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Vertebral Artery Dissection (VAD)

Vertebral Artery Dissection Definition

Vertebral artery dissection (VAD) is a condition where blood collects within the walls of the vertebral artery and hampers blood flow through it. Since the vertebral arteries carry oxygen rich blood to the brain, most symptoms are a result of reduced blood flow and oxygen supply to the brain. It can even lead to a stroke. Vertebral artery dissection is one of the most important causes of a stroke in people younger than 45 years of age. Most cases arise with injury, where trauma to the vessel wall compromises its integrity and allows blood to collect within it. Sometimes however, this can arise spontaneously with no previous history of head or neck trauma.

Vertebral Artery Dissection Incidence

Vertebral artery dissection is overall quite rare in the United States. Together with a similar condition affecting the neck artery (carotid artery), it only occurs in 2.6 out of every 100,000 people. Carotid artery dissection (CAD) is much more common than vertebral artery dissection (VAD). Collectively CAD and VAD are referred to as cervical artery dissection.

Women have a 3 times greater risk of verterbral artery dissection than men. Whereas gradual narrowing and a sudden blockage with a clot (artherothrombotic) is the main cause of strokes in the elderly, vertebral artery dissection is the main reason for people under 45 years of age with an average age of incidence being 40 years. The death rate is about 10% in vertebral artery dissection cases during the acute phase.

Vertebral Artery Anatomy

In order to understand diseases that affect the vertebral artery, it is important to have at least a basic knowledge of the anatomy of the vertebral artery.

Origin of the vertebral arteries

The aorta is the largest artery in the body. It emerges from the left side of the heart carrying oxygen rich blood to the entire body. Two branches that arise from the arch of the aorta is the subclavian arteries – one right and one left. These subclavian arteries supply oxygen rich blood to much of the head and neck along with the carotid arteries. Around the level of sixth and seventh cervical vertebrae (C6 and C7), the vertebral artery arises from the subclavian artery.

Course of the vertebral arteries

The vertebral arteries (one on each side) then run through small passages in the vertebral bones. These passages are known as the transverse formina. Essentially the vertebral arteries run up the back of the neck in much the same way as the carotid arteries run up the front of the neck. The vertebral arteries eventually climb up to the back of the skull. Both arteries enter the skull cavity through a small hole known as the foramen magnum.

The two vertebral arteries then merge inside the skull to form a single basilar artery. It is this basilar artery that supplies oxygen rich blood to the brain stem and then parts of the brain by joining with the Circle of Willis. Therefore any disruption in the flow of blood through the vertebral arteries will affect the brain stem and also the brain to some extent, although the brain is largely supplied by the carotid arteries.

Vertebral Artery Dissection Location

Based on its course, the vertebral artery can be divided into four different segments.

  • Segment I – from point of origin to C5/C6 vertebrae
  • Segment II – from C5/C6 to C2 vertebra
  • Segment III – from C2 to the foramen magnum
  • Segment IV – from the point of the foramen magnum till it merges with the other vertebral artery to form the basilar artery.

The most frequent location of a vertebral artery dissection is in segment IV, particularly with trauma. In a spontaneous dissection, the most commonly affected site is segment III although it may extend to segment IV.

Vertebral Artery Dissection Pathophysiology

Arteries have thick walls to withstand the high pressure of blood rushing out of the heart. The arteries are both elastic to stretch as necessary and have muscles in its walls that can contract or relax to alter the amount of blood flowing through it. In order to cater for these different properties, arteries have several different layers. The innermost lining is the tunica intima. The middle layer is the tunica media and the outermost layer is the tunica adventitia.

Normally all layers of the artery are intact. There are no kinks (sudden narrowings) or protrusions from the artery walls. Blood flows rapidly and smoothly within the artery. A dissection may arise when :

  • the inner lining of the artery wall is damaged thereby forming a flap and allowing blood to enter the deeper layers in the wall.
  • the tiny blood vessels within the artery wall (vaso vasorum) ruptures and blood collects within the layers of the artery wall.

A collection of blood is known as a hematoma. It is a space-occupying lesion. A very small hematoma can cause no symptoms. However, when it is large then there are several possible consequences such as :

  • Causing a partial or complete obstruction of the blood flow within the vertebral artery.
  • Bulging of the vertebral artery wall (pseudoaneurysm) which can then press against the surrounding nerves and brain tissue.
  • Allow for blood clots to form at the site (thrombus) which can then break off (embolus) and obstruct the artery further on.

Vertebral Artery Dissection Causes

A vertebral artery dissection can be classified into traumatic or spontaneous.

Traumatic Dissection

In this instance the dissection arises with trauma. The injury, either blunt force or sharp (penetrating), may be isolated to the head or neck or affect several site sin the body. It is most commonly seen with :

  • Motor vehicle accidents
  • Assault
  • Contact sports
  • Yoga and other activities involving unusual postures
  • Spinal manipulation
  • Spinal cord injury
  • Prolonged hyperextension of the neck, like looking up for long periods when painting a ceiling.
  • Severe respiratory tract infections with bouts of violent coughing

Spontaneous Dissection

A spontaneous dissection is a category for any verterbral artery dissection that arises without previous trauma. In most instances there is some inherent defect that has led to weakening of the wall. Sometimes it is minor trauma that has occurred in the past but remains unnoticed for weeks or months. It is rare for a spontaneous dissection to occur with no predisposing cause. A careful medical history will often reveal a cause or risk factor.

  • Hypertension (high blood pressure)
  • History of chronic headaches and migraines
  • Use of oral contraceptives
  • Diseases of the blood vessels and the arteries
  • Fibromuscular dysplasia

Being female is a major risk factor, and women are three times more likely to suffer with a vertebral artery dissection.

Vertebral Artery Dissection Symptoms

The symptoms largely depend on which part of the brain or brain stem is affected by the reduced blood flow or to some extent the compression of the enlarged vessel on the nerve tissue. The major symptoms includes :

  • Head pain which is typically described as dull or pressure or a throbbing headache
  • Vertigo
  • Poor balance and equilibrium

Other symptoms may not always be reported by patients at the outset or if it is mild. There may be a latent period of 3 days where the first symptoms like head pain starts with other symptoms only occurring after 3 days.

  • Facial pain and numbness
  • Loss of taste
  • Tongue deviation
  • Difficulty swallowing
  • Slurred speech and/or hoarse voice
  • Hearing loss (unilateral)
  • Hiccups
  • Double vision
  • Movement of objects in the visual field with head movement (oscillopsia)
  • Loss of sensation (pain and temperature) on one side of the trunk and limbs
  • One-sided weakness or paralysis of the limbs (less common)

Vertebral Artery Dissection Diagnosis

Reporting of symptoms and the presence of signs mentioned above along with a medical history indicating a possible cause or risk factor should raise the suspicion of a vertebral artery dissection. This is more significant in a person younger than 45 years of age, where stroke-like symptoms are evident. However, further investigations are usually warranted such as :

  • Cerebral angiography
  • Computed tomography (CT) scan
  • Magnetic resonance imaging (MRI) and angiography (MRA)
  • Vascular duplex scanning
  • Transcranial Doppler

Cerebral Angiography

This is considered the gold standard for vertebral artery dissection which means it conclusively diagnoses the condition and can identify the affected site. It involves inserting a catheter into a large artery and positioning it in the aorta. Radiocontrast dye is then injected. Through fluoroscopy, the blood flow through the vertebral arteries can then be visualized on x-ray images.

Vertebral Artery Dissection Treatment

Depending on the symptoms present, the patient needs to be rushed to the emergency room or the stroke department. The first measure is to stabilize the patient. Treatment focuses on clearing the clot and preventing new clot formation with the use of medication. High blood pressure is not reduced when found at the time of admission as the higher than normal pressure allows some blood to pass through an occlusion and nourish the brain tissue.

Medication

Medication to treat the blood clots include :

  • Anticoagulants to prevent new clots from forming.
  • Antiplatelet drugs that have a similar effect as anticoagulants and are used for when anticoagulants are not well tolerated.
  • Thrombolytics, also known as ‘clot busters’, to break down existing blood clots.

Vertebral Artery Dissection Prognosis

The outlook is much more positive if the dissection occurs in the parts of the vertebral artery outside the skull (extracranial). Close to 90% of these patients will have a complete recovery but are at risk of strokes at a later stage. When the part of the vertebral artery within the brain is affected then the outlook is poorer. There is a greater likelihood of complications and a higher chance of death.

References :

http://emedicine.medscape.com/article/761451-overview

http://radiographics.rsna.org/content/20/6/1687.full

http://www.bupa.co.uk/individuals/health-information/directory/c/cervical-artery-dissection