Lacuna magna (a.k.a. sinus of Guérin) is a congenital blind-ended pouch located dorsal to navicular fossa of penis separated by fold and both share an external common opening to external urethral meatus. This diverticulum is located above and parallel to the urethra.
Although it may be present in 30% of boys, only a few become symptomatic owing to the valve-like effect of the septum.
Dysuria, haematuria and spotting of underclothes probably to chronic distension and irritation of the diverticulum. Also, elongated stream of urine due to pressure effect of the sinus may occur.
Congenital anomaly develops embryologically from an incomplete fusion between the ingrowing ectoderm at the tip of the penis and the glandular urethra.
Best evaluated by antegrade urethrography in which entire distal urethra must be included on the radiographic films. Lacuna magna are usually seen as diverticulum filled with contrast in the most distal part of the urethra.
Lacuna magna can be missed on retrograde urethrogram because it may be compressed and bypassed by the balloon of the catheter. However, it can manifest as resistance at the beginning of catheter insertion.