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PA26.1-2 | Atherosclerosis & Aneurysms — SDL Guide (Part 4)

Berry Aneurysms and Other Aneurysm Types

A three-panel medical diagram compares berry aneurysms of the circle of Willis with hypertensive Charcot-Bouchard microaneurysms and mycotic aneurysms, including mechanisms and complications. — **Panel A:** Circle of Willis, anterior communicating artery, anterior cerebral artery, internal carotid artery bifurcation, middle cerebral artery bifurcation, berry saccular aneurysm, deficient tunica media, haemodynamic stress at bifurcation, subarachnoid haemorrhage callout. **Panel B:** Small perforating cerebral artery, lenticulostriate artery, thalamostriate artery, pons, cerebellum, lipohyalinosis, Charcot-Bouchard microaneurysm less than 1 mm, hypertensive intracerebral haemorrhage. **Panel C:** Berry aneurysm icon, hypertensive microaneurysm icon, mycotic aneurysm icon, septic embolus, infective endocarditis source, inflammatory destruction of media, common complications.

Berry (saccular cerebral) aneurysm:
• Small, rounded outpouchings at branch points of the circle of Willis (most common: junction of anterior communicating artery with anterior cerebral artery, internal carotid artery bifurcation, middle cerebral artery bifurcation)
• Pathogenesis: congenital absence/deficiency of the tunica media at arterial branch points + haemodynamic stress at bifurcations. Not atherosclerotic. Associated with: ADPKD (autosomal dominant polycystic kidney disease), connective tissue disorders, coarctation of aorta.
• Morphology: thin-walled saccular protrusion, 0.5–3 cm; contains no normal media
• Complication: subarachnoid haemorrhage (SAH) — sudden 'thunderclap' worst headache of life, neck stiffness, photophobia → high mortality and morbidity

Hypertensive (Charcot-Bouchard) microaneurysms:
• Tiny aneurysms (<1 mm) in small perforating arteries of the brain (lenticulostriate, thalamostriate, pons, cerebellum)
• Cause: chronic hypertension → lipohyalinosis of vessel wall → small outpouchings
• Consequence: hypertensive intracerebral haemorrhage (most common cause of ICH)

Mycotic aneurysms:
• Caused by septic emboli (infective endocarditis, bacteraemia) lodging in vasa vasorum or directly infecting the arterial wall → focal inflammatory destruction of the media
• Can affect any artery; often multiple

Summary table — aneurysm types:

Type	Location	Aetiology	Shape	Complication
Atherosclerotic	Infrarenal aorta	Atherosclerosis, male>65	Fusiform	Rupture, embolism
Dissecting	Ascending aorta	Cystic medial degen, HTN, Marfan	—	Tamponade, AR, stroke
Syphilitic	Thoracic aorta	Vasa vasorum endarteritis	Saccular	AR, coronary ostial stenosis
Berry	Circle of Willis	Congenital media defect	Saccular	SAH
Charcot-Bouchard	Brain perforators	Hypertension	Micro	ICH
Mycotic	Any	Septic embolus	Variable	Rupture, sepsis

SELF-CHECK

A 38-year-old tall, thin man with arachnodactyly and lens dislocation is found on echocardiography to have a dilated aortic root (4.8 cm). Which underlying pathological process in the aortic wall BEST explains his risk for aortic dissection?

A. Atherosclerotic plaque causing medial ischaemia via vasa vasorum compression

B. Endarteritis obliterans of the vasa vasorum secondary to chronic infection

C. Cystic medial degeneration with loss of smooth muscle cells and elastic fibre fragmentation

D. Hyaline arteriolosclerosis of the aortic wall secondary to hypertension

Reveal Answer

Answer: C. Cystic medial degeneration with loss of smooth muscle cells and elastic fibre fragmentation

Marfan syndrome (FBN1 mutation → defective fibrillin-1) causes cystic medial degeneration (necrosis) — the hallmark histological lesion of the aortic media in Marfan patients. This manifests as loss of medial smooth muscle cells, fragmentation and loss of elastic fibres, and replacement by myxoid/mucoid ground substance. The weakened media cannot resist haemodynamic forces → progressive aortic root dilation → Type A dissection risk. Option A describes atherosclerotic AAA mechanism; option B is syphilitic aortitis; option D is arteriolosclerosis in hypertension (affects small vessels, not the aorta).