Keyhole sign | Radiology Signs

Answer:

Keyhole sign is the combination of a dilated, elongated posterior urethra in continuity with a distended, thick-walled urinary bladder, seen on prenatal (fetal) or postnatal ultrasound, classically due to posterior urethral valves (PUV) — a congenital obstructive uropathy in male infants presenting with bilateral hydronephrosis, oligohydramnios (antenatally), a thick-walled trabeculated bladder, poor urinary stream, urinary tract infections, and renal impairment. The sign is suggestive but not entirely specific for PUV — it has also been reported with other causes of bladder outlet obstruction such as urethral atresia, so its positive predictive value is imperfect.

Why is it called so?

Named for the characteristic keyhole appearance on ultrasound, in which the distended bladder forms the round upper part of the keyhole and the dilated posterior urethra forms the narrow, slit-like lower part.

Pathophysiology

Posterior urethral valves are ectopic mucosal folds acting as a diaphragm-like obstruction in the posterior urethra, causing obstruction to urine outflow during voiding; this leads to ballooning and elongated dilatation of the posterior urethra proximal to the valve in continuity with the distended bladder, producing the Keyhole sign configuration on ultrasound. On voiding cystourethrogram (VCUG), the analogous finding of an elongated, dilated posterior urethra with abrupt narrowing at the valve is more commonly termed the “spinning top” appearance.

Alternative names: None

Other associated named signs: Spinning top urethra (the corresponding VCUG appearance of the dilated posterior urethra in PUV)

References

1. Bernardes LS, Aksnes G, Saada J, et al. Keyhole sign: how specific is it for the diagnosis of posterior urethral valves? Ultrasound Obstet Gynecol. 2009;34(4):419-423.
2. Fonseca EKUN, et al. Keyhole sign in posterior urethral valve. Abdom Radiol (NY). 2018;43(9):2493-2494.