The goal of this feasibility/pilot study is to investigate whether or not vascular ultrasound can be used to aid the diagnosis of May Thurner Syndrome, in adult patients presenting to the vascular department with unexplained left leg swelling. The main research objective it aims to answer are:

• The diameter variation of the left common iliac vein based on posture, using duplex ultrasound.
• To determine inter- and intra-observer variation of the diameter measurement of the left common iliac vein using duplex ultrasound.

Participants will have an ultrasound scan performed on their abdomen by three different clinical vascular scientists, and will have each scan performed whilst lying supine on an examination couch and again when the couch is tilted to 45 degrees.

Researchers will compare the results of symptomatic patients to asymptomatic patients, to see if there is a difference in the diameter of the left common iliac vein between these two groups.

Inclusion Criteria:

• Participant has the capacity to consent
• Participant is required to have a vascular ultrasound scan in the vascular ultrasound department
• Participant is 18 years of age or older at the start of the study.

Exclusion Criteria:

• Previous iliac vein intervention or treatment for iliac vein disease
• Previous diagnosis of iliac vein thrombus
• Pregnancy
• Unable to provide appropriate informed consent
• The participant does not require a vascular ultrasound as part of their routine care
• Under 18 years of age at the start of the study

On the study day / day of the ultrasound scan

No difference

Limitation due to non-visualization of the CIV in some participants - In one participant no readings/data could be obtained, and in three participants only partial data could be obtained.