Prostatic Artery Embolization: Technical Pearls

Abstract

Benign prostatic hyperplasia (BPH) is a condition that primarily affects men between the fourth and seventh decades of life, occurring due to enlargement of the prostate which subsequently causes compression of the prostatic urethra causing chronic obstruction of the urinary outflow tract. BPH can cause significant quality-of-life issues such as urinary hesitancy, intermittency, decreased urinary stream, a sensation of incomplete emptying, dysuria, urinary retention, hematuria, and nocturia. Several medical and surgical treatment modalities are available for the treatment of lower urinary tract symptoms and other BPH-related sequela; however, increasingly prostate artery embolization is being utilized in this patient population. Technical demands for this procedure in this population can be significant. This article describes the optimal techniques, tricks, and advanced imaging techniques that can be used to achieve desired technical outcomes.

Benign prostatic hyperplasia (BPH) is a condition that primarily affects men in the latter half of life, with symptoms appearing between the fourth and seventh decades of life. BPH is an enlargement of the prostate which compresses the prostatic urethra causing chronic obstruction of the urinary outflow tract resulting in a significant deterioration in the quality of life (QoL) of the patients impacted. The obstruction results from two elements: static and dynamic bladder outflow obstructions (BOOs). Dynamic BOO is attributed to increased smooth muscle tone, which is regulated by alpha-adrenergic action, while static BOO is due to an increase in the stromal tissue of the gland. This bulk increase is regulated by androgenic action. ^{1 2}

BPH can cause significant QoL issues such as urinary hesitancy, intermittency, decreased urinary stream, a sensation of incomplete emptying, dysuria, urinary retention, hematuria, and nocturia. ³ This constellation of symptoms is called “LUTS” or “lower urinary tract symptoms.” Several medical and surgical treatment modalities are available for the treatment of LUTS and other BPH-related sequela. The first-line medical treatment consists of alpha-blockers and 5α-reductase inhibitors, which work to reduce symptoms by antiadrenergic action or by decreasing the gland size, respectively. Medical management is used for mild to moderate LUTS. ⁴ Surgical management, on the other hand, provides rapid relief of symptoms. However, this approach comes with a unique set of peri- and postoperative morbidity and mortality, ³ a risk which is elevated in this cohort of patients due to increased comorbidities such as age, diabetes mellitus, cardiovascular disease, and renal dysfunction. In addition, both treatment modalities can cause an overlapping set of adverse effects, such as erectile dysfunction, libido changes, and ejaculatory disorders.

Recently, various innovative modalities of BPH treatment have been introduced, such as prostate artery embolization (PAE), Urolift, Rezum, and Aquablation. PAE is a relatively novel, efficacious outpatient procedure for treating moderate to severe LUTS, urinary retention, and BPH-related hematuria. ^{4 5}

PAE addresses both the static and dynamic components of BOO by causing necrosis and volume reduction (static component) while reducing the neuromuscular tone (dynamic component). ⁶ PAE is also an advantageous alternative in patients who have contraindications to surgery or are concerned about sexual side effects of invasive surgical procedures. ⁷

PAE is also technically challenging for various reasons outlined later and may be especially challenging to new operators as it has a steep learning curve. The key aspects the operator should be familiar with are the complex vascular pelvic anatomy relevant to prostatic vascular supply, preprocedure imaging options, intraprocedural imaging tools, ideal microcatheters and wires, embolic material and delivery, and tools for preventing nontarget embolization (NTE). This article aims to describe the optimal techniques, tricks, and imaging that can be used to achieve desired technical outcomes.

Pelvic Vascular Anatomy

PAE is a challenging procedure with a steep learning curve, partly due to the complex and highly variable pelvic vascular anatomy, small caliber of prostatic vessels, and variable origin of the prostatic artery (PA). Therefore, appropriate imaging techniques and catheters relevant to the pelvic vascular anatomy are crucial to maximizing the technical success rates of PAE.

The prostate gland consists of a transitional and a peripheral zone. The transitional zone is a highly vascular area around the urethra, the zone where BPH arises, causing urinary symptoms due to obstruction of the urethra. The vascular supply to the transitional zone remains the key target for a successful PAE. The PAs arise from the internal iliac artery (IIA) distribution in more than 95% of cases; hence, the understanding of IIA anatomy and branching pattern is essential.

Two studies have highlighted the incidence of the PA origin. The PA is most commonly a branch of the internal pudendal artery (IPA; Fig. 1a ; 27.9–34.1%) followed by the superior vesical artery (SVA; Fig. 1b ; 20.1–32.6%), gluteal pudendal trunk (17.8%), and obturator artery ( Fig. 1c ) in 12.6%. ^{8 9} The classification proposed by de Assis et al ⁵ provides a valuable resource for categorizing the origins of the PA. de Assis type I to type IV represent PA arising from the SVA, directly from the anterior division of internal iliac artery, obturator artery, or IPA, respectively; de Assis type V pattern is the least common, arising elsewhere. Prevalence rates in the population of de Assis types I to V are 28.7, 14.7, 18.9, 31.1, and 5.6%, respectively. ⁸ Rare origins (type V) include accessory obturator origin (from external iliac artery) and terminal IPA.

Fig. 1.

( a ) Origin of prostatic artery (PA) from the internal pudendal artery (IPA) (circle). A—Bladder branches of IPA. B—PA arising from IPA. C—Rectal collaterals from PA. D—IPA. ( b ) Origin of the PA from the superior vesicular artery (SVA). A—Supply to bladder, B—prostatic artery, C—rectal branches, D—IPA. Red circle = branch point. ( c ) Angiographic image showing origin of PA from the obturator artery. A—Supply to the bladder, B—obturator and PA, C—IPA, D—inferior gluteal branch. Origin highlighted by red oval.

The PROVISO mnemonic corresponding to P = internal pudendal artery; R = middle rectal artery; O = obturator artery; VI = inferior vesical artery and its prostatic branches; and S = superior vesical artery can be a valuable tool for identifying the vessels correctly on angiographic imaging. ¹⁰

Up to 60% of individuals have a single PA for both hemipelves. ⁴ In comparison, a cadaveric study showed multiple PA per side in 22.2% of cases. ¹¹ In about half of the cases, the configuration of the PA is bilaterally symmetrical. Identification and adequate embolization of both central and capsular branches of the PA are crucial, as failure to do so may lead to revascularization of the prostate, which can contribute to early recurrence of symptoms. ¹² Extraprostatic anastomoses are found in 39.1% of hemipelves, most commonly with the IPA, middle rectal artery, and SVA. ^{9 13} NTE can occur because of reflux into or inadvertent embolic delivery to the communications to penile, vesical, or rectal vasculature. Therefore, identifying and adequately handling these anastomoses are essential to prevent NTE. The tools for NTE prevention are covered separately later.

The PA is often tortuous in patients with BPH since the artery is forced to adapt its existing length to an enlarging gland. This intraglandular “cork screw” tortuosity can be used to confirm the catheterization of the correct artery on imaging. ¹⁴

Catheter and Wire Choice for PAE

Once the PA has been identified with an angiography or cone beam computed tomography (CBCT) from an internal iliac artery, various microcatheters can be advanced over a guidewire to cannulate the PA. The 2.1-Fr Maestro (Merit Medical, Utah) with a pre-shaped swan neck can be a valuable tool for tortuous origins. This microcatheter can also help facilitate the catheterization of type I branching patterns due to its swan neck design. ¹⁵ The Progreat 2.0 (Terumo Interventional Systems, Somerset, NJ) is another alternative for smaller caliber vessels. ¹⁶ Both microcatheters are also available in 150 cm length, allowing embolization from radial access.

Other catheters available in PAE include Tru-select (Boston Scientific) and Pursue (Merit Medical). In addition, sniper (Embolx, California), a 2.2-Fr balloon occlusion catheter, has the advantage of inflating the balloon at the tip of the catheter to prevent reflux and potentially reverse the flow in the collateral vasculature. ^{10 17} The choice of catheters depends on user preferences, availability, vascular anatomy, and risk of nontarget embolization. All these catheters are compatible with most 0.016-in microcoils. However, the compatibility of an individual catheter with available coils should be checked beforehand to avoid intraprocedural delays.

Similar considerations also influence the choice of the guidewire. Wires such as 0.014-in shapeable guidewire (Fathom; Boston Scientific Corporation, Marlborough, MA) and the 0.016-in double-angle Glidewire GT (Terumo Interventional Systems, Somerset, NJ) are widely used starter choices. ^{15 18 19} For larger vessels, 0.016-in shapeable guidewire (Fathom, Boston Scientific Corporation) may be utilized. ^{19 20}

Some of the most technically challenging arteries are type 1 prostatic arteries (shared origin with the SVA in a common trunk), primarily associated with atherosclerosis of the anterior division of the IIA. In such cases, making a double curve in the microwire (“C” or cobra shape), curved microcatheters or microwires, or advancing the 5-Fr catheter near the PA origin can help in the microcatheterization. ⁸ Type 4 (origin from IPA) and type 5 (origin from accessory IPA) are shorter arteries, and in such cases, embolic agent reflux is a significant concern, usually into pudendal or rectal territories.

Preprocedural Imaging

Given the rather complicated anatomical variations and equipment described earlier, it is prudent to optimize the use of preprocedural and intraprocedural imaging to achieve the desired technical endpoint.

Preprocedure imaging can be performed to identify the PA before the date of the procedure. Computed tomography angiography (CTA) or magnetic resonance angiography (MRA) are helpful investigations for procedure planning. ^{9 21} Preoperative CTAs can also be expanded to measure the baseline prostate volume. ²² While digital subtraction angiography (DSA) is the mainstay of intraoperative imaging, PAs often lack the pathognomonic DSA features for early adopters; this increases the importance of preoperative imaging for PA identification. ²³

Bilhim et al described the protocol for preprocedure CTA imaging for identification of the PA. The settings include power of 100 to 120 kV; 200 to 300 mA; and a matrix of 512 × 512 pixels with a 360 × 360 mm field of view. A voxel of 0.7 × 0.7 × 1.25 mm; collimation of 16 × 1.25 mm; and pitch of 1.3 are recommended. Iodine contrast agent injection of 100 to 120 mL (at a concentration of 350–370 mg/mL iodine) at an injection rate of 3.5 to 5 mL/s, using bolus triggering in the abdominal aorta (above the renal arteries) with saline flush (30 mL, at the same rate) before and after contrast injection is recommended. ²³ Using this protocol, the authors were able to accurately identify PA in over 95% of preprocedure CTA with additional specification that 35% of PA originated from the IPA, while 20% were from a common origin with the SVA, 15% from the common anterior gluteal, 10% from a common recto-prostatic trunk, and 10% from the obturator artery, which is in line with other reported studies. ^{8 9} Once the PAs have been identified on axial images, volume-rendered three-dimensional (3D) images are created with post-acquisition firmware. This can be useful to determine the optimal obliquities for visualization with fluoroscopy. Manipulation of 3D images can reveal the origins of the target arteries and the angles of origin to the arteries from which they arise. ²²

However, patients with advanced atherosclerotic disease can still pose technical challenges, and extreme cases of tortuosity and atherosclerosis may not be suitable depending on the operator's experience.

Access Site

PAE has traditionally been performed using a transfemoral artery (TFA) approach. However, recent studies have discussed the efficacy of TRA, with many showing no statistically significant difference in adverse events, radiation dose, procedure time, or clinical success rate compared to TFA. ^{24 25} Simultaneously, other studies have shown faster procedure times and decreased fluoroscopy time with transradial access. ¹⁵ When evaluating TFA versus TRA, it has been shown that while transradial access and transfemoral access PAE had similar technical success rates, TRA was associated with significantly decreased procedure time (110 minutes for TRA vs. 131 minutes for TFA) and fluoroscopy time (39 minutes for TRA vs. 48 minutes for TFA). Additionally, the same study found no difference in the rates of access site complications. ²⁵

In terms of the technical advantages of TRA versus TFA, TRA offers multiple advantages. First, TRA allows for direct advancement of catheters and wires to the iliac arteries from the aorta as opposed to the “up and over-technique” required to access the contralateral IIA in TFA. The practical advantage of TRA over TFA is that the patient is mobile and can walk or be discharged on the same day.

However, a disadvantage of radial access is the risk of cerebrovascular accidents due to traversing the aortic arch. ²⁶ This risk of stroke has been documented in percutaneous coronary intervention cases where radial access was utilized, albeit very rarely. ²⁷ Additionally, radial access increases the risk for radial artery occlusion or spasm, although, once again, very rarely. Finally, one practical disadvantage of radial access for PAE is the need for longer, steerable catheters to reach the prostate. ²⁶

Intraprocedural Imaging

Using DSA with a 5-Fr catheter can help visualize the origin of the prostate artery. The catheter should be placed at the common iliac artery trunk to avoid missing any branches of the PA that arise from the anterior or posterior divisions. Next, the anterior division of the IIA is catheterized, and angiographic views in the ipsilateral anterior oblique (35 degrees) and caudocranial (10 degrees) projection should be obtained to visualize the prostatic arteries and their branches. The PAs are then cannulated with microcatheters, and any spasm can be managed with 100 to 200 μg of nitroglycerin. ¹⁴

CBCT can be a great resource for identifying the pelvic vasculature, the origin of PA, and potential anastomotic communications of the PA. CBCT can be performed with 5-Fr catheter tip in IIA (proximal) or a microcatheter in the PA (distal). Proximal CBCTs provide the PA origin details and can also provide a good source of information about the anastomotic pathways. ¹⁰ Additional advantages CBCT can provide are an improved signal-to-noise contrast ratio and lower radiation dose compared to the DSA. The proximal CBCT can be an alternative to potentially multiple DSA at different angles. The following protocol reported is used in the senior (S.B.) author's institution: a 3D rotational selective angiography of the IIA is obtained using a 5-Fr catheter placed in the artery with 25 mL of iodinated dye injected for 10 seconds (2.5 mL/second). After a 5-second X-ray delay, a 6-second spin is performed. Finally, a 3D reconstruction is performed on an independent workstation, and a navigation path is created and displayed on the two-dimensional roadmap ( Fig. 2a–c ). ²⁸ The use of proximal CBCT in PAE has been shown to reduce overall radiation exposure and the total procedure time. ²⁸ An alternative protocol for proximal 5s CBCT is reported with the 5-Fr catheters in the internal iliac artery, above the bifurcation of the anterior and posterior branches. The following injection parameters are typically used: 22 to 26 mL of pure contrast, injected at 2 mL/second, with an X-ray delay of 6 to 8 seconds. ²⁹

Fig. 2.

( a ) Image from cone beam computed tomography (CBCT) with catheter tip in prostatic artery confirming supply to the left hemiprostate. Both hemiprostates are marked by white arrows. ( b ) Image from CBCT showing rectal collaterals (marked by a white arrow) from prostatic supply. ( c ) Roadmaps developed from intraprocedure CBCTs with catheter tip in internal iliac artery (IIA).

Distal CBCT is performed with the microcatheter tip in PA. The protocol for this CBCT includes injection of 2 to 4 mL of contrast into the PA based on the size of the gland, immediately followed by 5- or 6-second spin. Some operators prefer continuous injection of contrast through microcatheter connected to an injector while performing rotation for CBCT. This CBCT helps determine the ideal position to perform embolization and also provides valuable information on the patient's anatomy to confirm the coverage of the gland (especially transitional zone) and filling of any collaterals or nontarget vessels. ³⁰ A postembolization CBCT using the same protocol can also be used to confirm an adequate and appropriate distribution of the embolic solution.

Embolization

After microcatheter placement in the PA, a prostatic arteriogram is obtained by DSA to visualize the characteristic blush of the hemi prostate, as well as its central gland (anteromedial) and peripheral zone (posterolateral) prostatic branches ( Fig. 3a, b ). Once the PA has been catheterized, 100 to 200 μg of nitroglycerin diluted in saline is injected to prevent vasospasm and to increase the diameter of the artery to facilitate distal catheterization.

Fig. 3.

( a ) Urethral branches of prostate artery, supplying the transitional zone. ( b ) Peripheral branches of the prostate artery which supply the peripheral zone. Some peripheral branches may supply parts of the anal canal or rectum.

Once the microcatheter is positioned, embolic materials are injected. A variety of embolic materials are available for PAE. Permanent embolic such as Trisacryl gelatin microspheres (Embosphere, Merit Medical; Bead Block, Boston Scientific Corporation) in sizes ranging from 100 to 300 μm and 300 to 500 μm can be used. Increasingly, the preference is to use 300- to 500-μm particles since they have similar efficacy and lower NTE rates than 100- to 300-μm spheres. ³¹ At the senior author's (S.B.) institution, PAE is performed with 300 to 500 μm Embosphere Microspheres, using a mixture of 9 mL saline, 9 mL iodinated contrast material, and 2 mL embolic agent for a total mixture of 20 mL. The aliquot is injected at an approximate rate of 1 mL/minute. Intermittent flush with 3 mL of saline is conducted after injection of 1 to 3 mL aliquot, depending on the flow dynamics. The process is repeated until near the stasis endpoint. Use of other embolic materials such as polyvinyl alcohol (PVA) particles (Bearing nsPVA, Merit Medical Systems, Inc.; Contour, Boston Scientific Corporation) and polyethylene glycol has also been reported. ^{19 32}

High dilution (approximately 1:10 ratio of embolic to total volume) and slow injection (<2 mL/min) under guidance by fluoroscopy are necessary to avoid early proximal occlusion and to achieve the goal of diffuse gland particulate distribution. It typically takes 10 to 15 minutes to achieve the desired endpoint. ¹⁰ However, waiting 2 to 3 minutes after the initial stasis is essential to assess potential early recanalization. If the continued forward flow is seen, more embolic solution may be injected. Total stasis, filling of any collateral, the presence of reflux, and filling of a venous structure are considered appropriate target endpoints. Sometimes early stasis and associated reflux prevent continued embolic volume delivery. To overcome this limitation, one can use the PErFecTED technique (Proximal Embolization First, Then Embolize Distal). This technique allows the operator to inject embolic particles into the intraprostatic vessels without reflux into vessel of origin and has been shown to have more favorable clinical outcomes following PAE. ³³

It is imperative to perform a follow-up arteriogram if there is any concern of other vasculature feeding the prostate. If any accessory prostatic branch is not embolized, long-term clinical lower urinary tract symptom recurrence may reappear. ^{10 34} A study performed by Carnevale et al in 2019 showed that revascularization by the posterolateral branch and recanalization of the previously embolized PA was the most frequent patterns seen in patients who required repeat PAE. ³³ This result shows that embolization of the posterolateral branch in the initial procedure could potentially decrease the likelihood of revascularization of the prostate and recurrence of LUTS. ³⁴

Nontarget Embolization—Prevention

Anatomical identification of the PAs and surrounding collaterals is necessary to avoid untargeted ischemia to the bladder, rectum, anus, or corpus cavernosum. ²³ Anastomoses widely interconnect pelvic arterial supply. However, most are low flow and usually do not require protective embolization. ¹⁴ NTE can occur in high-flow anastomoses to clinically relevant regions such as the bladder, rectum, and penis ( Fig. 4a–c ). High-flow collaterals to these regions, such as those arising from the IPA, middle rectal artery, and superior rectal artery, are at higher risk of facilitating NTE. ¹³ Due to this distribution, NTE may lead to rectal and bladder ischemia or penile ulcers. ¹⁷

Fig. 4.

( a ) Digital subtracted angiography showing rectal collaterals from prostatic artery. Rectal supply is highlighted within the black oval. A—Prostatic artery. ( b ) Left: Bladder collateral marked by a white arrow. ( c ) Left: Accessory pudendal artery supplying the prostate, collateral marked by the arrow; Right: stasis after coil embolization (coil circled in red).

Using preoperative CTA or intraprocedural CBCT can significantly increase the identification rates of such collaterals and decrease the risk of NTE. ³⁰

If such high-risk collaterals are present, the operator can either move the catheter distally to the anastomosis before beginning embolization or perform protective embolization of the anastomosis. Protective embolization can be performed using coils or Gelfoam (Pfizer Inc, New York). ¹⁰ Coils appear to be the preferred tool for the prevention of NTE. A study with more than 120 patients evaluated the use of coil embolization to prevent NTE. Coils were placed in the middle rectal artery, penile communications, vesical artery, obturator artery, and intraprostatic arteriovenous fistula. It is recommended to wait for 2 to 5 minutes after coil embolization to ensure no flow through the embolized vessel. This technique significantly increased the fluoroscopy and procedure time. However, the change in dose product area was not significant. There was no statistically significant difference in minor and major ischemic complications at 1 and 3 months, showing the safety of the coil embolization to prevent NTE. ^{35 36} The coil/microcatheter compatibility should be assured to ensure the appropriate placement of the coils.

It should be noted that the possibility of NTE is only significantly reduced, not eliminated, after coil embolization of high-risk collaterals. Using 300- to 500-μm particles if using Embospheres or upsizing the particles will further decrease the risk of NTE. Using vasodilators such as verapamil (3–5 mg, concentration of 0.5 mg/mL) can also help reverse the flow and prevent nontarget embolization. ¹⁴

Balloon occlusion microcatheters can also potentially prevent nontarget embolization by flow reversal in the collaterals or anastomoses with the PA. In a retrospective case–control study, Ayyagari et al found decreased procedure time and fluoroscopy time, resulting in lower radiation doses for the operator and patient. ³⁷ However, both groups did not significantly differ in IPSS, PVR, or voiding trial success. Bilhim et al conducted a randomized control trial for the same technique, and the results did not show any statistically significant differences in the postoperative outcomes between the control and experimental groups. ³⁸ However, there is evidence in both these studies which suggests that balloon occlusion microcatheters may be effective at reducing NTE. Therefore, it is possible that while there is no efficacy benefit to this technique, it may make the procedure safer for the patient.

Unilateral versus Bilateral PAE, and Repeat PAE

Multiple studies have shown a consistently better clinical outcome, reduction in prostate size, and urodynamic parameters with bilateral embolization compared to unilateral embolization. ³⁹ However, it may not be possible to embolize bilaterally for various reasons. In such cases, unilateral embolization should still be performed, and patient expectations should be reset accordingly.

In repeat PAEs, each hemitransitional zone is often revascularized by more than one branch, making the procedure very technically challenging. ⁸ A recent study addressing repeat PAE showed that revascularization was most commonly seen in a previously nonembolized posterolateral prostatic branch, recanalization of the previously embolized PA, and revascularization of distal branches from SVA, IPA, or obturator arteries. ⁴⁰ Performing CBCT from IIA can help in these cases determine the origin of vascularization of the prostate.

Conclusion

PAE is a safe, effective outpatient procedure for treating BPH, which requires operator experience and a good selection of candidates. Important technical aspects include pelvic vascular anatomy, catheter/wire choice, and pre- and intraprocedural imaging, and embolization technique should be optimized to achieve the best outcomes.