Treatment of Midaortic Syndrome
James C. Stanley
The midaortic syndrome, as characterized by abdominal aortic coarctation or hypoplasia, is an uncommon disease. Although the midaortic syndrome has been the subject of earlier reviews, very few individual or institutional experiences comprise more than 10 patients.
Pathogenesis
The aortic narrowings or stenoses associated with the midaortic syndrome result from either a developmental fault or an inflammatory aortoarteritis. The narrowings may be either focal or diffuse. Interrenal coarctations are the most common variant, affecting 52% of patients, with the balance comprised of infrarenal coarctations (25%), diffuse aortic hypoplasia limited to the abdominal aorta (12%), and suprarenal coarctations (11%). The narrowed aortas that result from developmental faults or defects represent diminutive vessels that often have an hourglass shape in the regions of focal coarctation. These developmental narrowings usually exhibit marked subendothelial fibroplasia with increased basophilic ground substance in the media without evidence of acute or chronic inflammation. The stenoses that result from inflammatory aortoarteritis exhibit adventitial or periadventitial fibrosis and an associated inflammatory cell infiltrate, suggesting an active or chronic process. This inflammatory etiology is likely responsible for only the minority of the midaortic syndromes and usually occurs in patients with a variant of Takayasu disease.
Developmental abdominal aortic coarctations appear related to events occurring around the 25th day of fetal growth. At that time, the two dorsal aortas migrate toward each other, fuse, and subsequently lose their intervening wall, thereby forming a single vessel. An abnormal fusion or overfusion of the two embryonic dorsal aortas is supported by studies demonstrating decreased aortic diameters among older patients having single origins for the lowest pair of lumbar arteries. Multiple renal arteries, either unilateral or bilateral, are seen in more than 70% of patients exhibiting abdominal aortic narrowings, and this observation lends further support to a developmental etiology. The normal fusion of the two dorsal aortas occurs at approximately the same embryonic time that the multiple lateral branches to the metanephros usually disappear (thereby leaving a single renal artery in 65% to 75% of individuals). The persistence of this single renal artery has been attributed to its hemodynamic advantage over the adjacent metanephric vessels. It can be hypothesized that if an aortic narrowing exists the flow disturbances in the vicinity of this principal renal artery diminish its hemodynamic advantage, thereby allowing the adjacent metanephric channels to persist.
The cellular and molecular events that contribute to the developmental aortic faults may be viral mediated or related to the same disorder as seen in patients with neurofibromatosis. Viral-mediated events may impede transition of the fetal mesenchymal tissue to vascular smooth muscle or alter the organization and growth of this smooth muscle. This may impair development of the dorsal aortas in utero or the fused aorta during early infancy, thereby resulting in aortic narrowing. Support for this hypothesis is provided by the fact that certain viruses, including rubella, are cytocidal and inhibitory to cell replication, and that aortic hypoplasia has been observed in patients with gestational rubella. Patients with neurofibromatosis exhibit an unusually high frequency of arterial abnormalities, including abdominal aortic coarctations and renal artery stenoses. The primary vascular pathology in neurofibromatosis appears to be related to abnormal medial smooth muscle rather than entrapment or invasion of the arterial wall by neural elements. The responsible mechanism for aortic narrowings in neurofibromatosis remains unknown but is likely related to faulty vessel growth.
Panaortitis, as demonstrated by adventitial/peri-adventitial fibrosis and associated inflammatory cell infiltrates, is an uncommon cause of abdominal aortic coarctations. It has been proposed that these inflammatory associated narrowings represent a variant of Takayasu disease. However, this hypothesis is quite controversial and not supported by histologic findings nor the observation that most patients with Takayasu disease do not have multiple renal arteries (in contrast to those patients with a noninflammatory midaortic syndrome).
Diagnostic Considerations
The clinical sequelae associated with the developmental narrowings of the abdominal aorta generally become evident during the first or second decades of life. The classic clinical triad consists of severe hypertension, diminished or absent femoral pulses, and an abdominal bruit. Lower extremity claudication occurs in approximately 25% of these cases. There is no apparent gender predilection among patients
with the developmental etiology, in contrast to the male predominance for patients with thoracic isthmic coarctations and the female predominance for patients with inflammatory aortic stenoses. Arteriography has been essential for confirming the presence of this entity and for identifying coexisting visceral artery lesions. Direct catheter-based pressure measurements and/or noninvasive Doppler arterial studies have been used to determine the hemodynamic significance of the aortic disease.
with the developmental etiology, in contrast to the male predominance for patients with thoracic isthmic coarctations and the female predominance for patients with inflammatory aortic stenoses. Arteriography has been essential for confirming the presence of this entity and for identifying coexisting visceral artery lesions. Direct catheter-based pressure measurements and/or noninvasive Doppler arterial studies have been used to determine the hemodynamic significance of the aortic disease.
Indications and Contraindications
The prognosis for untreated patients with the midaortic syndrome is poor, and most patients die in early adulthood from cardiac failure or cerebrovascular accidents. In one review, 55% of untreated patients died at a mean age of 34 years. Thus, all hypertensive patients with coarctation or segmental hypoplasia of the distal thoracic/upper abdominal aorta must be considered at risk for serious complications of their disease and merit treatment.
Operative Technique
Endovascular Treatment
The underlying disease processes for both the developmental and inflammatory etiologies have limited the success of percutaneous transluminal angioplasty for patients with aortic coarctations. Balloon dilation of the stenoses resulting from the developmental causes is often accompanied by transient stretching of the vessel and immediate recoil when the balloon is deflated. This is likely due to the fact that the vessel wall contains an excess of elastic tissue. On occasion, overdilation of these diminutive vessels will lead to disruption. Similarly, the transmural fibrotic changes associated with the inflammatory lesions are not usually successfully treated with balloon dilation. Accordingly, experienced endovascular therapists have not recommend percutaneous angioplasty and/or stenting for the lesions responsible for the midaortic syndrome.
Open, Surgical Treatment
Thoracoabdominal aortoaortic bypass and patch aortoplasty, with concomitant renal and splanchnic revascularization as needed, have become the standard treatments. These procedures require extensive exposure of the aorta and its visceral branches. In most patients with proximal abdominal aortic disease, exposure is facilitated by a thoracoabdominal incision through the left sixth or seventh intercostal space extending from the posterior axillary line across the costal margin onto the abdomen. The abdominal component of the incision can be extended in an oblique fashion to the right of the umbilicus or along the midline to just above the pubis. The distal descending thoracic aorta is exposed following a circumferential incision along the periphery of the left hemidiaphragm. This is favored over a radial incision through the central tendinous portion of the diaphragm because it preserves the phrenic innervation. Extraperitoneal medial reflection of the abdominal viscera following incision of the lateral parietes adjacent to the left colon provides generous access to the proximal abdominal aorta and its visceral branches. For lower and midabdominal aortic disease, a transverse supraumbilical abdominal incision extended bilaterally to the posterior axillary lines in combination with medial rotation of the viscera is preferred to a midline incision with a transmesenteric approach.
Thoracoabdominal bypass in conjunction with renal and/or visceral revascularization has been the most common operative treatment in the past (Fig. 48-1
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