The true incidence of ALI is difficult to ascertain. Much of the literature is historical data with no recent updates and has been summarized in multiple texts [21, 22]. Scandinavia has been pivotal in population data regarding ALI. In 1984, Dryjski and Swedenborg [3] investigated the incidence of lower extremity ALI in Stockholm, with a population around 1.5 million. They found an overall annual incidence of nine per 100,000 people. This incidence was related to age; 0.4 per 100,000 for those 20–30 years old, with a peak incidence of 180 per 100,000 in patients over 90. More recently, the Swedish Vascular Registry identified the national incidence of ALI to be 13 per 100,000 people in 1998 [3]. Ljungman et al. focused on temporal trends in ALI over a 19-year period from 1965 to 1983 in Uppsala. They showed an annual increase in ALI from 2.7 to 3.9 %, which remained after age adjustments were made with a 2.7 % annual increase in men [23]. This increase over time was felt to be likely due to an aging population.

The other major epidemiological data for ALI have originated from British studies. Clason et al. showed in 1989 the incidence for lower extremity ALI in an area of Scotland to be 3.7 per 100,000 [24]. Later, the incidence was assessed from the entire county of Gloucestershire, England, from a single year in 1994. All data was prospectively gathered including hospital and general practice records, for a total population of 540,000. They found that the incidence during that period was one per 7000 and rose to one per 6000 when bypass grafts were included (14.3 per 100,000 and 16.7 per 100,000, respectively) [4].

Acute upper limb ischemia accounts for 16.6 %, approximately one-fifth of all ALI [5]. Upper extremity ALI occurs with an incidence of 1.2–3.5 cases per 100,000 people per year; however, this estimate is an underestimation as it only includes those that underwent intervention [5]. Dryjski and Swedenborg identified a risk of 1.13 per 100,000 people that included all admissions to the hospital. The patients who develop upper extremity ischemia tended to be slightly older than those with lower extremity ALI, with mean ages of 74 compared to 70 [25], and have a higher ratio of female to male at 2:1. The female to male preponderance was noted in a Danish study over a 13-year period that showed the incidence of upper extremity thromboembolectomy was 3.3 per 100,000 person-years among men and 5.2 per 100,000 person-years among women; however, they did not look at any patients that underwent conservative management [26].

Campbell et al. showed that 75 % of patients with ALI presented from home, with 10 % coming from another ward or 8 % from another hospital [27]. Only 3 % presented from a nursing home. They additionally noted that 14 % of ALI patients presented with bilateral lower extremity ischemia and that a similar number of left and right legs were affected. Furthermore, Campbell et al. showed that 40 % of patients had a delay in presentation with equal numbers due to patient, primary care physician, and transport delays [27]. Thirty-five percent had a delay in referral to a vascular specialist. There does not appear to be a significant seasonal variation with ALI; however, there is a trend toward a higher presentation in the winter [28].

Patients with ALI present in different stages of severity . Three stages were developed for ALI standardization (Table 1.1) [29]. Stage I is termed “viable.” The limb is not immediately threatened, without continuing ischemic pain, without neurologic deficit, and clear audible arterial signal in the pedal arteries. Stage II is termed “threatened.” Within this stage there are two levels, split for managing therapies. Stage IIa is marginally threatened and IIb is immediately threatened . Neither have clear audible signals in the pedal arteries. Those patients in IIa will have transient or minimal sensory loss which is usually limited to the toes. Those in IIb have persistent ischemic rest pain, sensory loss above the toes, and any motor disturbance. Stage III is termed “irreversible .” These patients have permanent neuromuscular damage with profound sensory loss and muscle paralysis, absent venous, and capillary flow distally. Typically there are skin changes such as skin marbling and muscle. The distribution of the stages of ALI at presentation is shown in Fig. 1.1 [1].

The etiology of lower extremity ALI is traditionally either embolism, in situ thrombosis with preexisting peripheral arterial disease (PAD), graft/stent thrombosis , trauma, or peripheral aneurysm with embolism or thrombosis . The frequency of these etiologies is shown in Fig. 1.2 [1, 27]. The timing of presentation depends on the severity of ischemia, which is linked to the etiology. Patients with embolism, trauma, and popliteal aneurysms present early (hours), compared to those with in situ thrombosis presenting later (days) [1]. Reconstructions—either bypass grafts, stents, or angioplasty sites—can present early or late given whether it is an acute thrombosis or in situ thrombosis with neointimal hyperplasia or atherosclerosis. This timing of presentation is generally related to the presence of or lack of collateral flow, something chronic PAD that typically affords individuals over time. The other differential diagnosis for ALI is shown in Table 1.2 [1, 31]. The typical sites for ALI involvement are shown in Table 1.3 [21, 32].