Types of Fibromuscular Dysplasia (FMD):
- Medial: Over 90% of FMD are characterized by medial fibroplasia, which is replacement of smooth muscle tissue with myofibroblasts that lay down disorganized collagen deposition. In between thick medial fibroplasia are interval thinning of aneurysmal degeneration due to smooth muscle loss. It produces the characteristic “string of beads” angiographic finding.
- Intimal: only about 1-2%, is characterized by tubular stenosis, rather than “string of beads”.
- Adventitial: <1%. Adventitia is infiltrated by lymphocytes and collagen, characterized by focal stenosis.
Association with intracranial aneurysms
FMD, regardless of involved vascular territory, is reportedly to have 12.9% incidence of at least one intracranial aneurysm. As such, all patients who have been diagnosed with FMD should undergo brain imaging to screen for brain aneurysms.
Renal fibromuscular Dysplasia (FMD)
- Most common form of FMD in some textbooks, but it is likely similar in prevalence with carotid FMD (see below).
- Most common in mid segment of the main renal artery
Symptoms of Renal FMD
- Hypertension
- Headache
- Renal insufficiency
Treatment for Renal FMD
Angioplasty without stenting is mainstay of therapy (as opposed to atherosclerotic stenosis, where primary stenting is typically needed for ostial disease)
Carotid Fibromuscular Dysplasia
Carotid FMD has a 9:1 female to male ratio. Incidence is nearly the same as renal FMD, although some early literature reports higher carotid propensity. The process is non-inflammatory and non-atherosclerotic. Therefore, the patient population tends to be younger than the typical patients with carotid and vascular disease.
FMD patient population:
- Predominantly female, with female-to-male ratio 9:1
- Younger than typical atherosclerotic patients, usually 40-50s, occasionally in 60s
- 65% will have FMD in bilateral carotid arteries
- 25%-50% will have concomitant renal FMD
- Concomitant ipsilateral intracranial cerebral aneurysms 7%-25%
Symptoms and signs
- Headache
- Pulsatile swooshing sensation
- Dizziness
- TIA/amaurosis/stroke
- Carotid dissection (more common in males)
- Associated with intracranial aneurysms
- Rupture in rare occasions
Treatment
- No surgical management is recommended if asymptomatic, despite how severe the stenosis
- Medical treatment with antiplatelet therapy is acceptable during first symptomatic episode
- If recurrent neurologic symptoms occur, angioplasty without stenting is mainstay of therapy, with selective stenting in case of dissection, recalcitrant stenosis, or thrombus formation
Vertebral Fibromuscular Dysplasia
- Very uncommon
- Characterized by stenotic regions, usually around 5th vertebral or higher
- Does not have the usual aneurysmal segments, hence less likely to have “string of beads” appearance
- Rarely symptomatic