A Review Of Ethnic Differences In Risk Factors For Diabetic Foot Ulcers

By Caroline A. Abbott, PhD, and Carine van Schie, PhD

Research in recent years has shown distinct ethnic differences in the prevalence of diabetic complications, including amputation and more recently, foot ulceration. Although the etiology of the diabetic complications among different ethnic groups is not completely understood, preliminary data has highlighted some interesting variations in the presentation of these complications. Projections of diabetes prevalence indicate the number of people with type 2 diabetes worldwide is set to double over a 25-year period.1 This epidemic of diabetes is due to an aging and increasingly obese population. These factors will particularly affect migrant ethnic populations, given the higher prevalance of obesity and the fact that the cohort of first generation migrants is just attaining retirement age.2,3 Migrant populations of African Caribbean and South Asian descent in the United Kingdom have a prevalence of type 2 diabetes which is three- and four-fold higher respectively than in the general population.2,3 This finding is similar to ethnic differences observed in the United States and other parts of the developed world.4,5 Work from our own and other groups indicates that rates of diabetes complications affecting the lower limb also appear to differ by ethnicity. Diabetes-Related Amputations: What The Research Reveals A few studies have described ethnic differences in diabetes-related amputation. Data from the U.S. shows a two- to three-fold elevation in risk for African Americans compared with White Americans.6,7 It’s possible this increased risk may be linked to inequalities in access to health care in the U.S., plus socioeconomic and environmental factors, such as education and smoking.8 In the U.K., paradoxically, amputation risk is actually lower in diabetic African Caribbeans and the risk is reduced by about two-thirds in African Caribbeans in comparison to Europeans. Low rates of smoking among African Caribbeans could account for part of this, but certainly, the lower prevalence of peripheral vascular disease and neuropathy in this ethnic group were the other key factors accounting for the low rate of diabetes-related amputation in African Caribbean men. Indeed, the rate of diabetes-related amputation in migrant South Asians (Indian, Pakistani, Bangladeshi origin) living in the U.K. is just one-quarter that of the Caucasian Europeans.10,11 In a very recent population-based case control study of ethnic variations in type 2 diabetes-related amputation risk in Greater Manchester, U.K., the reduced South Asian amputation rate was mostly explained by low rates of PVD and neuropathy, partly associated with low rates of smoking.11 What The Studies Say About Differences In Foot Ulcers To date, there are few population-based studies of ethnic differences in foot ulcer rates. Existing reports consistently show that foot ulcer rates for diabetic patients of South Asian origin are lower than those for White Europeans. Indeed, lower rates of foot ulceration have been demonstrated for South Asian patients with diabetes compared to White Europeans foot ulcers were twice as prevalent among White European diabetic patients (5.5 percent) compared to their Asian counterparts (2.7 percent).12,13 We have also examined ethnic rates in the Northwest Diabetes Foot Care Study (our population-based investigation of diabetes-related foot problems in the U.K.), in which we screened 9,710 diabetic patients as part of a standardized diabetes footcare and education service in the community.14 The Northwest region of the U.K. has a relatively high proportion of South Asian migrants. The prevalence of past/present foot ulcers in South Asian diabetic patients was only 1.5 percent, significantly lower than the 5.2 percent rate for Caucasian European patients with the same access to healthcare. The explanation for this data is not entirely clear. The U.K. Prospective Diabetes Study XII (UKPDS) has demonstrated that fewer newly diagnosed Asian and Black diabetic patients have abnormal vibration perception threshold compared to their Caucasian European counterparts. This indicates that ethnic minorities have a lower prevalence of neuropathy, which is a major risk factor for foot ulceration.15 We have confirmed this lower prevalence of neuropathy in South Asian and African Caribbean populations, using vibration and temperature sensation testing for diagnosis.16 It has also been suggested that inter-ethnic differences in ulceration rates may be associated with variations in footwear.17 Therefore, the risk factors for altered ulcer rates between the ethnic groups are clearly multi-factorial. However, let’s take a closer look at the role of biomechanics in differences found for ethnic foot ulcer rates. Understanding The Biomechanical Aspects Of Foot Ulcers Ulcer sites predominantly develop under the plantar surface of the toes, forefoot and midfoot, followed by the dorsal surface of the toes and heel.18 Several studies have emphasized the importance of high plantar foot pressures in the pathogenesis of diabetic foot ulceration.19-21 These increased foot pressures are caused by structural and functional changes of the foot, such as the development of prominent metatarsal heads, callus and limited joint mobility. The body responds to repeated microtrauma (high pressure) with callus formation in order to protect the skin from further damage. If callus formation becomes excessive, it will contribute to higher pressure and possibly to ulceration. Therefore, in these cases, you should remove the callus formation at a regular interval.22,23 Foot deformities, other than callus, are strongly associated and predictive of increased plantar pressures and foot ulceration.18,24,25 The development of clawtoes leads to prominent metatarsal heads, which are then left unprotected from harmful pressures. Plantar tissue thickness at the metatarsal heads is strongly related to plantar pressures, indicating that patients with thinner plantar fat pads have higher foot pressures.26 Loss of hallux function due to clawtoes or reduced joint mobility could severely alter pressure distribution in the forefoot during walking.24 It has also been suggested that limited joint mobility of the foot and ankle increases plantar pressure in diabetic patients.20,27,28 In addition, some preliminary results have indicated a relationship between reduced joint mobility and a history of ulceration. For example, researchers found a significant reduction of subtalar joint mobility in the ulcerated foot compared to the contralateral non-ulcerated foot in diabetic neuropathic patients.29 Similarly, another study found reduced ankle dorsiflexion and subtalar range of motion in diabetic patients with a history of plantar ulceration compared to patients without a history of ulceration and non-diabetic controls.30 Furthermore, researchers have shown a relationship between limited joint mobility of the first metatarsophalangeal joint and ulceration of the great toe.31 Since this is all data from retrospective studies, it is not clear whether the reduced joint mobility is part of the pathogenesis of foot ulceration or whether it may be related to the ulcer healing process, during which patients are restricted in their mobility. What About Joint Mobility And Plantar Pressure? It has been hypothesized that greater mobility of the joints in the foot can provide a biomechanical advantage during walking, in that it could facilitate shock absorption and thereby lower plantar pressure. However, too much joint motion can be a biomechanical disadvantage, since a hypermobile foot is unable to produce an effective push-off during the gait cycle. A greater “biomechanical efficiency” may be a factor in the lower incidence of foot ulceration in the South Asian diabetic population.32 A pilot study demonstrated a group of South Asian diabetic patients had a greater range of motion in the subtalar and first metatarsophalangeal joint and lower peak plantar pressures compared to a matched group of Caucasian European diabetic patients.32 Another preliminary report from the U.K. showed a reduced range of joint motion in both Caucasian European and South Asian diabetic patients compared to non-diabetic controls. However, there was no difference in subtalar joint range of motion between the South Asian and Caucasian European diabetic patients.13 Conversely, the South Asian diabetic patients had significantly lower peak plantar pressures compared to their Caucasian European counterparts. Researchers suggested better foot care (i.e., a higher prevalence of washing feet among the South Asian patients) and lower foot pressure possibly could account for the lower rate of foot ulceration they saw in the South Asian population. Using a similar protocol, Veves, et. al., reported ethnic differences in foot joint mobility and dynamic plantar pressure between Caucasian and African-American diabetic patients in a U.S.-based study.28 The range of joint motion of the African-American diabetic patients was similar to the Caucasian non-diabetic subjects, while the African-American non-diabetic subjects had the highest range of motion of all studied groups. In addition, the lowest peak plantar pressures were measured in the African American non-diabetic subjects, suggesting greater joint mobility is related to lower plantar pressures. Another report from the U.S. confirmed the ethnic differences in joint mobility and plantar pressure.20 In this study, Caucasian diabetic patients had higher plantar pressures and less joint mobility than African-American and Hispanic diabetic patients. Furthermore, the Caucasian patients accounted for the largest proportion of patients with a history of foot ulceration, which was consistent with the findings of less joint mobility and higher plantar pressures in this group. An average follow-up of 30 months of this cohort of patients confirmed the African-American and Hispanic patients had a reduced risk of ulceration compared to the Caucasians.21 When neuropathy, age and sex were added to the model, there was no difference in risk for ulceration between the different ethnic groups, indicating these were more important risk factors for foot ulceration.21 What The Evidence Suggests Although existing evidence suggests reduced joint mobility may develop in long-term diabetic patients and joint mobility has a potentially important role in increasing foot pressures, we couldn’t confirm this relationship in South Asian and Caucasian European non-diabetic and diabetic subjects.33 In our recent study, we noted substantial lower peak plantar pressures in the South Asian compared to the Caucasian European patients. Differences in joint mobility in the foot could not explain this. The results of this study suggest factors other than joint mobility, such as foot structure, are likely to be more responsible for the substantial difference in foot pressure. As the thickness of plantar padding has been shown to be one of the strongest factors related to dynamic foot pressures, it has been suggested that the amount of plantar tissue padding possibly could be an important factor in explaining the reduced foot pressures in South Asian subjects. Preliminary data from our group have shown an apparent significant difference in the amount of plantar padding between the two ethnic groups, with greater padding and lower foot pressures in South Asian neuropathic patients compared to Caucasian European neuropathic patients.34 Nonetheless, other factors (such as footwear) also could play an important role in protecting the foot from developing deformities. A difference in hygiene, such as frequency of feet washing, may also be important in explaining the lower prevalence of foot ulceration. In Conclusion Accumulating evidence shows a lower prevalence of foot ulceration and lower limb amputation in certain ethnic minority groups of diabetic patients compared to Caucasian European patients. When examining biomechanical potential risk factors for this phenomenon, it appears foot pressures in Asian, African-American and Hispanic diabetic neuropathic patients are lower compared to their European/Caucasian counterparts. The possible role of joint mobility in explaining the lower foot pressures (an important risk factor for foot ulceration) has been explored but no evidence supports this theory. It seems more likely that other foot structure characteristics, such as plantar tissue padding, are more important issues with regard to foot pressures and risk of ulceration. However, you should take into account differences in hygiene, footwear, the prevalence of neuropathy and PVD, and other factors strongly related to diabetic foot ulcer development when determining ethnic differences for ulceration risk. Dr. Abbott is a Lecturer in the University Department of Medicine within the Manchester Royal Infirmary at the University of Manchester in the United Kingdom. Dr. van Schie is a Research Fellow in the University Department of Medicine within the Manchester Royal Infirmary at the University of Manchester in the United Kingdom.



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