Introduction

Thalassemia major (TM) and sickle cell disease (SCD) are inherited chronic hemolytic anemias that significantly affect red blood cell function and structure. These disorders lead to ongoing hemolysis, triggering a cascade of complications including iron overload, skeletal deformities, recurrent infections, splenic dysfunction, and chronic pain syndromes1,2. Among these, neuropathic pain represents a particularly debilitating condition that results from damage or dysfunction within the somatosensory nervous system2,3. It is characterized by sensory abnormalities such as hyperalgesia (increased pain sensitivity), allodynia (pain in response to normally non-painful stimuli), and spontaneous burning sensations1,4.

Regular blood transfusions, which represent a cornerstone of thalassemia treatment, are known to cause micronutrient deficiencies—particularly in folic acid and vitamin B12—both of which play essential roles in maintaining nerve function5,6,7. Deficiencies in these nutrients have been associated with polyneuropathy, potentially manifesting as neuropathic pain symptoms. Additionally, chronic transfusion therapy leads to excessive iron accumulation, which is recognized to have neurotoxic effects on both the central and peripheral nervous systems. Iron-induced oxidative stress and neuroinflammation are thought to exacerbate peripheral nerve dysfunction, thereby contributing to the development of neuropathic pain3,4.

Despite the increasing recognition of neuropathic pain in these hematologic disorders, accurate diagnosis remains challenging. Standard neurological examinations often fail to detect neuropathic involvement, rendering electromyography (EMG) and nerve conduction studies (NCS) the gold-standard diagnostic tools for assessing structural nerve damage8. However, neuropathic pain is also reported in patients with normal EMG findings, underscoring the complexity of pain perception and the potential role of central dysfunction and sensitization mechanisms9.

Although extensive research has been conducted on vaso-occlusive pain crises and anemia-related complications in thalassemia and SCD, studies specifically addressing neuropathic pain in these conditions remain limited. Moreover, the relationship between neuropathic pain and disease-related factors—such as hydroxyurea therapy, iron overload, and depression—is not well understood. Given that chronic pain is frequently associated with psychological distress, particularly depression, a multidisciplinary approach is considered essential for optimal patient care10,11.

In this study, we aimed to investigate the presence of neuropathic pain and its associated factors in patients with TM and SCD, specifically focusing on individuals with normal nerve conduction studies in order to exclude confounding anatomical neuropathies. Instead of emphasizing general clinical differences between TM and SCD, we concentrated on identifying associations between neuropathic pain and key variables, including depression severity, hydroxyurea therapy, hemoglobin levels, platelet count, transfusion frequency, and iron overload.

Material and methods

Study design

This cross-sectional study was conducted at a single center between February 2022 and February 2023. Ethical approval was obtained from the Mustafa Kemal University Local Ethics Committee (Approval Number: 2025.03.19/58). The study was performed in accordance with the principles outlined in the Declaration of Helsinki. All participants provided both verbal and written informed consent before enrollment.

A total of 68 patients diagnosed with TM and SCD who visited the outpatient clinic were initially screened for eligibility. Of these, 11 patients were excluded due to abnormalities detected in their nerve conduction studies (NCS). The specific exclusions included:

  • 7 patients with sural nerve abnormalities (5 with TM, 2 with SCD),

  • 3 patients with carpal tunnel syndrome (all diagnosed with SCD),

  • 1 patient with mild sensorimotor axonal polyneuropathy (SCD patient).

Consequently, 57 patients (27 with TM, 30 with SCD) with normal NCS findings were included in the final analysis. Eligible patients had normal neurological examination findings and no history of:

  • Stroke-like episodes,

  • Seizures,

  • Infectious diseases affecting the nervous system.

Comprehensive demographic and clinical data were collected and analyzed in relation to age, gender, education level, employment status, smoking history, hemoglobin, hematocrit, platelet count, ferritin levels, presence of depression, and medical history including previous splenectomy, chelation therapy, and hydroxyurea treatment.

Assessment of depression and neuropathic pain

Depression levels were assessed using the Beck Depression Inventory (BDI), a widely validated screening tool for depression in clinical settings10. The Turkish version of the BDI, which has been tested for validity and reliability, was administered. Depression severity was categorized as follows:

  • Symptom-free: 0–9 points,

  • Mild depression: 10–16 points,

  • Moderate depression: 17–29 points,

  • Severe depression: 30–63 points.

Neuropathic pain was evaluated using the Douleur Neuropathique 4 (DN4) questionnaire, a validated tool for detecting neuropathic pain and polyneuropathy12. The DN4 consists of 10 items, with total scores ranging from 0 to 10, and a score of ≥ 4 was used as the diagnostic threshold for probable neuropathic pain13,14. For descriptive purposes, DN4 scores were also grouped in ranges (e.g., 0, 1–3, 4–6, ≥ 7), but all analyses and figures have now been revised to consistently reflect the 0–10 scoring system.

Electrophysiological assessment (nerve conduction studies)

All patients underwent standardized nerve conduction studies (NCS) to rule out peripheral neuropathy. NCS was performed using a Keypoint electromyography (EMG) device in a climate-controlled environment (room temperature 22–24 °C, limb temperature > 34 °C).

Motor and sensory NCS were conducted on the following nerves:

  • Median and ulnar nerves (upper limbs),

  • Peroneal and tibial nerves (lower limbs),

  • Sural nerve (sensory assessment).

NCS procedures followed standardized polyneuropathy evaluation protocols based on the methods described by Oh (15). Polyneuropathy diagnosis required the presence of at least two electrophysiologic abnormalities at non-entrapment sites.

Statistical analysis

All statistical analyses were performed using SPSS software (version 25; IBM Corp., Armonk, NY, USA), with statistical significance set at p < 0.05. Descriptive statistics were used to summarize the data, with categorical variables presented as frequencies and percentages (%) and continuous variables as means and standard deviations (Mean ± SD). Chi-square (χ2) tests were employed to assess differences between groups, with Fisher’s exact test used for small sample sizes. When significant differences were identified through chi-square analysis, the z-ratio test was conducted for further pairwise comparisons. The Spearman correlation coefficient (rs) was applied to evaluate relationships between continuous and ordinal variables.

Results

Descriptive statistics of socio-demographic and clinical features of the patients are summarized in Table 1. The study included 27 patients with TM and 30 patients with SCD. The mean age was significantly lower in the TM group (29.22 ± 4.86 years) compared to the SCD group (33.03 ± 7.85 years, p = 0.020). Gender distribution and education level were similar between the groups (p > 0.05). Hemoglobin levels were significantly lower in TM patients (8.21 ± 0.92 g/dL) compared to SCD patients (10.03 ± 1.45 g/dL, p < 0.001), whereas ferritin levels were markedly higher in TM patients (2678 ± 1689 ng/mL vs. 580 ± 689 ng/mL, p < 0.001). Platelet counts did not differ significantly between groups (p = 0.423).

Table 1 Socio-demographic and clinical characteristics of patients with thalassemia major (TM) and sickle cell disease (SCD). Neuropathic pain was evaluated using the DN4 questionnaire (0–10 scale). For descriptive purposes, DN4 scores were categorized into 4 groups: 0, 1–3, 4–6, and ≥ 7. Values are presented as mean ± SD or n (%).
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Regarding neuropathic pain, DN4 scores were distributed across the full 0–10 scale. In the TM group, 70.3% of patients scored 0, while 25.9% scored between 1 and 3. Only 3.7% scored within the 4–6 range, and none reached ≥ 7. In contrast, among SCD patients, 50.0% scored 0, 16.7% scored 1–3, 16.7% scored 4–6, and 16.6% scored ≥ 7. Although higher DN4 scores were more frequent in the SCD group, this difference did not reach statistical significance (p = 0.069).

No significant relationship was found between smoking status and neuropathic pain (p = 0.81). Depression severity was distributed similarly between groups (p = 0.632), although moderate-to-severe depression was more frequent in the SCD group (23.3% vs. 14.8%). Chelation therapy was universally applied in TM patients (100%), whereas only 16.6% of SCD patients received this treatment (p < 0.001). Conversely, hydroxyurea therapy was used by 86.6% of SCD patients but not by any TM patients (p < 0.001). Splenectomy rates were similar between the two groups (p = 0.289) (Table 1).

In the subgroup analysis of TM patients, no statistically significant differences in neuropathic pain scores were found in relation to sex, smoking status, chelation therapy, or splenectomy (p > 0.05). However, in the subgroup analysis of SCD patients, a statistically significant difference in neuropathic pain scores was observed between those who had undergone splenectomy and those who had not (p = 0.011).

Figure 1 illustrates the distribution of DN4 neuropathic pain scores across the two patient groups. The median DN4 score was 0 in the TM group and higher in the SCD group, with several patients reaching scores ≥ 7. Although neuropathic pain was more prevalent and severe in the SCD group, the difference between groups did not achieve statistical significance (p = 0.069) (Fig. 1).

Fig. 1
figure 1

Box-and-whisker plots of DN4 neuropathic pain scores (0–10 scale) in patients with thalassemia major (TM) and sickle cell disease (SCD). The boxes represent the interquartile range (IQR), the horizontal line indicates the median, and the whiskers show the minimum and maximum values. DN4 scores were generally higher in SCD patients compared to TM patients, although the difference did not reach statistical significance (p = 0.069).

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To further investigate the relationships between neuropathic pain and clinical variables, correlation and regression analyses were performed using scatter plots. Across the entire cohort (n = 57), neuropathic pain scores showed a strong positive correlation with depression scores (rs = 0.80, p < 0.001), and a weak but statistically significant positive correlation with ferritin levels (rs = 0.29, p = 0.028). Hemoglobin levels demonstrated a weak inverse, but statistically non-significant correlation (rs = –0.24, p = 0.070). No significant relationships were observed between neuropathic pain and age (rs = −0.05, p = 0.719) or platelet count (rs = 0.10, p = 0.469) (Fig. 2).

Fig. 2
figure 2

Scatter plots showing the relationships between DN4 neuropathic pain scores (0–10 scale) and clinical variables, including depression severity, ferritin, hemoglobin, age, and platelet count.

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Figure 3 demonstrates the relationship between hydroxyurea treatment and DN4 neuropathic pain scores. Patients with SCD who were receiving hydroxyurea had significantly higher neuropathic pain scores compared to those not on therapy. Correlation analysis revealed a moderate positive relationship between hydroxyurea use and neuropathic pain severity (rs = 0.363, p < 0.01) (Fig. 3).

Fig. 3
figure 3

Comparison of DN4 neuropathic pain scores (0–10 scale) between patients with and without hydroxyurea treatment. Neuropathic pain scores were significantly higher in patients receiving hydroxyurea therapy, and correlation analysis confirmed a moderate positive association between hydroxyurea use and DN4 scores (rs = 0.363, p < 0.01). Diamonds represent outliers beyond the interquartile range.

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Figure 4 demonstrates the relationship between hydroxyurea use and DN4 neuropathic pain scores. Patients receiving hydroxyurea had higher DN4 scores compared to those not on therapy, with median values of 7.0 and 5.0, respectively. Outlier values were observed in both groups, as indicated by diamonds. Correlation analysis confirmed a moderate positive relationship between hydroxyurea use and neuropathic pain severity (rs = 0.80, p < 0.001) (Fig. 4).

Fig. 4
figure 4

Relationship between hydroxyurea use and DN4 neuropathic pain scores (0–10 scale). Mean DN4 scores are shown by the green line, and median values are indicated numerically above each group (No = 5.0, Yes = 7.0). Diamonds represent outlier values beyond the interquartile range. Patients receiving hydroxyurea had higher DN4 scores compared to those not on therapy.

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Discussion

This study highlights the significant impact of neuropathic pain in patients with TM and SCD, contributing to the growing body of evidence that these hematologic disorders are associated not only with systemic complications but also with neurological impairments. Our findings are consistent with previous studies emphasizing the high prevalence of neuropathic pain in these populations, suggesting that disease-related factors such as chronic anemia, iron overload, treatment regimens, and metabolic disturbances may contribute to dysfunction in the peripheral and central nervous systems, including sensitization mechanisms13,15.

Neuropathic pain is a complex and multifactorial condition that arises from either structural damage or functional dysfunction of the peripheral or central nervous system. It is often associated with chronic diseases and typically presents with burning sensations, tingling, allodynia, and hyperalgesia, which significantly impair quality of life16,17. In patients with SCD, inflammatory processes triggered by vaso-occlusive crises—which are specific to SCD—and in TM, chronic iron overload, might have contributed to sensitization mechanisms and persistent pain. Increased levels of pro-inflammatory cytokines, including TNF-α, IL-1, and IL-6, have been linked to neuronal hyperexcitability and pain amplification in both conditions18. In SCD, neuropathic pain can arise from both direct nerve injury related to vaso-occlusive ischemia and functional abnormalities such as central or peripheral sensitization due to repeated nociceptive input18,19. In TM, neuropathic symptoms have been reported and may be influenced by long-term iron deposition, treatment effects, or micronutrient imbalance; however, definitive causal pathways remain uncertain13,20. Together, these observations suggest that, although both groups experience neuropathic pain, the underlying mechanisms likely differ21.

Chronic pain conditions, including neuropathic pain, are closely associated with psychological distress, particularly depression. Previous studies have shown that thalassemia patients often experience depression, which is exacerbated by disease-related disability, social limitations, and iron overload22. In our study, in the SCD group, neuropathic pain scores correlated significantly with depression severity (rs = 0.551, p = 0.01), indicating that patients with higher depression severity reported more intense neuropathic pain. These findings are consistent with prior research suggesting that chronic pain can act both as a consequence and a driver of psychological distress23. SCD patients are also at high risk for depression due to the unpredictable nature of vaso-occlusive crises, frequent hospitalizations, disease-related fatigue, chronic pain, and socio-economic burdens such as limited educational and occupational opportunities. Prior studies have demonstrated that chronic pain in SCD increases the likelihood of depressive symptoms, and such psychological distress can enhance pain perception through central nervous system dysfunction, including sensitization. Our results support this association, as patients with higher depression scores—regardless of diagnosis—tended to report more severe neuropathic pain. These findings highlight the need for future research evaluating the effectiveness of routine psychological assessment and integrated pain–mental health interventions in patients with TM and SCD.

Toret et al. previously reported that thalassemia patients struggled with educational, occupational, and social challenges due to their physical limitations, increasing their vulnerability to depression23. Mednick et al. emphasized that demographic, medical, and psychosocial factors play a crucial role in the development of psychological distress, underscoring the need for a multidimensional approach to patient care24. The interaction between chronic pain and depression is well documented, with studies suggesting that depressive symptoms can amplify pain perception through central nervous system dysfunction25,26,27,28. These findings support the importance of incorporating routine mental health assessments into the clinical management of patients with hematologic disorders.

Our study found a moderate association between hydroxyurea treatment and neuropathic pain in SCD patients. However, hydroxyurea is not classified as a neurotoxic agent, and current evidence does not support a direct causal relationship. This association is more likely to reflect greater disease severity, as patients with more frequent vaso-occlusive crises and related complications are more commonly prescribed this therapy. Further studies—including mechanistic animal studies and clinical investigations in other patient populations receiving hydroxyurea—are warranted to explore any potential links. Additionally, some patients not receiving hydroxyurea are managed with chronic transfusion therapy, which may also influence pain perception. In our study, half of the SCD patients who were not on hydroxyurea were receiving chronic transfusions, suggesting that treatment modality and disease severity may act as overlapping or confounding factors. Hydroxyurea has been widely used to reduce the frequency of vaso-occlusive episodes in SCD by increasing fetal hemoglobin levels and reducing hemoglobin S polymerization29,30. However, its long-term effects on pain-related nervous system dysfunction, including sensitization, remain unclear. A previous study using the painDETECT questionnaire found that 37% of SCD patients experienced neuropathic pain and reported a strong association with hydroxyurea use19. Although hydroxyurea is not classified as a neurotoxic agent, its prolonged use may have indirect effects on peripheral nerve function and pain modulation. Proposed mechanisms include vascular changes, oxidative stress, and neuroinflammatory responses, warranting further research31.

Iron overload, a common complication in TM, has well-documented neurotoxic effects. Chelation therapy is essential for managing iron accumulation; however, its relationship with neuropathic pain remains controversial. Some chelating agents, such as deferasirox (DFX), have been associated with hepatic and renal toxicity, which might indirectly contribute to nervous system dysfunction19,32,33,34. In contrast, our study did not find a significant association between chelation therapy and neuropathic pain, suggesting that other factors—such as disease severity and chronic inflammation—may play a more dominant role.

Thalassemia patients, particularly those with beta-thalassemia major (b-TM), exhibited a high prevalence of peripheral neuropathy, which significantly affected their quality of life. Studies by Paola et al. and Orhurhu et al. reported varying degrees of sensory neuropathy in thalassemia patients, many of whom had abnormal findings on nerve conduction studies or physical examination. In contrast, our study specifically included patients with normal NCS and neurological examination results, aiming to explore functional rather than structural alterations in pain processing20,35. In our study, 16.6% of SCD patients and 15.6% of TM patients showed abnormalities in NCS, but these individuals were excluded from the final analysis. Despite normal EMG findings, many patients reported neuropathic pain, suggesting involvement of mechanisms such as small fiber neuropathy or sensitization. We distinguished “damage,” referring to detectable abnormalities, from “dysfunction,” referring to altered signaling without pathology. Sensitization is recognized to occur both centrally and peripherally, leading to heightened pain perception. Given the inclusion criteria of normal NCS and physical examination, the neuropathic pain observed in this study likely reflects dysfunction rather than overt nerve injury, although subclinical damage may still be present.

Another important contributor to neuropathic pain in SCD is the occurrence of frequent vaso-occlusive crises (VOCs). Repeated ischemia–reperfusion injury can induce neuroinflammation and promote dysfunction in both nervous systems. Prior studies have demonstrated that frequent VOCs increase the risk of chronic pain syndromes with neuropathic features. Although our study did not include VOC frequency due to retrospective limitations, future prospective studies should incorporate this variable to clarify its role18,31.

Correlation and regression analyses demonstrated significant associations between neuropathic pain and several clinical variables in our cohort. The positive correlation with depression severity supports existing evidence that chronic pain and psychological distress are deeply interconnected—a relationship documented in previous studies showing depression as both a result and a driver of neuropathic pain36,37. Higher ferritin levels were associated with increased neuropathic pain scores. While this may reflect iron-induced oxidative stress and neuroinflammation, it is also possible that ferritin served as a surrogate marker of disease severity and transfusion burden, similar to the interpretation of hydroxyurea use27,38. The inverse relationship with hemoglobin suggests that more severe anemia may exacerbate neuropathic symptoms, as has been noted in similar hemolytic anemias39. These associations remained robust after excluding zero-value cases, reducing concerns of statistical artifacts or floor effects. Platelet count did not associate with pain, indicating that thrombocytosis alone may not significantly influence neuropathic pain in these populations.

In subgroup analyses, only SCD patients exhibited a statistically significant difference in neuropathic pain scores based on splenectomy status. Although prior studies have focused on splenectomy’s impact on infections and hematologic outcomes, little is known about its role in pain modulation40. Our finding may reflect underlying mechanisms such as microvascular alterations or immune-mediated neural sensitization, which align with evidence involving central sensitization in chronic SCD pain syndromes41,42. In contrast, no categorical demographic or treatment-related differences—including sex, smoking, chelation therapy, or splenectomy—were found within the TM group, suggesting a more homogeneous pain profile across these clinical variables. This underscores the need for disease-specific models to evaluate neuropathic pain, as mechanisms likely differ between TM and SCD despite overlapping clinical features.

Limitations

This study has some limitations. First, the sample size was relatively small, which may have limited the generalizability of the findings. Moreover, the study population did not include pediatric patients, which restricts the applicability of the findings to younger age groups. Future studies including children and adolescents with sickle cell disease (SCD) would provide valuable insight into early-onset neuropathic alterations. Second, the absence of a control group reduced the ability to compare patients with healthy individuals. Third, conventional nerve conduction studies (NCS) were limited in detecting small fiber neuropathies, which are frequently implicated in neuropathic pain syndromes. Additionally, the frequency and severity of vaso-occlusive crises in patients with SCD were not systematically recorded, which restricted evaluation of their contribution to neuropathic pain development. Furthermore, sickle cell genotypes (e.g., HbSS, HbSC, HbSβ-thalassemia) were not documented for all patients, precluding genotype-based subgroup analysis, although genotype might influence both disease severity and pain characteristics. Future studies should include quantitative sensory testing (QST), skin biopsy, and genotype-stratified analyses to better elucidate the mechanisms and risk factors underlying neuropathic pain in these patient populations.

Conclusions

In conclusion, this study demonstrates the burden of neuropathic pain in patients with TM and SCD, emphasizing its significant association with depression and its moderate correlation with hydroxyurea treatment. These findings underscore the importance of comprehensive patient care, including neurological evaluation, individualized pain management, and integrated mental health support. As neuropathic pain remains under-recognized in patients with SCD, early detection and multidisciplinary approaches are essential. The observed relationship between hydroxyurea use and neuropathic pain warranted further investigation to determine whether it reflected a direct medication effect or served as a marker of greater disease severity. Future research involving larger cohorts and advanced diagnostic modalities is needed to clarify underlying mechanisms and develop targeted strategies that address both the physical and psychological dimensions of neuropathic pain in these hematologic populations.