Cobb’s Collar
Alternate Example
A
B
C
D
In the 1960s, Cobb described a number of cases of narrowing in the bulbar urethra that appeared to be congenital, eventually named Cobb’s Collar. Seen on VCUG as an indentation on the bulbar urethra distal to the muscular incisura and verumontanum, this anomaly can exist as a normal common variant, but rarely contributes to urethral obstruction. Its embryonic origin is not entirely clear, and may result from persistence of the urogenital diaphragm and/or the cloacal membrane. There are a number of theories regarding its development as a posterior urethral obstructive lesion, the contemporary accepted theory described in the late 1990s by Dewan et al. who reported this entity as synonymous to type III posterior urethral valves, the latter perhaps better termed as membranes (refer to atlas PUV discussion). Type III PUV consist of a rare inframontane bulbar urethral membrane with a central rather than posterior aperture that contributes to congenital bulbar urethral narrowing. Hereafter termed congenital narrowing of the bulbar urethra, this entity is identified on VCUG as a circumferential impression within the bulbar urethra seen during voiding. It is described as either minor, presenting as a normal common variant, or significant, contributing to posterior urethral obstruction. The normality of this findings is suggested by the fact that when patients undergo retrograde urethrography the narrowing is not evident. The membrane has been visualized during cystoscopy as membranous or fibrous. Indeed, this entity is commonly seen during VCUG as a minor indentation within the bulbar urethra, and is most commonly accepted in the radiologic community as a normal, common variant. Nomenclature of this entity has evolved with cystographic and pathologic investigation. Though still commonly termed Cobb’s Collar, other synonymous terms for this same entity are congenital narrowing of the bulbar urethra (as described above), and Moorman’s ring (Moorman showed constricting bulbar urethral lesions in older men, highlighting that bulbar urethral findings may be either muscular or membranous, and more likely muscular in the adult population). Again, Dewan et al. have described this entity as synonymous with PUV type III, and have further investigated and described the entity of posterior urethral valves as a spectrum of congenital obstructing posterior urethral membranes (COPUM). Reference: Dewan PA, Yunfan L (2006) Congenital urethral obstruction: the video-endoscopic perspective. Br J Urol 98: 953-959 Krishnan A, deSouza A, Konijeti R, Baskin LS (2006) The anatomy and embryology of posterior urethral valves. J Urol 175: 1214-1220.
A. Cobb’s collar
B. Prostatic urethra
C. Anterior urethra
D. Incisura
Differential Diagnosis
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Urethral Diaphragm
Posterior Urethral Valve