The majority of nonpregnant patients with deep vein thrombosis have proximal thrombosis with involvement of the calf veins (58%–87%), whereas isolated proximal vein thrombi are uncommon (0%–13%).1–5
However, in our review of deep vein thrombosis during pregnancy, 71% of the cases were restricted to the proximal veins, and 64% of these were in the iliofemoral region. These observations strongly suggest that the anatomic distribution of deep vein thrombosis in pregnant women, and perhaps the pathophysiology of the condition, may indeed differ from that reported in the general population.
The pathophysiology of iliofemoral deep vein thrombosis in both nonpregnant and pregnant patients requires further exploration. In a large retrospective study of venography performed in nonpregnant patients with deep vein thrombosis, Ouriel and colleagues36
reported that the left to right ratio was 1.3:1 for infrainguinal deep vein thrombosis but 2.4:1 for iliac vein thrombosis. The authors speculated that in a substantial proportion of nonpregnant patients with iliac vein thrombosis, there may be undetected venous webs in the iliac vein (May–Thurner syndrome). In an observational study of the management of deep vein thrombosis in pregnant patients, Voke and associates26
similarly reported that the more proximal the thrombosis, the more likely it was to be on the left side. In addition, iliofemoral thromboses were also more likely to be reported in the third trimester of pregnancy.
In our current study, iliofemoral deep vein thromboses were also predominantly in the left leg (95%). We might speculate that among pregnant women, a May–Thurner-like syndrome brought on by compression of the left iliac vein by the gravid uterus (at the point where it crosses the right iliac artery) plays a major role in the increased incidence of iliofemoral deep vein thrombosis in late pregnancy. However, given that deep vein thrombosis occurs with equal frequency in all three trimesters of pregnancy,13
this hypothesis would presumably not apply to deep vein thrombosis observed in early pregnancy.
The higher prevalence of isolated deep vein thrombosis of the proximal veins seen in this study, relative to previous studies of nonpregnant patients, is clinically important. Patients with proximal deep vein thrombosis have a high risk of pulmonary embolism (40%–50%).37
Untreated or unrecognized pulmonary embolism can result in maternal morbidity and mortality.38
Commonly used protocols for compression ultrasonography to diagnose deep vein thrombosis in pregnant patients (e.g., two-point compression imaging) may be limited in their ability to detect isolated iliofemoral thrombi and may therefore be inadequate. Several authors have demonstrated the feasibility of using Valsalva manoeuvres and assessing flow changes with respiration throughout pregnancy to assess the patency of proximal veins.39–41
However, the sensitivity of these manoeuvres for detecting isolated iliac vein thrombosis in pregnant women with suspected deep vein thrombosis is unknown.
There are obvious limitations to a study of this nature. Our analysis was derived from pooling small observational studies and excluded articles in languages other than English. As such, reporting bias and selection bias are possibilities. We tried to minimize reporting bias by selecting case series of three or more patients from one site. In all six studies (case series or cohorts), determining the anatomic distribution of deep vein thrombosis, the aim of the current study, was not the primary objective. Therefore, selective patient exclusion was unlikely. That said, deep vein thrombosis of the lower extremity was assessed with compression ultrasonography in more than half of the cases. Compared with venography, compression ultrasonography is relatively insensitive for the diagnosis of deep vein thrombosis of the calf and isolated iliac vein thrombosis.6
Therefore, we might have underestimated the prevalence of isolated calf vein or iliac vein thrombosis. This might have affected our findings with respect to proximal and distal distribution of the thromboses.