The findings in this study are the first to determine the prevalence and other associated factors of neuropathic pain in Nigeria sickle cell disease patients. This is also the first study to use a validated screening tool for neuropathic pain in SCD patients in Nigeria.
Previous studies from other climes have shown that neuropathic pain is present in SCD patients. Data from our study show that neuropathic pain exists in Nigerian SCD patients. In this present study, the PDQ questionnaire was used to evaluate for neuropathic pain in SCD and about 30% of the SCD population had evidence of neuropathic pain. This value falls within the range of the hypothetical figures we initially proposed, and it is surprisingly higher than values obtained from epidemiologic studies in chronic pain population [20–23]. However, it is similar to some chronic conditions and even lower in some others [24–28]; this however, unequivocally positions SCD as a high-risk condition for the development of neuropathic pain. The prevalence of NP in our study is equally slightly higher than the 17.9% reported by Ramsay et al. in Jamaica and the 19.3% found by Cregan et al. in adolescents, but comparable to the 25–40% estimates from studies focusing on SCD patients with chronic pain [17–19]. When viewed through an African perspective, this prevalence figure carries profound implications. For decades, the narrative of SCD pain in Africa has been dominated by the acute, nociceptive vaso-occlusive crisis. Our findings disrupt this monolithic view. The high prevalence suggests that a significant proportion of what is often dismissed as "background" or "unexplained" chronic pain in African SCD patients may, in fact, have a neuropathic basis. This aligns with the observations of Noubiap et al. and Farooq et al [15, 16]., who lamented the near-total absence of research on neurological complications like NP in African SCD populations. The high prevalence in our study could also be as a result of the cumulative effect of a lifetime of undertreated painful episodes in a resource-constrained setting, or a lack of access to disease-modifying therapies that might mitigate neuronal damage.
In one of their research works, Ballas and Darbari argued that though neuropathy was present in SCD, neuropathic pain was rarely reported [28]. Symptoms outside allodynia, hyperalgesia and sensitivity to cold and heat were mere neuropathy and not neuropathic pain [29]. However, our data showed that SCD patients do exhibit neuropathic pain symptoms including allodynia and hyperalgesia. Previous studies showed that SCD patients exhibit symptoms of neuropathic pain which may alter pain and sensory processing with evidence of central and peripheral sensitization [30–34]. Experiments performed in SCD mice also support the findings of altered pain and sensory processing [35–37]. A recent study reported that the brain network in SCD underlies the hypersensitivity in them during pain crisis [38]. In another review, Ballas and Darbari acknowledged the existence of neuropathic pain in SCD [39].
Our findings show that age is significantly associated with neuropathic pain. This is consistent with previously published works [19, 40]. In the general population, neuropathic pain is known to increase with age [41, 42]. This may be a result of increased co-morbidities encountered in the aged. However, gender was not associated with neuropathic pain in our study. Though a slightly higher number of females had neuropathic pain compared to males, it was not significant. This may appear to be different from what is reported in literature [43], or perhaps those females have more health-seeking attitudes. Blood transfusion is one of the common treatments administered to SCD patients, especially in the prevention of neurological deficits like stroke.
Blood transfusion is also known to be used for the prevention of frequent painful crises [44]. However, its role in chronic pain is less well-known. In our study, there was a significant association between the units of blood transfused and neuropathic pain. Our findings show that approximately 80% of the SCD patients have had at least one unit of blood transfusion. A study by Curtis et al reported an increased neuropathic and nociceptive pain quality among SCD patients on chronic transfusion indicating worse pain [45]. Similarly, there was also a significant positive correlation between the units of blood transfused and neuropathic pain. Thus, it may mean that chronic blood transfusion may also play a role in neuropathic pain in SCD patients. The univariate association between a higher number of blood transfusions and NP is a critical finding that warrants further research. Chronic transfusion therapy is a standard intervention for primary and secondary stroke prevention and, in some cases, for intractable pain. However, in our cohort, a high transfusion burden may likely mean proxy marker for a more severe disease phenotype, which translate to more frequent and severe vaso-occlusive insults, leading to cumulative ischaemic damage to peripheral nerves and the central nervous system, thereby predisposing them to NP.
On hydroxyurea use, our data did not show any significant association between its use and neuropathic pain. Hydroxyurea is known to reduce painful episodes in SCD [46]. Previous studies have been reported to be positively associated with neuropathic pain in SCD [19]. The reason for this may not be fully known, but in our case, many of the patients were not on hydroxyurea, only about 37% were on hydroxyurea. Thus, analyzing its relationship with neuropathic pain is somewhat tricky. A review of the literature does not reveal any data supporting the use of hydroxyurea as a treatment for neuropathic pain. While pain in SCD is rather complex, the use of pain relievers is rather inadequate. The majority of the patients have not used morphine before to manage their pain. Also, while pregabalin and gabapentin are the mainstay of treatment for neuropathic pain, approximately 2% of SCD patients are on the drugs in this study. This low use of neuropathic pain drugs was also reported by Brandow et al [19]. The use of neuropathic pain drugs has been reported to be beneficial in SCD [47–49]. The American Society of Hematology 2020 guidelines for sickle cell disease recommend the use of gabapentinoids, SSRIs, and TCAs for the management of neuropathic pain (SCD-related chronic pain with no identifiable cause beyond SCD) albeit for adults only [50]. The almost non-existent use of gabapentinoids in our study is a major therapeutic gap that may reflect systemic barriers including cost, concerns about monitoring, limited availability etc. It also underscores a critical disconnect between international best practices and the realities in many African healthcare systems, where pain management remains heavily reliant on NSAIDs and often scarce opioids, which are largely ineffective for neuropathic pain. It is hoped that the use of neuropathic pain drugs will be adopted alongside conventional pain therapies for the management of pain in SCD patients.
Neuropathic pain in SCD can have some significant clinical implications. These include a worsening quality of life [51], negative psychological impact [52] and with association to some inflammation markers. It was associated with worse sleep and greater stiffness in another study [53]. This invariably can have adverse effect on social functioning, increasing psychological distress. Thus, identifying SCD patients with neuropathic pain is important in order to improve their quality of life.
Limitations
This study has some limitations. Firstly, being a cross-sectional study limits our ability to understand the course of the pain. Secondly, the painDETECT questionnaire was the only screening tool used and it is not validated in SCD patients under 14 years of age, hence, that population of patients are unaccounted for in the prevalence. In addition, a neurological examination was not done to help further understand the aetiopathology of the pain. In addition, a nerve conduction study was also not done. As a result of the subjective nature of pain, recall bias could be an issue; the patient could also have received some form of management before a presentation.