Compression of the dorsalis pedis artery: a novel cause of blue toe syndrome

Introduction

Blue toe syndrome (BTS) is an uncommon condition defined as the development of painful, ‘blue or violaceous discoloration of one or more toes in the absence of trauma, cold-induced injury or generalized cyanosis.’ There are many causes of BTS documented in the literature; ¹ it is a manifestation of ischemia with capillary damage and leakage of hemosiderin into the soft tissues. In general, this is the result of decreased arterial blood flow due to mechanical obstruction secondary to thromboembolism, vasospasm, vasculitis, impaired venous return or abnormalities in the circulating blood (Box 1). Several medications are also known to cause BTS, the most common of which is warfarin, probably through cholesterol embolization. ² While many of these pathologies can be differentiated with a full history and examination, supported by simple blood tests, bedside investigations or radiological imaging, there are unusual causes of BTS which can present diagnostic uncertainty. Such causes include calciphylaxis, a syndrome of vascular calcification, thrombosis and skin necrosis seen almost exclusively in patients with end-stage renal failure. ³ By far, the most common cause of BTS, however, is peripheral embolization, the source of which may be cardiac (due to endocarditis or mural thrombus), atherosclerotic lesions, emboli from vascular aneurysms or secondary to arrhythmias such as atrial fibrillation. ⁴ We present the novel case of a patient presenting with BTS secondary to embolic disease, the source of which was found to be a stenosed dorsalis pedis artery, externally compressed by an aberrant extensor hallucis brevis tendon.

Case history

A 53-year-old man presented to our department via his general practitioner with recurrent episodes of discoloration of the right great toe. The patient described intermittent ‘blue’ discoloration of the toe with tenderness and some blistering of the skin which had previously been treated with both anti inflammatory agents and antibiotics to no effect. His past medical history included non-insulin-dependent diabetes and gastroesophageal reflux but no cardiovascular disease of note, although the patient was a smoker of 20 cigarettes per day. The remainder of his history was unremarkable and, specifically, the patient gave no history of intermittent claudication or palpitations, no symptoms suggestive of malignancy or infection and no family history of peripheral vascular disease.

On examination, the patient was found to have a cool, blue-purple great toe with discolored patches to the skin over the second to fourth toes on the right foot. The patient was in sinus rhythm. All peripheral pulses were palpable with at least biphasic signals on hand-held Doppler assessment; the ankle-brachial pressure index was 1.09 on the left and 0.95 on the right. After a normal thrombophilia screen, a presumptive diagnosis of peripheral embolization was made. The patient was admitted for heparinization and investigations. Electrocardiogram, 24-hour tape study, transthoracic echocardiogram and computed tomography of the thoracic and abdominal aorta failed to identify a definite source of emboli. An arterial duplex scan showed no evidence of peripheral arterial disease or embolic source. The patient was commenced on aspirin and Clopidogrel and discharged home.

The patient remained asymptomatic on clinical review at one year, at which point his anti-platelet therapy was discontinued. One month after stopping his year-long clopidogrel treatment (during which time he was symptom-free other than some occasional aching in the right great toe), the patient represented with identical symptoms and examination findings. Again, he was admitted and heparinized and an angiogram was arranged and undertaken by our Department of Interventional Radiology. This study (Figure 1) displayed a small peroneal artery and an unusual, apparently extrinsic, narrowing of the right dorsalis pedis artery, which it was felt could be a source of the emboli-causing symptoms. All other vessels appeared completely normal and provocative maneuvers revealed no further abnormality. A duplex ultrasound (Figure 2) confirmed significant stenosis with a soft tissue band causing extrinsic compression of the artery. As the nature of the band was not clear, dedicated pedal magnetic resonance scanning was undertaken (this had previously been avoided due to patient claustrophobia). OPEN IN VIEWER

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Figure 2

Duplex ultrasound confirming significant arterial stenosis with increased velocity ratio

This imaging modality confirmed the arterial compression and showed that the patient's extensor hallucis brevis tendon passed superficially to the dorsalis pedis artery at the level of the intermediate cuneiform bone with fascial attachment and tethering to the bone (Figure 3), corresponding to the site of narrowing on the angiogram. After discussion at our multidisciplinary meeting, the patient was referred to our orthopedic colleagues and, after review, it was felt that a tendon release procedure could be undertaken to prevent further embolic events, although currently the patient has chosen a non-operative course and has remained symptom-free on dual antiplatelet therapy. OPEN IN VIEWER

Discussion

While dorsalis pedis artery entrapment has recently been described in the literature as a cause of atypical foot claudication, ⁵ there has been no prior documentation of compression of this artery causing embolization and BTS. The dorsalis pedis artery is the continuation of the anterior tibial artery and runs across the tibial side of the dorsum of the ankle joint into the first intermetatarsal space, where it divides into two branches: the first dorsal metatarsal artery and the deep plantar artery which contributes to the plantar arch. The artery is accompanied by two veins, as well as the extensor hallucis longus tendon on the tibial side and the first tendon of extensor digitorum longus and the deep peroneal nerve on the fibular side. It is crossed near its termination by the extensor hallusis brevis tendon, and compression against the navicular bone allows palpation of the dorsalis pedis pulse, often used as an assessment of the presence or absence of peripheral vascular disease.

The extensor hallucis brevis tendon, often considered to be the medial extension of the extensor digitorum brevis, arises from the superolateral surface of the calcaneus and inserts into the dorsum of the base of proximal phalanx of the hallux, where it acts to extend the great toe. In our patient, the extensor hallucis brevis tendon crossed the dorsalis pedis arteiy more proximally than is usually described (prior to division of the vessel) and showed abnormal tethering to the intermediate cuneiform bone. As in the case described by Weichman et al., ⁵ these combined abnormalities resulted in an inability to ‘displace the artery to the plantar surface of the foot’ and subsequent compression. We believe that this compression and vessel narrowing resulted in thrombosis and microembolization to the digital arteries. No other source of peripheral embolization or possible cause of BTS was demonstrated in this patient, and although impossible to determine without doubt, we hypothesize that this aberration in foot anatomy was the likely source of emboli, resulting in impaired blood flow, discoloration and tenderness. Although a tendon release procedure would be technically possible and we believe would permanently prevent the return of symptoms, our patient has currently opted for conservative management and has remained stable on dual antiplatelet therapy. In summary, this case presents a novel but important cause of BTS for which surgical intervention in the form of tendon release may be indicated. This diagnosis should be considered when other causes of BTS have been excluded and it is hoped that future case literature will help determine the optimum management of these patients.

Declarations

Conflicts of interest: None.