Is the Modified Allen’s Test a Useful Tool for Evaluating the Vascular Dominance of the Forearm?

Abstract

Background

The modified Allen’s test (MAT) is commonly used to evaluate the collateral blood supply from the ulnar artery before harvesting a radial or ulnar forearm free flap. We hypothesized that the outcome of the MAT would be inversely proportional to the vessel’s diameter. Therefore, we investigated the correlation between MAT results and arterial diameter to determine whether the MAT could serve as a decisive factor for evaluation of distal forearm circulation.

Methods

Patients who underwent forearm based free flap were identified by retrospective chart review between September 2012 and April 2023. The diameter of the radial and ulnar arteries was measured by computed tomography angiography (CTA). Correlation between capillary refill time measured during the Allen test and vessel size was analyzed.

Results

A total of 25 patients met the inclusion criteria. The radial artery was significantly larger than the ulnar artery on both sides. However, correlation analysis showed no association between the diameters of the radial and ulnar arteries and the MAT results.

Conclusion

The MAT does not adequately represent the diameters of the radial or ulnar artery; it merely indicates the presence of collateral circulation between them. Preoperative CTA or ultrasonography would be recommended to identify forearm vascularity precisely.

Graphical Abstract

INTRODUCTION

Since its introduction by Dr. Yang and Dr. Gao in the early 1980s, the radial forearm free flap (RFF) has become a cornerstone in reconstructive surgery.1 The flap offers thin, pliable soft tissue and can include the radius for osteocutaneous reconstruction when necessary. Moreover, it can be harvested with a generously sized pedicle of sufficient length. The dissection of the pedicle is relatively straightforward, as the radial artery is located superficially. The functional and aesthetic outcomes of reconstructions using the RFF have been documented extensively, demonstrating its versatility and reliability.2,3 However, risk of circulation failure of distal forearm following radial artery harvest cannot be excluded.

Vascular inflow to the forearm originates from the brachial artery, which bifurcates into the radial and ulnar arteries. These arteries and their branches form anastomoses at the hand, creating the superficial and deep palmar arches. Typically, the superficial palmar arch is formed by the ulnar artery and the superficial branch of the radial artery. Cadaveric and hemodynamic studies have shown that the radial artery is the primary source of blood flow to the hand, with its size and arterial flow patterns confirming this role.4,5 Although the ulnar artery has larger diameter where it branches off from the brachial artery, the radial artery exhibits a larger diameter at the wrist level.6 Plethysmographic studies also indicated that the radial artery plays a significant role in the perfusion of the fingers, particularly the thumb.4

The modified Allen’s test (MAT) is a straightforward, cost-effective, and noninvasive method for assessing the vascular connection between the radial and ulnar arteries at the level of the palm. It is commonly used to confirm adequate hand circulation following invasive procedures involving the radial artery. According to the Poiseuille’s law, the flow velocity of blood is inversely proportional to the fourth power of the radius of the vessel. We hypothesized that the flush time measured during the MAT would correlate with the diameter of the vessel. If the MAT could determine vascular dominance in the hand, it would enable proper risk evaluation ahead of flap harvest. The purpose of this study was to elucidate the relationship between the outcomes of the MAT and the vessel size analyzed by computed tomography angiography (CTA) scans.

METHODS

A retrospective review of electronic medical records was conducted for patients who underwent reconstructive surgery with RFF between September 2012 and April 2023. Only patients who had undergone preoperative CTA were included in the study. The data collected included patient demographics, etiology, details of preoperative vessel examination, surgical complications and underlying conditions that may affect flap perfusion.

Preoperative vessel examination

The vessel diameter on 3D CTA was measured by a single examiner using INFINITT PACS M6 software (INFINITT Healthcare Co., Ltd., Seoul, Korea). Measurements were taken of the radial and ulnar arteries on both sides at the level of the distal radioulnar joint, specifically at the point where the radial or ulnar artery would be ligated during the elevation of a forearm flap (Fig. 1). Concurrently, the examiner checked for anatomical abnormalities such as an unexpected course, absence, or stenosis of the forearm arteries. The MAT was administered by plastic surgery residents in a conventional manner without use of equipment such as sonography or plethysmography, patient placed the back of their hand on a table. The examiner then compressed both the radial and ulnar arteries at the wrist, after which the patient exsanguinated their hand by making a fist. Maintaining pressure on the ulnar artery, the examiner released the radial artery and timed the return of blood flow to the hand and fingers in seconds. This procedure was then repeated for the ulnar artery.

Fig. 1. Anatomical site for vessel caliber assessment. (A) The 3D reconstructed angiography of left arm. The red line indicates the point of measurement at the wrist level, at the distal radioulnar joint. (B) At the marked point, radial (arrowhead) and ulnar (arrow) are identified and their diameter is measured.

Data were analyzed using SPSS for Windows version 23.0 (IBM Corp., Armonk, NY, USA). The Student’s t-test and Pearson’s correlation test were performed. The level of statistical significance was set at 0.05.

Ethics statement

The study was approved by the Institutional Review Board (IRB) of Inha University Hospital (IRB No. INHAUA 2023-10-022). Informed consent was waived because of the retrospective nature of the study.

RESULTS

Twenty-five patients met the inclusion criteria. All patients underwent RFF reconstruction in the head and neck region. The mean age of the patients was 59.68 years (standard deviation, 7.94; range, 44–78 years), with 21 men and four were women. In 21 cases (84.0%), the superior thyroid artery was selected as the recipient vessel. for the remaining four patients, the facial artery was used in three cases (12.0%), and the lingual artery in one case (4.0%). The primary indication for reconstruction was squamous cell carcinoma (n = 23, 92.0%). Other reasons included low-grade carcinoma of the buccal mucosa (n = 1) and traumatic facial deformity (n = 1). Seven patients (28.0%) had a history of hypertension, and an equal number had diabetes. Additionally, two patients had a different origin of malignancy, one had experienced a cerebral infarction, and one had a history of coronary artery occlusion. There was one instance of flap failure. No other flap-related complications, such as partial necrosis or venous congestion, were observed (Table 1). Patient sex was not correlated to age or underlying disease, which might affect results.

Table 1. Patient demographics and clinical characteristics.

Variables		Values
Sex
	Man	32 (78.0)
	Woman	9 (22.0)
Etiology
	Squamous cell carcinoma	38 (92.7)
	Basal cell carcinoma	1 (2.4)
	Carcinoma of buccal mucosa	1 (2.4)
	Absence of nose	1 (2.4)
Recipient vessel
	Superior thyroid artery	30 (73.2)
	Facial artery	7 (17.1)
	Lingual artery	2 (4.9)
	Branch of carotid artery	1 (2.4)
	Transverse cervical artery	1 (2.4)
Underlying disease
	Hypertension	13 (31.7)
	Diabetes	10 (24.4)
	Cerebral infarction	2 (4.9)
	Malignancy	2 (4.9)
	Coronary artery occlusion	1 (2.4)
Complications
	Flap necrosis	0 (0)
	Venous congestion	0 (0)
	Flap loss	1 (2.4)

Values are presented as number (%).

Arterial diameter measurements were obtained from CTA scans (Table 2). The radial artery was significantly larger than the ulnar artery at the point of measurement on both sides (P < 0.001) (Table 2). There was no correlation or inverse correlation between arterial diameter and time to vessel opening. Additionally, demographic factors that might be related to vessel size were analyzed. Sex, comorbidities, and age showed no demonstrate correlation with vessel size.

Table 2. Average flush-out time and diameters of the RA and UA.

Variables	Right RA	Right UA	Left RA	Left UA
Flush-out time, sec	4.25 ± 1.35	4.56 ± 1.80	4.24 ± 1.28	5.02 ± 2.44
Diameter, mm	2.15 ± 0.27a	1.87 ± 0.28a	2.07 ± 0.22b	1.90 ± 0.25b

RA = radial artery, UA = ulnar artery.

^a,bMarks indicate a significant difference.

The flushing time of the radial and ulnar arteries was measured by conventional MAT (Table 2). The mean flushing time was consistently shorter in the radial arteries of both arms, with the shortest time observed in the right radial artery (4.24 seconds) and the longest in the left ulnar artery (5.02 seconds). However, these differences did not reach statistical significance. Vessel opening time between 2 to10 seconds is considered normal range for Allen’s test. In our study, four patients exhibited a reflushing time longer than 10 seconds. Nonetheless, an abnormal Allen’s test result did not correlate with an increased risk of flap loss or necrosis. Furthermore, the size of the arteries in these cases did not differ significantly from those in patients with reperfusion times within the normal range. No abnormal vascular anatomy was observed in patients with prolonged flushing times.

DISCUSSION

Distal forearm circulation has been investigated by cardiac surgeons and interventional radiologists, as well as by reconstructive surgeons. Allen’s test and its modifications, being simple and inexpensive, are the primary means of evaluating the collateral circulation of the hand. However, the subjectivity and low sensitivity of the test have been criticized by multiple practitioners.4,5,6,7,8,9,10,11,12

With the evolution of diverse modalities, researchers have attempted to verify the results of the MAT. Jarvis et al.7 examined the collateral circulation of the ulnar artery using Doppler ultrasound to suggest a more reliable cutoff value for Allen’s test. However, they were unable to propose a value with adequate sensitivity and accuracy. At a 5 second cutoff, the diagnostic accuracy was at its highest, but the sensitivity was only 75.8%. Nuckols et al.8 employed Doppler imaging and plethysmography to evaluate digital blood flow when the radial and ulnar arteries were compressed. They argued that their method provides more conclusive, objective results compared to the conventional, subjective Allen’s test.

In this study, we analyzed the relationship between vessel diameter and the time required to restore blood flow after releasing the vessel during the MAT. Poiseuille’s equation was applied to estimate flow velocity.13 However, no significant correlation was found between these two vessel size and measured time. This implies that measuring the reperfusion time of the hand with Allen’s test does not provide an accurate assessment of the hand’s vascular dominance. Therefore, relying solely on Allen’s test to evaluate circulation of the forearm could increase the risk of ischemia and donor site morbidity.

Recent research suggests that Allen’s test is quite subjective and has a low predictive value. We recommend additional modalities other than Allen’s test for evaluating the connection between the radial and ulnar arteries in the context of invasive procedures to minimize the risk of ischemia and donor site morbidity. It has been reported that 4–10% of patients exhibit anatomical variations of the radial artery, which may include an abnormal origin, an unexpected course, tortuous configuration, hypoplasia, and stenosis.14

This study has several limitations. Data was collected in retrospective manner. Secondly, only a small number of patients were included since CTA is not a routine procedure prior to forearm based flap. Analyzing larger data would allow more power in statistical analysis. Finally, cases were limited to head and neck cancer reconstruction patient. Further study with divers etiology would provide further insights. However, Allen’s test was re-evaluated based on fluid dynamics. Measurement of vessel diameter of computed tomography scan is a simple method. Nevertheless, further data such as intraoperative flow velocity and blood volume would augment verification power. Also, multiple researches were reviewed and compared to the result of our data. As a result, the role of modalities used for evaluation of distal forearm perfusion was re-established.

In conclusion, as a simple, inexpensive method, the MAT has been widely used as an initial assessment before procedures involving the radial artery. However, the reperfusion time, measured in seconds does not accurately indicate the predominant blood supply to the distal forearm. To confirm the vascular supply to the distal forearm and identify any aberrant anatomy, modalities other than MAT, such as preoperative CTA or ultrasonography may be recommended.