Overall, DVT is considered to be a complication occurring at a variable rate in acute SCI, with incidence rates depending on several factors including diagnostic methods used, study population, characteristics (e.g., age, acuity of injury, location of SCI), associated risk factors, and modality of thromboprophylaxis (Chung et al. 2014; Merli et al. 1993). There are inconsistencies in the scientific literature regarding the incidence of this complication as most occurrences of DVT are asymptomatic. Thus, the use of various diagnostic imaging screening methods is of utmost importance as clinical criteria alone are insufficient (Meissner 1998). However, the incidence of DVT varies even with the use of non-clinical methods of detection (Winemiller et al. 1999). Nevertheless, DVT in acute SCI has been reported as having the highest rate of incidence within the first 3 months following injury (Chung et al. 2014), with a variable incidence rate reported in older studies ranging from 49% to 100% during the first 12 weeks (Merli et al. 1993); however, most DVT events occur within the first 2 weeks (Chen & Wang 2013; Merli et al. 1993). Table 1 shows the incidence of DVT among individuals with SCI; between 2010 and 2019, reports place the incidence of DVT between 1.6% and 45%; this range is based on studies in which individuals with SCI did or did not use thromboprophylaxis methods.

The formation of venous thrombosis has been reported to occur as early as 72 hours post-injury, peaking between days 7 and 10, as detected by impedance plethysmography, venous flow dopplers, and lung scanning. However, this statistic is also based on some studies which did not indicate a specific method of assessment, and as such should be interpreted with caution (Merli et al. 1993). Using color duplex sonography, Germing et al. (2010) detected the formation of DVT even sooner, reporting that 38% of individuals in their study developed DVT within the first 36 hours after hospitalization following injury. However, the occurrence of DVT and PE is rare within the first three days after injury (Raslan et al. 2010). PE has also been reported to have the highest likelihood of occurrence within the first 3 months following SCI (Chung et al. 2014); however, fatal PE has been found to be a rare occurrence after the initial 3-month duration following SCI (Sugimoto et al. 2009). Interestingly, some studies have shown that the incidence of DVT seems to be lower in Eastern (e.g., India and Pakistan) SCI populations compared to those in the West, possibly due to a difference in diet and genetic risk factors (Rathore et al. 2008; Saraf et al. 2007). Despite prophylaxis being widely used since the 1980s to prevent and treat occurrences of VTE, it remains a major health complication for acute SCI individuals that results in significant morbidity and mortality (Furlan & Fehlings 2007).

The high risk of DVT in acute individuals with SCI is due to the simultaneous presence of three factors of Virchow’s triad: hypercoagulability, stasis, and intimal (inner vessel layer) injury (Aito et al. 2002). VTE usually begins with a calf DVT (Cogo et al. 1998; Nicolaides et al. 1971; Philbrick & Becker 1988). Other contributing factors include partial or total limb paralysis and absence of spasticity which is a signifi­cant independent risk factor for DVT (Do et al. 2013). VTE affects blood flow, reduces the capacity of the vessels, and increases venous resistance. These as a result promote a cascade of metabolic derangements resulting in activation of the coagulation cascade and venous thrombosis (de Campos Guerra et al. 2014).

Approximately 20% of DVTs extend into the proximal veins (Brandstater et al. 1992; Kakkar et al. 1969; Lagerstedt et al. 1985); over 80% of symptomatic DVTs involve the popliteal or more proximal veins. Non-extending distal (i.e., calf) DVTs rarely cause PEs and as such are rarely worrisome (Kakkar et al. 1969), although they may account for over 80% of the incidence of DVT (Germing et al. 2010). Proximal (i.e., knee or above) DVTs often lead to PEs and are a cause for concern (Kakkar et al. 1969). Selassie et al. (2011) noted that individuals who developed a PE had a twofold increase in the risk of in-hospital death compared to those who did not develop a DVT. Distal DVT, which is more common, is associated with post-thrombotic phlebitis and venous valvu­lar insufficiency (Do et al. 2013).

Post SCI pulmonary emboli incidence is 4.6-14% and is mostly asymptomatic or unrecognized. However, in 1.7-4.7% of the cases, it is large and fatal (de Campos Guerra et al. 2014).