropelvic junction obstruction, calculi, retroperitoneal fibrosis, external compression
• Complete recovery of function only if obstruction relieved within ~7 days; little recovery after 12 weeks
• Symptom to Diagnosis, 4th Ed., p. 3331-3333

4. Infectious / Inflammatory

• Recurrent bacterial infections destroy renal parenchyma
• Leads to a small, scarred kidney with irregular contour and calyceal distortion
• One of the most common causes of unilateral acquired renal atrophy (along with reflux and renovascular disease)

• Granulomatous destruction of the kidney; eventually the whole kidney calcifies and becomes nonfunctional ("autonephrectomy" - "putty kidney")
• The end result can appear as a completely non-visualized or densely calcified structure on USG

5. Post-surgical / Traumatic

• Previous nephrectomy (donor transplant, prior surgery for stone/tumor) - always ask the patient
• Renal trauma with subsequent vascular injury or chronic atrophy

6. Positional / Technical (not truly absent)

• Ectopic kidney - the left kidney may be pelvic, iliac, or cross-fused to the right side (crossed renal ectopia)
• In crossed fused ectopia, both kidneys lie on the same side - the right kidney appears "normal" but is actually the fused product of both kidneys
• The USG operator may simply not have looked in the pelvis

Why the Right Kidney Remains Normal

Key Distinguishing Features

Next Steps

1. Review history - childhood UTIs, hypertension, prior surgery, TB, trauma
2. Serum creatinine + GFR - usually normal due to compensatory hypertrophy
3. CT scan (with/without contrast) - superior to USG; can confirm absence, identify an ectopic kidney, assess vasculature, and show scarring patterns
4. DMSA renal scan - best for functional assessment and detecting scarring
5. Doppler USG of renal arteries - assess for RAS
6. Cystoscopy - if agenesis suspected, absence of ureteral orifice confirms it