Available online 22 May 2024

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Primary lateral sclerosis (PLS) is a motor neuron condition marked by pure upper motor neuron (UMN) degeneration. PLS represents around 3% of all motor neuron diseases. Classically the prognosis of PLS is less severe than those of amyotrophic lateral sclerosis (ALS). This explains the necessity to distinguish both conditions as early as possible. The key hallmark between the two diseases is the involvement of the lower motor neuron (LMN) system which is classically considered spared in PLS contrary to ALS. Although it seemed clinically easy to distinguish PLS from ALS with the aid of clinical and complementary examinations, there is a large body of evidence highlighting that the LMN system might be impaired in PLS. This led us to suggest that PLS might be considered as an almost pure UMN ALS phenotype.

Section snippetsClinical findings of LMN involvement are not so rare

LMN involvement is clinically defined by the presence of cramps, fasciculations and amyotrophy [9]. There were several reports mentioning these signs in patients with a diagnosis of PLS: one of the most illustrative pieces of evidence came from the study of Le Forestier et al. who, in a cohort of 20 patients, noticed cramps among 16/20, fasciculations among 18/20 and amyotrophy among 13/20. Moreover, 9/20 patients disclosed all three signs at physical examination [7]. Muscle testing might also

Discussion

All this evidence could support a continuum between PLS and ALS and that PLS would be a UMN-predominant ALS phenotype characterized by a late occurrence of slow progressive LMN involvement (Table 1) [33].

Nevertheless, we are aware that some findings could, on the contrary, plead for two different entities. Transmagnetic stimulation (TMS) led to disclose striking differences owing to neurophysiological findings in favor of hyperexcitability in ALS, contrary to PLS associated with

Disclosure of interest

The authors declare that they have no competing interest.

Authors contribution

All authors contributed equally to the inception and drafting of this review paper.

Acknowledgements

We are grateful to the ERN Motoneuron for its support.

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