Neurovascular hand symptoms in relation to cold exposure in northern Sweden: a population-based study

Living in a cold climate is associated with numerous adverse health effects (Hassi et al. 2005; Makinen and Hassi 2009). A number of studies have documented increased wintertime morbidity and mortality, especially in the respiratory and cardiovascular systems (Gasparrini et al. 2015; Raatikka et al. 2007; The Eurowinter Group 1997). Cold exposure can also cause specific injuries to the extremities, such as frostbite and chilblains, and is associated with long-term neurological and vascular dysfunction (Carlsson et al. 2016; Imray and Castellani 2012). Different pathophysiological mechanisms of such cold injuries have been studied and it has been shown that reduced tissue temperature due to ambient cold exposure can lead to several detrimental vascular effects, such as vasoconstriction, ischemia-induced endothelial damage, hemoconcentration, and the creation of free radicals (Thomas and Oakley 2002). Some of these effects are believed to contribute to vascular hand symptoms, such as Raynaud’s phenomenon (Garner et al. 2015). When it comes to the peripheral nerves, apart from direct damage from cooling, the ischemic vascular effects mentioned above can also cause nerve dysfunction through effects on the vasa nervorum, the small arteries that supply blood flow to the peripheral nerves (Thomas and Oakley 2002).

Cold sensitivity has previously been defined as an exaggerated or abnormal reaction to cold exposure causing discomfort or the avoidance of cold (Kay 1985), and it has often been studied in conjunction with hand injuries, such as digital amputations or hand-arm vibration injury (Carlsson et al. 2010; Lithell et al. 1997). The pathophysiological mechanisms in cold sensitivity might involve both neurological and vascular aspects (Hope et al. 2014; Thomas and Oakley 2002). To the authors’ knowledge, cold sensitivity has not previously been investigated in large-scale population-based studies. However, one Swedish study included a small sample of subjects from the general population (N = 94), (Carlsson et al. 2010).

Regarding exposure to cold, temperatures at or below 10 °C are defined as ambient cold exposure in occupational health standards (International Organization for Standardization 2008). The experience of being cold can also be defined from a subjective standpoint regardless of the ambient temperature (Makinen and Hassi 2002). Cold exposure can occur during both work and leisure time and is often associated with aggravating environmental conditions, such as wind, rain, or snow (Keim et al. 2002). Indoor work with cold storage, contact with cold objects, and immersion in cold water can also contribute (Baldus et al. 2012). In addition, the effects of ambient cold exposure are modified by individual factors, such as sex, age, nutritional status, pre-existing diseases, medication, thermal insulation of clothing, and activity level (Raatikka et al. 2007). Swedish national statistics report that 22% of working men and 11% of working women in Sweden are occupationally exposed to cold for more than one quarter of their working hours (Swedish Work Environment Authority 2014). Living in the northern parts of Sweden can also result in significant leisure-time cold exposure, but national statistics for such exposure are not available. Large-scale, questionnaire-based studies of cold-related health effects have been undertaken in Finland (Makinen et al. 2009; Nayha et al. 2011; Raatikka et al. 2007), but such studies are lacking in Sweden.

The aim of the present study was to describe the self-reported ambient cold exposure in northern Sweden and to relate the level of cumulative cold exposure to the reported occurrence of sensory and vascular hand symptoms. We hypothesize that cold exposure is positively related to reporting such symptoms.

The present study shows that cold exposure is common in northern Sweden, both during work and leisure time. However, the correlation between leisure time and occupational exposure is rather low (r _s = 0.27, p value < 0.01), indicating that the study subjects could be highly exposed in one situation but not in the other. In addition, the exposure to cold is generally greater among men than women and is more pronounced in the rural alpine regions as compared to the urbanized coastal regions, which can be explained by certain outdoor occupations, such as forestry and farming being more common, but also due to outdoor leisure-time activities being more popular in these regions. In addition, the lowest mean temperatures in the study area were found in the alpine regions. Taken together, both behavioral and climatic factors contribute to intensive cold exposure in these areas.

Neurovascular hand symptoms, such as reduced sensory perception, increased cold sensitivity, and Raynaud’s phenomenon, were frequently reported and were generally associated with a higher cumulative cold exposure, on a population level. These findings suggest that cold exposure, even in the absence of overt cold injury, such as frostbite or chill blains, might affect the sensory and vascular function of the hands. In addition, previous frostbite to the hands was strongly associated with both cold sensitivity and Raynaud’s phenomenon, which might indicate a common pathophysiological mechanism for these conditions. We hypothesize that cold exposure might cause neurovascular dysfunction in exposed body parts, such as the hands, and give rise to the aforementioned symptoms. There is some, although limited, laboratory (Hope et al. 2014) and epidemiological (Block and Sequeira 2001; Mackiewicz and Piskorz 1977) data to support this concept. Even though cold exposure was generally more pronounced among men, cold-related symptoms were more frequently reported by women. This might be interpreted as a particular susceptibility among women, and this warrants further research.

Previous studies have reported that men are generally more exposed to cold climate, especially those working in farming, forestry, industry, or construction (Makinen et al. 2006). The same pattern was seen in the present study. The anthropometric data, tobacco use, and disease spectrum in our cohort (Table 1) roughly corresponded with other recent Swedish investigations (Backman et al. 2014; Eriksson et al. 2010; Lindberg et al. 2006), which indicates that our study has included a representative sample of the population. Other authors have reported that frostbite is common in circumpolar areas (Ikaheimo and Hassi 2011), with a prevalence of serious frostbite of 1.5% for men and 0.5% for women in Finland (Raatikka et al. 2007), which is similar to our results. Regarding sensory hand symptoms, a recently published study on heavily cold-exposed Swedish military conscripts showed a significant decrease in sensory perception after winter training, which was evident even among those without any reported cold injury (Carlsson et al. 2016). The prevalence of Raynaud’s phenomenon in our study was high compared to previous studies on a general population, especially among men. In a recent review, the prevalence of Raynaud’s phenomenon was estimated to be 0.8–6.5% in men and 2.1–5.8% in women, with the exact figure depending on the region in which the study was conducted and how a case was defined (Garner et al. 2015). However, in the aforementioned Finnish study (Raatikka et al. 2007), the prevalence of Raynaud’s phenomenon was 11.9% for men and 12.4% for women, which compares well with our results showing a occurrence of 11.0% for men and 14.0% for women. Such a high occurrence might indicate secondary etiologies, such as hand-arm vibration injury or auto-immune diseases, but the cold climate in Sweden and Finland might also trigger Raynaud’s phenomenon and other hand symptoms more easily than in a warmer climate, a notion that is supported by epidemiological data (Block and Sequeira 2001).

There are several limitations to our study. The cross-sectional design does not allow causal relations to be established. The rather low response rate (35.9%) may limit the generalizability of the results and increase the uncertainty in prevalence estimates. There is also the possibility that symptomatic subjects might be more prone to respond to a questionnaire of this kind, and this might lead to an overestimation of both exposure and symptoms. The number of retired respondents was high, which might weaken any possible associations with occupational factors. The study region comprises a large area with a mean monthly temperature during the study period that spanned from about −9 to 5 °C, meaning that there is reason to suspect a variance in ambient cold exposure that has not been adjusted for in the analyses. Thus, the results in our study serve as an indication of health effects due to cold exposure in northern Sweden, but must be cautiously interpreted with regard to these limitations in study design and response rate.

Despite these limitations, this is the first population-based study on cold exposure and related health effects in Sweden, and it includes more than 12,000 participants. Despite the rather low response rate, the high occurrence of cold-related symptoms in the cohort still indicates that cold exposure poses a significant negative impact on health in northern Sweden. Our analysis revealed no major differences between responders and non-responders regarding geographical region, which was the expected main determinant of the cold exposure variables (Online Resource 1). Thus, we believe that the present study has included a representative sample of the population and that the possible bias produced by a low response rate has not affected the exposure data to any great extent. The cold exposure in our study was estimated from a subjective standpoint, but the close association between cumulative cold exposure and the occurrence of frostbite (which we consider as a definitive marker of cold exposure) supports that this approach to collecting exposure data was relevant (Table 3). This first study within CHINS will provide a baseline cohort for questionnaire-based case–control studies focusing on different cold-related health effects.

The present study demonstrates that the cold environment in northern Sweden might be an underestimated health risk. Our hypothesis that cold exposure is positively related to reporting of neurovascular hand symptoms was supported by our findings. In addition, such symptoms were common not only in conjunction with an overt cold injury. Our results warrant further study on pathophysiological mechanisms and suggest the need for confirmatory prevalence studies to support national public health planning.