Pitfalls in Calculating the Incidence of Guillain-Barre Syndrome During the Pandemic

Abstract

We read with interest the article by Blanco-Ruiz et al. on a retrospective study of the prevalence of Guillain–Barre syndrome (GBS) in Spain in 2018–2021 to assess whether or not it has increased during the pandemic [1]. The total number of Spanish GBS cases in 2018, 2019, 2020, and 2021 was 832, 861, 670 and 784, respectively [1]. It was concluded that the incidence of GBS before and during the pandemic was similar to other countries and that the incidence decreased during the pandemic [1]. The study is appealing, but some points should be discussed.

The first point is that only a single ICD code (G61.0) was used to filter out patients diagnosed with GBS during the observation period [1]. However, GBS is only a generic term for several subspecifications. These include acute inflammatory demyelinating polyneuropathy (AIDP), acute motor axonal neuropathy (AMAN), acute motor and sensory axonal neuropathy (AMSAN), Miller–Fisher syndrome (MFS), pharyngo-cervico-brachial (PCB) GBS, GBS with involvement of one or more cranial nerves, flail arm/leg syndrome, and Bickerstaff brainstem encephalitis (BBE) [2]. These entities can be coded by ICD codes other than G61.0. Possible other codes are G61.9 (polyradiculitis), radiculopathy (M54.1), and polyneuropathy (G62.9). Is it conceivable that the prevalence figures calculated for the years of interest are inaccurate because they do not include these alternative ICD codes? Is it conceivable that the clinical presentation of GBS during the pandemic is different from before, which may be why these patients were not coded as G61.0? Assuming that some of the GBS diagnoses were missed, it would be interesting to repeat the analysis including the alternative ICD codes and compare it with the results of the index study.

The second point is that a decrease in prevalence in 2020 and 2021 does not necessarily mean that GBS due to SC2I predominates or that SC2I has not increased the prevalence of GBS. Theoretically, it is conceivable that the number of patients with GBS due to triggers other than SARS-CoV-2 has decreased in 2020 and 2021 due to the reduced opportunities for transmission due to the lockdowns, social distancing and other protective measures, and that the majority of cases registered in 2020 and 2021 are actually due to SC2I or SC2V. As temporary visits to restaurants or mass events had become impossible, infections with these pathogens are also likely to have decreased. The most common infectious triggers of GBS are Campylobacter jejuni, Haemophilus influenzae, cytomegalyvirus, and Mycoplasma pneumoniae [3]. The increase in incidence in 2021 compared to 2020 could be due to the fact that isolation and protective measures were abandoned after the introduction of SC2V. Is it also conceivable that the incidence of GBS has decreased because less severe cases were not hospitalized due to the restrictions and therefore did not enter a registry?

The third point is that the effect of the SARS-CoV-2 vaccination (SC2V) was not included in the analysis. Since SC2V was only available from the beginning of 2021, it is conceivable that the lower prevalence of GBS compared to 2018 and 2019 is due to the vaccination effect. We should therefore know whether the prevalence of (SC2I) has also decreased in 2021 due to the vaccination effect. How did the authors differentiate between these two pathophysiological scenarios? How many of the GBS patients in 2021 were caused by SC2V and how many by Sc2I?

Finally, what is the reason that the incidence in 2021 was high in December but low in January [1]? If the incidence was truly due to SC2I, one would expect it to be higher in all months of the cold season.

To summarize, this interesting study has limitations that put the results and their interpretation into perspective. Addressing these limitations could strengthen the conclusions and reinforce the study's message. All open questions need to be clarified before readers can uncritically accept the conclusions of the study. In order to compare the prepandemic prevalence and incidence figures with those of the pandemic, it would be important to include patients coded with ICD codes other than G61.0. As long as the causes of GBS are not included in the analysis, no definitive conclusions can be drawn about the prevalence and incidence dynamics during the pandemic.

Josef Finsterer: investigation, conceptualization, methodology, validation, writing – review and editing, writing – original draft.

The author has nothing to report.

The author has nothing to report.

The author declares no conflicts of interest.