The S curve: a novel morphological finding in the internal carotid artery in patients with fibromuscular dysplasia.

Fibromuscular dysplasia (FMD) is a non-atherosclerotic vascular disease commonly affecting the renal and internal carotid arteries (ICAs). A previously unrecognized finding is a redundancy of the mid-distal ICA in FMD patients causing an 'S'-shaped curve. Carotid artery duplex ultrasounds were reviewed in 116 FMD patients to determine S-curve prevalence. FMD patients with an S curve were matched to four control patients divided equally into two groups: (1) age and sex-matched and (2) age ≥70 and sex-matched. S curves were present in 37 (32%) FMD patients. Of these, nine (24%) had angiographic evidence of FMD in their ICA only, 13 (35%) had renal artery FMD only, and 15 (41%) had both ICA and renal FMD. Two patients in the age and sex-matched group had S curves (odds ratio 16.86, 95% CI 3.92-72.48; p<0.0001) while 12 (16.2%) patients in the age ≥70 and sex-matched group had S curves (odds ratio 2.42, 95% CI 1.16-5.03; p=0.016). In conclusion, the S curve is a novel morphological pattern of the mid-distal ICA. While the S curve may not be specific, its presence in individuals <70 years old should alert the clinician to the possibility that FMD is present.