Anastomoses are connections between blood vessels that provide alternative (collateral) pathways for blood flow. They are critical for maintaining tissue perfusion when primary vessels are occluded.
Definition
An anastomosis is a communication between two blood vessels, either naturally occurring or surgically created.
Collateral circulation: The network of anastomotic channels that can enlarge to supply blood to an area when the primary vessel is occluded.
Types of Anastomoses
Arterial Anastomoses
| Type | Description | Example |
|---|---|---|
| End-to-end | Two arteries join directly | Palmar arches (superficial and deep) |
| End-to-side | One artery joins the side of another | Anterior spinal artery receiving segmental feeders |
| Side-to-side | Adjacent arteries connect via branches | Coronary collaterals |
| Arcades | Series of connected arches | Mesenteric arcades (intestinal arteries) |
| Network (rete) | Complex interweaving network | Circle of Willis |
Venous Anastomoses
| Type | Description | Example |
|---|---|---|
| Superficial-deep | Perforating veins connect superficial and deep systems | Lower extremity perforators |
| Portal-systemic | Connections between portal and systemic veins | Esophageal, rectal, umbilical |
| Intracranial | Connections between cerebral veins | Veins of Troland, Labbe |
Arteriovenous Anastomoses
Direct connections between arteries and veins, bypassing capillaries:
- Glomus bodies: In fingertips, involved in temperature regulation
- Aggregate vessels: In the skin for thermoregulation
- Shunts: In the kidney (juxtamedullary)
True vs. Functional Anastomoses
| Type | Description | Example |
|---|---|---|
| True anastomosis | Direct connection between vessel lumens | Palmar arch |
| Functional anastomosis | End-artery with no anastomosis | Renal artery (functional end artery) |
| Potential anastomosis | Preexisting but not usually functioning | Coronary collaterals |
Collateral Circulation
Development of Collaterals
Preexisting collaterals:
- Present from birth
- Small diameter (< 100 microns)
- Minimal flow under normal conditions
Collateral remodeling (arteriogenesis):
- Triggered by pressure gradient across the anastomosis
- Increased shear stress activates endothelium
- Monocyte recruitment and matrix remodeling
- Smooth muscle proliferation (20-fold diameter increase)
- Functional collateral within 2-4 weeks
Factors Affecting Collateral Development
| Positive Factors | Negative Factors |
|---|---|
| Chronic ischemia | Diabetes mellitus |
| Gradual occlusion | Advanced age |
| Exercise training | Smoking |
| Normal endothelial function | Hyperlipidemia |
| Younger age | Hypertension |
Collateral Flow Grades (Rentrop)
| Grade | Description |
|---|---|
| 0 | No visible collateral filling |
| 1 | Faint filling of side branches only |
| 2 | Partial filling of the epicardial vessel |
| 3 | Complete filling of the epicardial vessel |
Key Anastomotic Networks
Circle of Willis
The most important anastomotic network in the brain:
Component arteries:
- Anterior communicating artery
- Anterior cerebral arteries (A1 segments)
- Internal carotid arteries
- Posterior communicating arteries
- Posterior cerebral arteries (P1 segments)
Function:
- Collateral flow from one side of the brain to the other
- Collateral flow from anterior to posterior circulation
- Protects against ischemic stroke
Anatomic variations:
- Complete circle: Only 20-30% of individuals
- Hypoplastic posterior communicating: 25-30%
- Absent anterior communicating: 1-2%
Palmar Arches
Superficial palmar arch:
- Formed by the ulnar artery (dominant)
- Completed by the superficial palmar branch of the radial artery
- Supplies the fingers via digital arteries
Deep palmar arch:
- Formed by the radial artery (dominant)
- Completed by the deep branch of the ulnar artery
- Supplies the metacarpals and thumb
Clinical test: Allen test assesses patency of the arches
Mesenteric Arcades
| Level | Arches | Location |
|---|---|---|
| Primary | 1 arch | Near the mesenteric border |
| Secondary | 2-3 arches | Mid-mesentery |
| Tertiary | 3-5 arches | Near the intestinal wall |
Straight arteries (vasa recta):
- Terminal branches from the arcades to the intestinal wall
- Functional end arteries (anastomoses are minimal within the wall)
Lower Extremity Anastomoses
| Location | Anastomotic Network |
|---|---|
| Hip | Cruciform anastomosis (medial/lateral circumflex, inferior/superior gluteal) |
| Knee | Genicular anastomosis (descending genicular, superior/inferior genicular) |
| Ankle | Malleolar anastomosis (anterior/posterior tibial, peroneal) |
| Foot | Dorsal and plantar arches |
Coronary Collaterals
- Septal collaterals: Between LAD and PDA septal branches
- Epicardial collaterals: Between coronary artery territories
- Apical collaterals: LAD to PDA at the apex
- Vieussens ring: Conus branch of RCA to LAD
Portosystemic Anastomoses
Connections between the portal venous system and the systemic venous system:
| Location | Portal Vein | Systemic Vein | Clinical Effect |
|---|---|---|---|
| Esophagus | Left gastric vein | Esophageal veins (azygos) | Esophageal varices |
| Rectum | Superior rectal vein | Middle/inferior rectal veins | Hemorrhoids |
| Umbilicus | Paraumbilical veins | Epigastric veins | Caput medusae |
| Retroperitoneal | Splenic/mesenteric veins | Lumbar/renal veins | Retroperitoneal collaterals |
Azygos-Hemiazygos Collaterals
Anastomoses between the azygos system and the SVC/IVC provide collateral flow in SVC syndrome:
- Hemiazygos to azygos connection
- Lumbar veins to azygos
- Intercostal veins
Functional End Arteries
Vessels that lack sufficient anastomoses to maintain tissue viability after occlusion:
| Organ | End Artery |
|---|---|
| Heart | Most intramyocardial branches |
| Kidney | Renal lobar arteries |
| Brain | Lenticulostriate arteries (basal ganglia) |
| Spleen | Splenic trabecular arteries |
| Lung | Bronchial arteries (peripheral) |
| Intestine | Vasa recta (within the wall) |
| Eye | Central retinal artery |
| Uterus | Arcuate arteries (inner myometrium) |
Clinical Significance
Ischemic Protection
Well-developed collaterals protect against:
- Stroke: Patients with good circle of Willis collaterals have smaller infarcts
- Myocardial infarction: Collaterals limit infarct size by 30-50%
- Peripheral artery disease: Collaterals may prevent limb loss
Critical Stenosis
Collaterals become functional when a primary stenosis exceeds:
- 50-70% diameter reduction
- Resting flow maintained by distal vasodilation
- Maximum flow limited beyond 80-90% stenosis
Therapeutic Collateral Enhancement
Experimental approaches:
- Growth factor therapy: VEGF, FGF
- Gene therapy: HIF-1 alpha
- Cell therapy: Endothelial progenitor cells
- External counterpulsation: EECP
- Exercise training: Enhances natural collaterals
Surgical Anastomoses
| Type | Description | Use |
|---|---|---|
| End-to-end | Direct connection | Bowel, vessel repair |
| End-to-side | One vessel to side of another | Coronary bypass |
| Side-to-side | Lateral connections | Arteriovenous fistula for dialysis |
| Patch | Graft expanded by patch | Vessel reconstruction |
Anastomotic Complications
| Complication | Cause | Prevention |
|---|---|---|
| Leak | Incomplete sealing | Fine technique, tissue sealants |
| Stenosis | Neointimal hyperplasia | Large anastomosis, drug-eluting techniques |
| Thrombosis | Technical error, hypercoagulability | Anticoagulation, improved flow |
| Pseudoaneurysm | Infection, tension | Tension-free technique |