Fossa Navicularis Indwelling Large Calculus

A 31-year-old man presented to the outpatient urology clinic with abdominal pain, recurrent burning voiding and urinary dripping. The patient suffered of intermittent painful micturition over the past three months. No past surgical history. The physical examination was irrelevant. The initial laboratory investigations revealed normal kidney functions, subtle leukocytosis and microscopic haematuria.

Urinary tract conventional radiography examination elicited no renal, ureteral or urinary bladder radiopaque calculi.

Abdominal and pelvic ultrasonography (US) performed and elicited no urinary calculi. US showed right renal calyces and pelvis and proximal right ureter modest dilatation. post-voiding study elicited urine residue vast amount. Post voiding study repeated and elicited almost the same post voiding vast amount (≈ 449 cm³). The patient mentioned that he used to void urine with difficulty with weak urine stream all over the past three months.

 Urinary tract non-contrast CT (NCCT) examination performed and elicited no defined urinary tract hyperdense calculi or other abnormality apart from modest dilatation of the right renal calyces and pelvis and proximal right ureter. Soft conventional radiography of the pelvis including the penile urethra elicited a large elongated faint radiopaque calculus at the glans penis.

Penile US and duplex confirmed the presence of a large elongated echogenic structure occupied the fossa navicularis consistent with calculus. Distal penile urethra proximal to the calculus exhibited subtle dilatation.

Background

Most urethral calculi originally arise either from the urinary bladder or the kidneys and halted at segments of urethral stricture or deformity. These calculi are either found at the posterior or less frequently anterior urethra and commonly result in acute urinary retention. No imaging literature valuable data about fossa navicularis indwelling primary calculi.

Clinical Perspective

A 31-year-old man presented to the our institute’s urgent care center with abdominal pain, recurrent burning voiding and urinary dripping. The patient gave a history of intermittent painful micturition over the past three months. No past surgical history. The physical examination was irrelevant. the initial laboratory investigations revealed normal kidney functions, subtle leukocytosis and microscopic haematuria.

Imaging Perspective

The anterior urethra distal physiologic dilatation at the glans penis is called is the fossa navicularis [1].

The urethra is an unusual location for either halting or do novo formation of calculi. Urethral urolithiasis accounts for approximately 0.3% of overall urolithiasis [2].

Urethral calculi are almost strictly exclusive male pathology. The urethral calculi are either primary or migrant [3].

The less frequent primary urethral calculi originally formed within urethral lumen probably due to certain anatomic deformity [4]. The more frequent secondary or migrant urethral calculi form in the upper urinary tract or urinary bladder and pass to get lodged into the urethra [5].

The current case calculus almost entirely casts the fossa navicularis and measures approximately 18×12 mm. Such size proves its primary urethral formation and excludes its migration from the kidneys, ureters or urinary bladder .

The primary urethral calculi formation is attributed to disorders predisposing to urinary stasis and recurrent urinary infections [6].

Primary urethral calculi manifest with impaired voiding, urethral discharge, haematuria and eventually urine retention [7].

The urethral migrant calculi are common among elderly males with bladder outlet obstruction secondary to benign prostatic hyperplasia, prostate cancer, and urethral strictures [8].

The majority of urethral calculi composed of calcium oxalate (approximately 86-90%). Struvite calculi constitute approximately 6% while uric acid calculi approximately 2% [9].

Conventional radiography has poor sensitivity (29%) and specificity concerning urolithiasis [10].

US urinary calculi detection sensitivity is variable (approximately 61%–90%) [11].

US has the privilege of wide availability, absence of ionizing radiation and cost-effectiveness, yet is highly operator dependent [12].

Combined conventional radiography-US combination is less sensitive for urinary calculi detection compared to NCCT [13].

Calculi of less than 200 Hounsfield units (HU) are not visible on urinary NCCT [14].

Urate and cystine calculi possess approximately 100-500 HU attenuation range. Calculi containing calcium possess attenuation higher than 700 HU [15].

Almost all types of urinary calculi are radio-opaque on CT. CT enables accurate estimation of calculus size which helps to predict its fate and determine appropriate management [16]. CT calculus size estimation accuracy is leveraged by using bone window and magnification. CT evaluation of the urethra and urethral calculi is unfavourable owing to inevitable exposure of the testicles [17].

In conclusion, US is the modality of choice for urethral calculi detection and evaluation. The current case calculus large size and configuration denote it is primarily formed at the fossa navicularis.

Outcome

Owing to its large size and distal urethral location, the calculus was not able to be pushed back into the urinary bladder through the posterior urethra. Simple meatotomy under general anaesthesia performed and the calculus grabbed by stone forceps and extracted through urethral meatus. Calculus analysis elicited primary calcium oxalate composition.

Written informed patient consent for publication has been obtained.