Validation of a bedside sensory tool for detecting mechanical sensitization

Background & Aims: postherpetic neuralgia (PHN) is persistent pain more than 3 months after herpes zoster , often requiring multimodal treatment due
to the different underlying mechanisms generating and maintaining the neuralgia, such as sensitization and deafferentiation. To guide drug treatment,
quantitative sensory testing (QST) is a helpful tool to identify clinical phenotypes associated with specific pathophysiological processes paving the way for
precision medicine. However, the time-intensive nature and the costs of QST restricts the applicability in routine clinical practice, highlighting the need for
faster, cost-effective methods to define clinical phenotypes. In this study, we propose and validate a time and cost effective bedside (BS-)tool to identify
patients with a mechanical sensitization phenotype.
Methods: a cross-sectional study was performed in 42 PHN patients of the UMC Utrecht, undergoing laboratory QST based on the DFNS(German Research
Network on Neuropathic Pain)-protocol and consecutively the BS-tool protocol. This included a Bailey Tip Therm, a Semmes-Weinstein 300 grams pinprick
monofilament (assessing temporal summation (TS) and mechanical pain sensitivity (MPS)), and a rubber brush (assessing dynamic mechanical allodynia
(DMA)). The correlation between the QST and BS-tool results were analyzed using Spearman’s rank. Cut-offs for sensitization were based on literature and
expert opinion. A sensitivity-specificity analysis was performed, comparing the results of the BS-tool to QST.
Results: 55% of the patients were female, with a mean age of 70 years (range 27-91). The mean numeric pain rating scale (NPRS) was 5 (SD 2.1). PHN
occurred most commonly in the trigeminal nerve (35.7%) or thoracic dermatomes (54.8%). The BS-tool and QST showed moderate to very strong correlation
on the affected side (DMA: ρ=0.833, MPS: ρ=0.598, MPT: ρ=-0.670, TS: ρ=0.752). Signs of mechanical sensitization were observed in 25 patients (59,5%)
with the BS-tool and in 31 (73.8%) using the QST-tool (p=0.058). Mechanical hypersensitivity rates on the affected side were the same for both tests. Windup or hypersensitivity distant to the injured site were more often identified with QST (QST: 38.1%, BS: 9.5%; p=0.003). The BS-tool had a sensitivity of
74.2% and specificity of 81.8% (AUC=0.780) for any signs of mechanical sensitization. The discriminatory power increased for higher NPRS-scores (NPRS ≥
4: AUC=0.865, NPRS ≥ 7: AUC=0.867), and if patients were not able to discriminate cold (AUC=0.917).
Conclusions: the BS-tool shows good correlation with mechanical QST-variables of the DFNS protocol on the affected side and is able to discriminate
between PHN-patients with and without signs of mechanical sensitization. The discriminatory power improves with peripheral measurements, higher pain
scores and disrupted discrimination for cold.
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