Megalourethra
Megalourethra is a nonobstructive urethral penile urethral dilation. It is associated with abnormal development of the corpus spongiosum and corpora cavernosum. It is commonly seen in prune-belly syndrome and therefore abnormal development of the mesoderm has been proposed.  Two forms have been described, scaphoid and fusiform. The fusiform form is associated with malformation of the corpus spongiosum, and the scaphoid form is associated with malformation of the the corpora cavernosum.   There is a high association with upper tract anomalies including vesicoureteric reflux, bladder diverticulum and megacystis-microcolon-hypoperistalsis syndrome and VATER association.  Both anterior and posterior urethral valves have been reported.  85% of cases are reported to have other malformations, necessitating an appropriate work-up. Surgical repair is for cosmetic reasons, and erectile function is variable depending on the degree of associated corpus spongiosum or cavernosum hypoplasia.  By urethrography, the distinction between megalourethra and anterior urethral diverticulum can be arbitrary.  A useful method to distinguish them is the absence of obstruction in megalourethra. A diverticulum may have a narrow opening and may fill with urine and obstruct the adjacent urethra.
 
Reference:
Campbell’s Urology 8th ed. Saunders
 
V. Sreetharan*, B.C. Sommerlad, L. Kangesu, A case of congenital megalourethra Journal of Plastic, Reconstructive & Aesthetic Surgery (2006) 59, 550–552
 
Ozokutan BH, Kucukaydin M, Ceylan H, et al Congenital scaphoid megalourethra: report of two cases.  International Journal of Urology  (2005)  12,  419–421
 
 
Differential Diagnosis (Click on the image to go to a specific discussion)
 
A
A. Megalourethra
B. Decompressed posterior urethra
 
One should note that there is no specific obstructing lesion causing the abnormally distended urethra.  The process is the result of lack of supporting structures.
 
Anterior urethral valve
Anterior diverticulum
A B