Priapism is defined as a sustained, undesirable erection lasting greater than 4 h which is not associated with stimulation or sexual interest. Generally, the aetiology of priapism is due to either high-flow or low-flow states, with idiopathic or pharmacologic causes as the most common cause in adults. High-flow priapism is generally secondary to traumatic injury to the pelvis, with increased arterial flow to the penis causing a sustained erection. Low-flow causes are many, to include haemoglobinopathies, pharmacologic, neoplastic and neurologic causes. Priapism is subsequently classified into either ischemic or non-ischemic causes, with ischemic causes generally secondary to low-flow states with resultant decreased overall blood flow throughout the penis. Ischemic priapism is a medical emergency, with treatment indicated within 24–48 h due to risks of tissue loss and necrosis. Other potential consequences of priapism include severe patient discomfort and pain, risk of urinary retention, as well as potential vascular compromise (if traumatic in origin). This spectrum of pathologic aetiologies renders evaluation of priapism as a medical emergency, as the source is often immediately unknown prompting urgent initiation of the patient's diagnostic work-up.1
Ultrasound is the most common primary imaging modality to visualise the penis, with MRI used as an adjunct to ultrasound for problem solving when the clinical and sonographic features remain non-diagnostic. Ultrasound is readily accepted for initial screening due to its high availability, low cost and real-time capabilities. High-frequency, 7.5–12 MHz, linear transducers are used because they yield images with better resolution, compared to lower-frequency transducers. Transverse and longitudinal grayscale images of the corpora spongiosum and both cavernosa are captured along with Doppler images of the both cavernosal arteries in order to document flow characteristics.2
Subsequently, lesions which are identified with US are further characterised in multiple planes with grayscale and focal Doppler interrogation. Metastatic disease usually appears as either a single focal hypoechoic lesion or multiple hyperechoic structures, both of which demonstrate hypervascularity.3
With its excellent soft-tissue contrast, multi-planar capabilities and excellent spatial resolution, MRI can also confidently be used to identify and characterise primary and metastatic penile neoplasms. MRI distinguishes between corpora cavernosa and spongiosum depending upon the rate of blood flow through them at the time of imaging. Most commonly, the cavernous spaces are intermediate signal on T1 and high signal on T2 sequences. The corpora cavernosa demonstrate the same signal because they share common blood flow via interconnecting fenestrations. The spongiosum, however, is isolated from the cavernosa, and its signal varies slightly from them. The cavernosal bodies are surrounded by an inner and outer fibrous layer, the tunica albuginea and Buck's fascia, respectively; which are low in signal intensity on both T1 and T2 weighted images. Cavernosal arteries are seen on coronal T2 images as a hypointense foci within the medial aspect of the corpora cavernosa. T1, T2 and postcontrast fast suppressed T1 images are routinely regarded as the most useful sequences to obtain when evaluating for malignancy of the penis.4
Metastasis to the penis is relatively rare, despite its rich blood supply.4–6
However, despite its rarity, penile metastasis generally portends an advanced stage and a poor prognosis.4 5
Penile metastasis usually arises secondary to a surrounding primary genitourinary organ malignancy in 70–75% of cases,4–6
with rectosigmoid colon primaries accounting for another 13%.6
The most frequent primary cancer to metastasise to the penis is cancer of the bladder (27%) followed by primary prostatic cancer (generally adenocarcinoma from the glandular portion of the organ).4 5 7
Eberth is credited for documenting the first case of penile metastasis in 1870, and since that time, there have been about 460 additional cases reported in the literature worldwide.6 8
Retrograde venous flow via communications between the dorsal venous system of the penis and venous plexuses draining the pelvic viscera is believed to be the primary route of malignant spread to the penis.5 6
Other routes of malignant spread to the penis include retrograde lymphatic flow, arterial spread, direct extension from aggressive low-lying rectal cancers and implantation secondary to surgical instrumentation.6 9
The corpora cavernosa are the most frequently reported sites of penile metastasis.5
Penile metastasis most commonly presents as multiple infiltrative lesions, indurated nodules (60%) or in episodes of priapism (40%).4–6 9
An even rarer presentation is the distinct entity known as ‘malignant priapism’ which was first described in 1938 by Peacock. Malignant priapism is defined as the invasion of malignant cells into the cavernous sinuses and draining veins without affecting the arterial supply. The blockage of venous drainage leads to an erection from inability of the patent, distended cavernous sinuses to drain.4
Another mechanism which may contribute to malignant priapism includes irritation of the neural pathways of erection by local tumour infiltration. Both high- and low-flow states of priapism occur and high-flow priapism (high-flow in the cavernosal arteries with reversal of flow in diastole) is believed to be more common.4 6
Regardless of exact mechanism, vascular invasion of the penis likely portends a poorer outcome in metastatic disease when it originates from urothelial carcinomas and adenocarcinoma of the prostate.7
Biopsy or corporeal aspirations are the most common procedures to yield a histopathalogical diagnosis.6
Imaging remains the most sensitive method for determining cavernosal metastasis and is also extremely useful in preoperative staging.9
On MRI, penile metastasis usually has a low signal intensity on T1 which is isointense to the adjacent corpora cavernosa. The metastatic focus is heterogeneously hypo- to iso-intense and clearly contrasted against the hyperintense signal of the cavernosal bodies on T2 weighted imaging. Metastatic foci commonly enhance on postcontrast T1 images more than the adjacent cavernosum. In addition, MRI is quite useful for evaluating the primary malignancy if confined to the genitourinary tract, prostate or rectosigmoid colon secondary to their inclusion within the typical field of view. Metastatic lymph node involvement may additionally be noted and can assist with surgical staging.6 9
Management of metastatic lesion in the corpora cavernosa includes local excision, partial or total penectomy, radiation, and/or chemotherapy. The average prognosis is 1 year (range of 47 weeks to 7 years) from the time cavernous metastasis is first discovered.5
- Malignant Priapism is an uncommon cause of painless priapism which may be seen in older individuals.
- MRI and Ultrasound can be readily used to elucidate the cause of painless priapism.
- Imaging is helpful in staging the patient's disease as well as identifying additional silent disease within the region of interest.