Diagnosis and Reasoning
This middle aged gentleman has presented with an interesting mix of symptoms: left sided tinnitus and unsteadiness, suggesting at a neurological deficit; and numbness in the right upper arm, and pain suggestive of arm claudication, hinting at vascular compromise.
The examination reveals several alarming signs: a colder right hand, diminished right-sided radial and brachial pulses, a significant discrepancy in systolic blood pressures between both arms, and a bruit at the root of the neck; these confirm that significant vascular compromise of the right arm is present.
Furthermore, note the deviation to the right in the Unterberger stepping test. In this test, the patient is asked to undertake a few stationary steps with their eyes closed; if the patient rotates towards one side or the other (rather than staying in the same place), this is suggestive of a labyrinthine lesion on that side.
A diagnosis which unifies these disparate findings would be a vascular pathology that compromises both the posterior circulation of the brain and the circulation of the right upper extremity.
In turn, the possibility which immediately springs to mind is subclavian steal syndrome (SSS), a rare condition where occlusion of the subclavian artery proximal to the vertebral arterial origin gives rise to reversal of blood flow in the vertebral artery, compromising the vertebrobasilar circulation, and that of the ipsilateral upper extremity.
It should be appreciated that chronic aortic dissection involving the proximal aorta can also potentially give rise to similar vascular and neurological symptoms; as opposed to acute dissection, chest pain may be minimal or absent.
Note however that a one year duration of dissection would be rather unusual, but is not unheard of.
Carotid Duplex Ultrasonography is a sensitive and non-invasive screening test for SSS, and is a good initial investigation. This demonstrates retrograde flow through the right vertebral artery, suggesting that SSS is indeed present.
As proximal vertebral artery stenosis or occlusion can also give rise to this sonographic finding, confirmation via computerized tomography angiography (CTA) or magnetic resonance angiography (MRA) should be strongly considered.
In turn, this patient's CT angiogram confirms the presence of SSS, clinching the diagnosis; furthermore the proximal aorta appears structurally normal, definitively ruling out dissection.
It is important to appreciate that SSS can give rise both cerebral ischemia and infarction, although the latter is fortunately rare. Give the persistent nature of his neurological symptoms, it is thus justifiable to order an MRI of the brain; this in turn confirms the presence of multiple ischemic lesions.
Note that this patient needs further evaluation as to the underlying cause of SSS; in this respect, extraluminal causes (such as a cervical rib or a large compressive aneurysm) are ruled out by the CT films.
Thinking further, the family history of coronary vascular disease makes atherosclerosis a likely possibility. Note that the presence of an otherwise pristine extracranial vascular tree does not exclude this etiology.
Coronary angiography is not indicated, as there is no history of angina or other cardiac symptoms,.
Percutaneous Transluminal Angioplasty (PTA) is the preferred method of treatment in these patients; subclavian endarterectomy is not performed anymore, and has largely been replaced by extrathoracic bypass surgeries.
Propranolol would have been indicated if thoracic aortic dissection were present. Corticosteroids are the first line treatment of Takayasu arteritis; note that while this possibility is yet to be definitively excluded, this patient's age, gender and angiographic findings argue against it.
Subclavian Steal Syndrome (SSS) is a rare phenomenon where stenosis or occlusion of the subclavian artery proximal to the origin of the vertebral artery results in retrograde blood flow in the ipsilateral vertebral artery.
To understand this phenomenon, one needs to appreciate that in the human body, blood flows in the path of least resistance; furthermore, according to Poiseuille's Law, resistance offered to blood flow is inversely proportionate to the width of the vessel.
Therefore, whenever a stenosis or an occlusion is encountered, blood attempts to sidestep that route and take another path which is wider and offers less resistance, disregarding the length of the latter vessel.
Thus, in individuals with SSS, blood is driven away from the occluded/stenosed proximal subclavian artery, travels up to the brain, circulates through the arterial circle, and then descends via the ipsilateral vertebral artery into the distal subclavian artery which supplies the upper limb and shoulder.
The term "steal" refers to this redirection of blood into the distal subclavian artery at the expense of the vertebrobasilar circulation.
SSS is usually a benign condition and most diagnosed patients are asymptomatic.
When symptoms do occur, they manifest as features of cerebral, vertebrobasilar, and/or brainstem ischemia; these include visual loss (which may range from total blindness to unilateral visual field loss or amaurosis fugax), vertigo, syncope, diplopia, dysarthria, ataxia, dysphagia, pulsatile tinnitus, numbness or tingling of the face, transient hemiparesis, or sensory hemianesthesia.
Atherosclerosis is the single most common etiology for SSS, accounting for more than 75% of cases; therefore the risk factors for the condition are the same as those for coronary artery disease in general and include smoking, hypertension, diabetes mellitus, hyperlipidemia, and a family history of vascular disease.
Note that it is rare for atherosclerotic plaques to habitat the subclavian artery; this explains the rarity of the disease and its predominance in elderly males.
In contrast, there is a subgroup of young patients with SSS, largely consisting of females of Asian origin, in whom the underlying causative agent is Takayasu arteritis.
Thrombosis of the subclavian artery due to an underlying coagulopathy and extrinsic compression (such as in thoracic outlet syndrome) are other, rare risk factors for SSS.
Usually, SSS is a clinical diagnosis which can be arrived at via a detailed history and thorough physical examination.
However, even in the presence of the aforementioned supportive signs and symptoms, other causes for symptomatology including aortic dissection, and cerebral or cerebellar pathology should always be excluded before a diagnosis of SSS is made.
A pivotal finding in these patients is a discrepancy in the bilateral assessment of pulse and blood pressure.
Differences of up to 50 mmHg in systolic blood pressures between the brachial arteries are seen in patients with predominant arm ischemia; slightly lower values are seen in individuals with predominant cerebral symptoms.
Auscultation may also reveal a carotid or a suprascapular bruit.
Once the clinical possibility of SSS is suspected, diagnostic imaging aiming to demonstrate reversal of flow through the ipsilateral vertebral artery is the next step.
Duplex ultrasonography with color flow is non-invasive and safe, and is widely used as an initial screening tool; in many individuals, this is capable of demonstrating retrograde blood flow in the vertebral artery.
However, note that not all retrograde vertebral flow seen on ultrasound examinations constitutes true subclavian steal. Some have been shown to stem from proximal vertebral artery stenosis or occlusion.
Thus, confirmation via computed tomographic angiography (CTA) or magnetic resonance angiography (MRA) should be considered; both permit excellent anatomical resolution and potential assessment of not only the subclavian artery but the other supra-aortic vessels as well.
Note that digital subtraction angiography (DSA) is considered the "gold standard" study to diagnose SSS and other pathologies involving the vertebral artery; endovascular treatment may be attempted at the same time.
The primary goal in the treatment of SSS is to reestablish anterograde blood flow in the affected vertebral artery and thus alleviate the vertebrobasilar symptoms.
Traditionally, this task has been achieved by vascular surgeries including, carotid-subclavian bypass, carotid-subclavian transposition, axillo-axillary bypass or endarterectomy.
However, percutaneous transluminal angioplasty (PTA) has widely gained popularity since it was first introduced a little more than three decades ago, and in many units is now the preferred method of treatment.
The added benefits of PTA include a significant reduction in mortality and morbidity and the ability to perform the procedure in a day case setting under local anesthesia.
However, it should be noted that these advantages of PTA come at the expense of long term patency rates.
Stenting and atherectomy of the subclavian artery are the latest additions to the pool of treatment options; the initial results so far have shown to been promising.
Since the primary pathology in most cases of SSS is atherosclerosis, educating patients regarding lifestyle and dietary modifications is of paramount importance.
Take home messages
1. Subclavian Steal Syndrome (SSS) is a rare condition where reversal of blood flow in the ipsilateral vertebral artery occurs due to an obstruction of the subclavian artery proximal to the vertebral origin.
2. Most patients with SSS are asymptomatic; where symptoms do occur they are commonly neurological, or less commonly involve the upper extremity.
3. Percutaneous transluminal angioplasty (PTA) has largely replaced traditional surgery in the recent years as the preferred method of treatment for SSS.
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