Undiagnosed bilateral complete cervical rib with subclavian artery aneurysm presenting as acute ischaemic limb following high-altitude expedition

Discussion

Spontaneous arterial thrombosis, although less common than venous, has been reported after an ascent to HA.1 However, thrombosis in upper limbs is six times less common than lower limbs.2 HA-associated hypercoagulability presents in two ways. There is an initial transient phase of hypercoagulability, which lasts about a week. A second delayed phase of hypercoagulability is seen in people staying at the HA for prolonged periods. It usually peaks at around 5 months of stay and stays throughout the period of HA stay.3 In a study of Indian Army troops exposed to HAs for prolonged periods of time, one case of peripheral arterial thrombosis has been described. A 30 times higher risk of spontaneous vascular thrombosis was reported among young male soldiers following mean stay of over 10 months at altitudes from 3000 m to 6500 m (extreme HA). Manifestations varied from deep vein thrombosis and pulmonary thromboembolism to stroke, mesenteric, splenic, portal vein thrombosis, retinal artery and peripheral arterial thrombosis.4

The mechanism by which hypoxia contributes to thrombophilia remains unclear. It is probably the result of a complex set of interaction between different steps in the coagulation cascade. Patient factors such as cold and exercise influence the haematologic responses during ongoing hypoxic exposure. Studies have identified increases in red blood cell and platelet counts and in various coagulation factors, as well as reductions in natural thrombolysis in humans and animals exposed to hypoxia, all of which may contribute to thrombosis.1

Bilateral, complete cervical rib is an extremely rare anomaly. An analysis of multiple studies shows that the prevalence of cervical ribs ranges from 0% to 3% in the population. About 30% of cervical ribs are complete ribs fused to the first rib by a true joint or a fibrous attachment.5 In a study conducted in India, the incidence of bilateral cervical rib is reported to be 0.44%.6 Arterial thoracic outlet syndrome (TOS), a rare occurrence (<1% of all types of TOS), represents subclavian artery thrombosis or aneurysm formation within the thoracic outlet.7 Subclavian artery aneurysm is a very rare complication of cervical ribs.8 Most of them are either asymptomatic or present with symptoms of claudication owing to thrombosis. Rarely do they embolise to distal cerebral and limb circulation.9

Although arterial TOS has a characteristic presentation, other causes of embolus viz. Cardiac should be ruled out. Cardiac conditions such as mitral valve disease and atrial fibrillation are common causes of embolus to the upper limb. Other causes of upper limb ischaemia such as Takayasu’s arteritis, various vasculitis and connective tissue disorders and atherosclerotic disease along with dissections should be considered as differentials. In our case, exposure to cold and hypoxia associated with HA was an additional risk factor for spontaneous thrombosis.

Decision to repair an artery after decompression and excision of cervical rib depends on various factors. Symptomatic patients with subclavian artery aneurysms tend to have recurrent thromboembolic complications that may lead to digit or limb loss. However, arterial repair may be indicated in asymptomatic patients with subclavian artery thrombosis due to reported risk of retrograde propagation with resultant stroke.10

The Scher staging classification of TOS complications can act as a guide to direct the type of surgical intervention needed.11 Although regression of poststenotic dilatation after TOS decompression has been reported, presence of any sign of intimal damage such as mural thrombus or embolism, and aneurysmal change warrants surgical reconstruction or repair. The supraclavicular approach is best suited for vascular reconstruction along with excision of cervical rib. A large aneurysm or a more distal aneurysm way requires an infraclavicular approach. Except a few rare scenarios, most cases of subclavian artery repair need a conduit for replacement. The commonly used conduits are: great saphenous vein, femoral vein, ringed Poly Tetra Fluoro Ethylene (PTFE) or Dacron conduits. The 5-year patency rates vary from 90% to 100%. Operative morbidity is less frequent and mostly involves brachial plexus injury and complications related to pleural entry.