The transradial approach (TRA) was first introduced by a cardiologist for coronary angiography [37]. Previous studies, including the RIVAL (radial versus femoral access for coronary angiography and intervention in patients with acute coronary syndromes) and MATRIX (radial versus femoral access and bivalirudin versus unfractionated heparin in invasively managed patients with acute coronary syndrome) trials, have provided evidence that TRA reduces morbidity, mortality, length of hospital stay, and costs, in addition to increasing patient satisfaction due to early mobilization compared to the transfemoral approach (TFA) [47,86]. Because of its safety and comfort, TRA has become the preferred technique for percutaneous cardiovascular interventions by most patients and physicians [54,66,87]. Neurointerventionists have extensive experience and familiarity with TFA, but TRA is minimally used due to a lack of familiarity and practical experience. The limitations associated with a large catheter, owing to the small diameter of the radial artery and absence of a specific radial device for neurointervention, make TRA unfavorable as a first-line approach. Consequently, TRA has not yet been widely adopted for neurointervention.

Despite the existing limitations, TRA continues to be used in neurointerventions. Levy et al. [60] reported 132 cases of cerebral angiography using TRA and suggested that TRA could safely replace TFA. TRA has been used not only for cerebral angiography but also for other endovascular treatments, such as aneurysm coiling, flow diversion stent insertion, onyx embolization, carotid artery stenting (CAS), and mechanical thrombectomy. This review discusses the technical skills, detailed applications, advantages, limitations, and complications associated with TRA to improve its awareness among neurointerventionists (Fig. 1).

The right subclavian artery originates from the aortic arch via the innominate artery. The subclavian artery branches further into the vertebral artery, internal thoracic artery, thyrocervical trunk, and costocervical trunk and passes along the lateral margin of the first rib to become the axillary artery. The axillary artery is divided into three parts : the superior thoracic artery branches in the 1st part; the thoracoacromial artery and lateral thoracic artery branch in the 2nd part; and the subscapular artery, anterior humeral circumflex artery, and posterior humeral circumflex artery branch in the 3rd part. The axillary artery passes through the lower margin of the teres major muscle to become the brachial artery.

The deep brachial, superior ulnar collateral, and inferior ulnar collateral arteries arise from the brachial artery. The brachial artery passes through the cubital fossa and bifurcates into the ulnar and radial arteries (Fig. 2A). The ulnar artery runs along the ulna in the medial side of the forearm, and the radial artery runs along the radius in the radial side of the forearm. The ulnar artery gives rise to the common interosseous artery in the proximal portion and ends in the superficial palmar arch. The radial artery gives off the radial recurrent artery, palmar carpal branch, and superficial palmar branch, which anastomoses with the superficial palmar arch and terminates in the hand to form the deep palmar arch.

Knowledge of the branches and course of the axillary artery is necessary to avoid wire perforation during retrograde catheterization. The axillary artery starts from the lateral border of the first rib as a direct continuation of the subclavian artery and is divided into three segments. Distally, the axillary artery continues as the brachial artery at the lateral edge of the teres major muscle tendon. The first portion of the axillary artery runs from the lateral edge of the first rib to the superior border of pectoralis minor. It only has a single branch in this portion, the superior thoracic artery. The second part begins at the upper border of the pectoralis minor muscle and ends at the lower border of the pectoralis minor muscle. It has two branches, the thoracoacromial artery and lateral thoracic artery. The third portion begins at the lower border of the pectoralis minor muscle and ends at the lateral border of the teres major muscle. It gives off three branches in this portion, the subscapular artery and the anterior and posterior humeral circumflex arteries (Fig. 2B).

ARSA is a rare anatomical variation that occurs in 0.16% to 1.8% of the population [13,57,70], in which the right subclavian artery arises directly from the posterior aspect of the aorta instead of originating from the brachiocephalic artery and has a retroesophageal course to the right thoracic outlet. The right vertebral artery almost always arises from the right subclavian artery, but occasionally the right vertebral artery can arise from the right common carotid artery (CCA). The presence of an ARSA significantly increases the difficulty of transradial catheterization of the aortic arch, and often requires switching to the TFA.

Furthermore, the presence of a severely tortuous course and multiple curves of the right subclavian artery and brachiocephalic artery makes vessel selection of the carotid arteries more difficult because of a reduction in torquability of the wire or catheter and difficulty in crossing a guiding catheter [50]. To overcome the difficulty in maneuvering the catheter through severely tortuous arteries during TRA, thicker guide wires (0.038 inch wire) can be used to straighten the tortuous vessels, allowing the guiding catheter to gain sufficient support and lower resistance.

Before performing a neurointervention via TRA, a procedural plan should be established by considering patient-specific factors. The most important patient-specific factors are the vascular anatomy of the forearm, diameter of the radial artery, aortic arch configuration, procedure to be performed, and the patient’s medical and social history. The vascular anatomy of the forearm can vary from patient to patient [91]. For example, there could be a rudimentary radial artery with three roots, replacement of the radial artery by the anterior interosseous artery, lower division of the radial artery, and lower division of the radial artery with a double recurrent radial artery. Performing TRA may be difficult if the radial artery is hypoplastic or a loop is formed in the pathway.

The mean diameter of the radial artery is reportedly 2.38-3.1 mm [40,55]. The diameter of the radial artery is relatively smaller in women than men (approximately 2.8 mm and 3.2 mm, respectively), making it technically challenging to use the TRA [46,55,92]. While planning the surgery, it should be noted that the small diameter of the radial artery compared to the femoral artery limits the use of devices with a large caliber. To overcome this limitation, topical application of 30 mg lidocaine ointment and 40 mg nitroglycerin cream or subcutaneous administration of nitroglycerin before radial artery puncture could help in significantly increasing the diameter of the radial artery and enhancing the TRA success rate [11,31]. When a spasm occurs in the radial artery due to stimulation during puncture, it can be released by injecting nitroglycerin through the sheath.

The configuration of the aortic arch can influence the selection of target vessels. In contrast to the TFA, selecting the left vessel in the TRA is challenging. Selecting the left vertebral artery is generally problematic because of the angle and distance between the innominate and left subclavian arteries. Factors that complicate the selection of the left internal carotid artery (ICA) for TRA include the aortic arch type, height of the right subclavian artery, turn angle of the left CCA, distance between the innominate artery and left CCA, angulation of the right subclavian artery, and angulation of the left subclavian artery [22].

TRA is not recommended in cases with the following medical history : patients who may require a radial artery donor for coronary artery bypass graft surgery or extracranial artery for intracranial artery bypass surgery, patients with end-stage renal disease, and the potential for developing future arteriovenous fistula or Raynaud’s phenomenon. The radial nerve descends parallel to the radial artery in the proximal third of the forearm, gradually approaches the mid-third of the forearm, and descends behind the brachioradialis muscle tendon in the distal third forearm. TRA is generally considered safe because the radial nerve is away from the puncture site; however, radial nerve palsy can rarely occur as a complication [44]. Therefore, the access route should be determined after considering the risk of palsy in patients who perform an activity in which manual use is essential (e.g., pianist, writer, surgeon). By considering these factors during planning, physicians can simultaneously maximize the TRA success rate and minimize complications. Additionally, it should be noted that TFA may be required when the risk associated with TRA is considered high.

The right radial artery is commonly used because of operator convenience and easy navigation of the catheter into the CCA and ICA. During this procedure, the patient is placed in the supine position and an arm extension board is inserted under the right arm. The patient’s arm can be positioned in several ways. Abducting the arm to 70-90° enables easier navigation to the subclavian artery. However, this may make catheter exchange uncomfortable and increase the risk of the catheter falling off the table. Consequently, surgeons prefer to abduct the arm to 15-20°. The wrist is extended using a towel roll or folded sheet, and the puncture site is sterilized and draped. For local anesthesia, lidocaine (1-1.5 mL) is injected into the subcutaneous tissue near the puncture site. A puncture can be attempted under ultrasound guidance or palpation of the radial artery. Accessing the radial artery under ultrasound guidance improves the success rate compared to access under palpation [11,53]. A 20- or 21-gauge needle or micropuncture kit is then introduced into the radial artery using the Seldinger or through-and-through technique. Subsequently, a 0.018 or 0.025-inch hydrophilic microguidewire is advanced into the distal radial artery and a hydrophilic 5- to 8-Fr sheath with a tapered dilator is inserted along the microguidewire [5]. Since a 6-Fr guiding catheter is generally suitable for neurointervention, a 6-Fr sheath is often used to reduce complications associated with the radial artery [5,38,83]. After removing the dilatator, antispasmodic agents (verapamil, nitroglycerin, or nicardipine) are slowly administered through the sheath before introducing the catheter [59]. Roadmap or angiography is used to evaluate the patency of the radial artery, vascular anomalies, and anastomosis of the ulnar artery.

The guiding catheter is coaxially navigated into the subclavian artery over a 125 cm Simmons 2 catheter and 180 cm J-shaped 0.035-inch hydrophilic guidewire. A Simmons curve can be constructed in three ways. The first method for creating a Simmons curve is to place the guidewire in the descending aorta and then advance the catheter over the wire until the second curve is positioned under the apex of the arch. After retracting the guidewire, the catheter is turned forward or backward to create a Simmons curve. The second method involves bouncing the guidewire off the aortic valve and tracing the catheter using the guidewire. However, in some patients with insufficient or unstable valves, wires or catheters may pass through the valves and reach the left ventricle. Cardiac monitoring is required because arrhythmias may occur. The final method, which can be used to approach the right vertebral artery, right CCA, or left CCA with a bovine arch, involves navigating the guidewire directly into the target vessel without reforming the Simmons catheter. Thus, 4-vessel catheterization is possible. However, selecting the left vertebral artery may be challenging depending on the relationship between the left subclavian artery and orifice of the left vertebral artery. After placing the Simmons catheter in the target vessel, the guiding catheter approaches the target vessel through the catheter. During this process, the Simmons catheter may be pulled back because of the rigidity of the guiding catheter; however, this issue can be overcome by using a 0.038-inch guidewire which provides more strength and support (Fig. 3).

After completing the endovascular treatment, the sheath is removed and bleeding is stopped. When performing hemostasis on the radial artery, minimal pressure should be applied to reduce the rate of radial artery occlusion. Gentle manual compression at the access site for 15-20 minutes is recommended. Several types of wristband devices are used for radial compression to facilitate hemostasis. These devices are placed 1-2 cm proximal to the access site and compressed for 2-6 hours. VasoStat Hemostasis Device (Forge Medical, Bethlehem, PA, USA) presses the puncture site with a convex oval surface with minimal pressure. In contrast, TR Band (Terumo, Toyko, Japan) and udeSync (Merit, Malvern, PA, USA) uses a balloon to deliver compressive forces over a large area near the puncture site. Gauzes with elastic bandages can be used to control bleeding. If radial artery occlusion is suspected upon removal of the hemostatic device, temporary compression of the ipsilateral ulnar artery may help in recanalization [10].

TRA offers five advantages over the TFA [16,19,47,54]. The first is superior patient comfort and satisfaction. After performing the procedure via the TFA, it is necessary to rest in the supine position for 4-6 hours. This posture restriction causes inconvenience to conscious patients in terms of restricting activities such as toileting and eating [63]. However, since hemostasis is relatively easy in TRA, there are no restrictions on posture and ambulation after the procedure [88]. The second and third benefits are reduced hospital stay and cost. A reduction in the postoperative rest and observation periods can decrease the hospitalization period and associated medical expenses. The fourth advantage is decreased access route complications [25]. The RIVAL trial showed that the risk of major bleeding at the access site was 0.2% for TRA and 0.3% for TFA [47]. In a study comparing TRA and TFA in cerebral angiography, Wang et al. [88] showed that the rates of symptomatic stroke, puncture site hematoma, pulmonary embolism, pseudoaneurysm, and arteriovenous fistula were significantly increased in TFA. Finally, TRA may be easy and safe in obese patients [88].

However, this approach also has some limitations. The first is that TRA is technically more challenging than the TFA [88]. TRA is generally known to have a longer learning curve. The smaller diameter of the radial artery compared to the femoral artery, higher vulnerability of the radial artery to spasm, tortuosity of the radial-brachial artery, subclavian artery to the aortic arch curve, and non-intuitive Simmons catheter movement make TRA a difficult technique [74]. Liu et al. [64] reported that the learning curve of TRA was steeper than that of TFA. However, Hess et al. [42] demonstrated faster adaptation to TRA among novices than among experienced operators [64]. The second disadvantage is that the diameter of the radial artery is smaller than that of the femoral artery. Owing to the small diameter, there are limitations regarding the catheters and techniques that can be used without causing vessel injury. Large-diameter catheters can be inserted after administering vasodilators; however, care must be taken to avoid vascular damage. Finally, it is challenging to select the left vertebral artery or ICA. Zussman et al. [94] reported that success rates of selecting the left ICA and vertebral artery were 89% and 59%, respectively. Target vessel selection in TRA can be attempted through various combinations of catheters and wires; however, selecting the left vertebral artery, in particular, remains challenging.

Although TRA has a low complication rate, it may lead to complications such as radial artery spasm, vessel perforation, local hematoma, radial artery occlusion, and radial artery pseudoaneurysm. However, most complications can be successfully treated with conservative management without permanent disability.

Radial artery spasm is a relatively common complication of TRA that can cause patient discomfort, ischemia, and limited access to devices. Radial artery spasms reduce the vessel diameter, which can block catheter entry, trap the catheter during the procedure, and make catheter manipulation difficult, resulting in procedure failure. Risk factors for radial artery spasms include anatomical variations of the radial artery, small vessel diameter, vessel tortuosity, female sex, younger age, large-diameter arterial sheath, multiple punctures, and long procedure time [43]. To reduce the incidence of radial artery spasms, reassuring the patient by providing sufficient information in advance and administering vasodilator drugs is important. If radial artery spasms occur during the procedure, intravenous pain or sedatives (such as morphine, fentanyl, or midazolam) can be administered to relieve pain and anxiety, and vasodilators, including nitroglycerin, verapamil, or nifedipine, can be administered through the sheath [14]. However, if the spasm persists, nitroglycerin may be injected subcutaneously along the radial artery. Finally, the operator needs to be patient and not forcefully manipulate the catheter to avoid vessel damage.

The operator should be familiar with the anatomical variations of the radial artery, such as radial loops and high radial insertion (separation of the radial artery from the elbow or armpit), to reduce the risk of vessel perforation. Most vascular variations can be safely navigated; however, a lack of knowledge, ignorance, or carelessness about the variant nature of vessels can increase the risk of perforation. If perforation of the radial artery is suspected, radial angiography should be performed immediately, and the procedure should be continued by advancing a catheter (diagnostic or guide) across the perforation site to seal the perforation site and control bleeding. In cases of an expanding hematoma, a blood pressure cuff should be rapidly inflated at the hematoma site to 20 mmHg below the systolic pressure and deflated every 15 minutes until the bleeding is stable [79]. If hemostasis fails, there is a sporadic risk of compartment syndrome, which requires a vascular surgery consultation for a potential myofascial incision and decompression. Additionally, due to the shape of the Simmons catheter, it is prone to knotting or kinking during excessive manipulation, particularly in very tortuous subclavian arteries. In such cases, gentle rotation of the catheter counterclockwise or clockwise to loosen the knot, advancing a stiff hydrophilic wire or long sheath through the kink, and snaring the catheter through the TFA may be considered [23,70].

Among the complications of the TRA, radial artery occlusion is clinically asymptomatic because of the anastomosis with the superficial palmar artery, which originates from the ulnar artery. The RIVAL trial (randomized, multicenter) revealed that radial artery occlusion was confirmed by ultrasound in only 0.2% of patients (n=6/3507), and none of the patients required surgical treatment [18]. Kassimis et al. [49] reported that up to 50% of radial artery occlusions may spontaneously recanalize within 1-3 months. However, continued radial artery occlusion may limit the use of TRA in future interventions.

The risk factors for radial artery occlusion include a long procedure time, large-diameter sheath, small radial artery size, higher number of puncture attempts, long compression time, and high compression pressure [24]. A systematic review reported no significant difference in the incidence of radial artery occlusion between 5-Fr and 6-Fr catheters. However, the use of 5-Fr sheath showed greater benefit due to a higher proportion of women with a small radial artery diameter in the 5-Fr group [76]. Another study claimed that the incidence of radial artery occlusion increased with an increase in the ID of the radial artery to outer diameter of sheath ratio (4% vs. 13%, ratio <1.0 vs. >1.07, respectively) [78].

An increase in the number of radial artery puncture attempts increases the risk of radial artery occlusion. Ultrasound-guided puncture and subcutaneous nitrate injection can reduce the number of puncture attempts [80]. Additionally, prophylactic heparin significantly reduces the incidence of radial artery occlusion in a dose-dependent manner, regardless of intravenous or arterial administration [72]. Ipsilateral ulnar artery compression may also reduce radial artery occlusion. Two randomized controlled studies have shown that ulnar artery compression reduced radial artery occlusion on day 30 from 3.0% to 0.9% and 5% to 0%, respectively, without hand ischemia when compared to the standard method [56,73].

Using the TRA poses a challenge for neurointerventionists. Due to numerous obstacles associated with the TRA, acquiring relevant knowledge and sufficient experience is necessary. Although performing neurointervention through the TRA has limitations, it also offers several advantages, such as patient comfort and a low rate of complications. It is crucial to know the advantages and disadvantages of the TRA and TFA and apply them appropriately on a case-by-case basis. Future research in the field of TRA for neurovascular interventions should focus on refining the training methods for acquiring technical skills, developing specialized devices, optimizing patient selection criteria, exploring combination approaches, conducting cost-effectiveness analyses, and performing comparative studies. These efforts will contribute to a deeper understanding of the potential benefits of TRA, its limitations, and its role in improving patient care and outcomes in neurointerventional procedures.