Primary Lateral Sclerosis and Early Upper Motor Neuron Disease: Characteristics of a Cross-Sectional Population

Could PLS represent a UMN-predominant ALS syndrome?

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.

Primary lateral sclerosis: application and validation of the 2020 consensus diagnostic criteria in an expert opinion-based PLS cohort

BACKGROUND

Validation of the 2020 consensus criteria for primary lateral sclerosis (PLS) is essential for their use in clinical practice and future trials.

METHODS

In a large cohort of patients diagnosed with PLS by expert opinion prior to the new criteria with detailed clinical baseline evaluation (n=107) and longitudinal follow-up (n=63), we applied the new diagnostic criteria and analysed the clinical phenotype, electromyography (EMG), diagnostic accuracy and prognosis, adding neurofilaments and MRI as potential biomarkers.

RESULTS

The criteria for definite PLS were met by 28% and those for probable PLS by 19%, whereas 53% did not meet the full criteria at baseline, mainly due to the time, EMG and region criteria. Patients not meeting the criteria had less generalised upper motor neuron involvement but were otherwise similar in demographic and clinical characteristics. All patients with definite and probable PLS maintained PLS diagnosis during follow-up, while four patients not meeting the criteria developed clinical lower motor neuron involvement. Definite PLS cases showed improved survival compared with probable PLS and patients who did not meet the criteria. Despite a clinical PLS phenotype, fibrillation potentials/positive sharp waves and fasciculations in one or more muscles were a frequent EMG finding, with the extent and prognostic significance depending on disease duration. Serum neurofilament light and a multiparametric MRI fibre integrity Z-score correlated with clinical parameters and were identified as potential biomarkers.

CONCLUSION

Validation of the 2020 PLS consensus criteria revealed high diagnostic certainty and prognostic significance, supporting their value for research and clinical practice.

Primary Lateral Sclerosis: An Overview

Primary lateral sclerosis (PLS) is a rare neurodegenerative disorder which causes the selective deterioration of the upper motor neurons (UMNs), sparing the lower motor neuron (LMN) system. The clinical course is defined by a progressive motor disability due to muscle spasticity which typically involves lower extremities and bulbar muscles. Although classically considered a sporadic disease, some familiar cases and possible causative genes have been reported. Despite it having been recognized as a rare but distinct entity, whether it actually represents an extreme end of the motor neuron diseases continuum is still an open issue. The main knowledge gap is the lack of specific biomarkers to improve the clinical diagnostic accuracy. Indeed, the diagnostic imprecision, together with some uncertainty about overlap with UMN-predominant ALS and Hereditary Spastic Paraplegia (HSP), has become an obstacle to the development of specific therapeutic trials. In this study, we provided a comprehensive analysis of the existing literature, including neuropathological, clinical, neuroimaging, and neurophysiological features of the disease, and highlighting the controversies still unsolved in the differential diagnoses and the current diagnostic criteria. We also discussed the current knowledge gaps still present in both diagnostic and therapeutic fields when approaching this rare condition.

Current challenges in primary lateral sclerosis diagnosis

INTRODUCTION

Primary lateral sclerosis (PLS) is a rare, adult-onset and slowly progressive motor neuron disorder whose clinical core is characterized by upper motor neuron (UMN) dysfunction. Its formal diagnosis is clinically based and disease duration-dependent. Differentiating PLS from other disorders involving UMN can be challenging, particularly in the early stages.

AREAS COVERED

Our review covers and discusses different aspects of the PLS field, including the diagnostic criteria and its limitations, its differential diagnosis and their major pitfalls, and the actual role of neurophysiology, neuroimaging, genetics, and molecular biomarkers. Symptomatic treatment of the different manifestations is also addressed. The authors searched MEDLINE and Scopus. They also searched the reference lists of articles identified by our search strategy and reviewed and selected those deemed relevant. They selected papers and studies based on the quality of the report, significance of the findings, and on the author's critical appraise and expertise.

EXPERT OPINION

It is important to investigate novel molecular biomarkers and plan multicenter clinical trials for PLS. However, this will require a large international project to recruit enough patients, particularly given the diagnostic uncertainty of the current clinical criteria. A better understanding of PLS pathophysiology is crucial for designing disease-targeted therapies.

Does limited EMG denervation in early primary lateral sclerosis predict amyotrophic lateral sclerosis?

Objective: We assessed whether a cohort of patients with primary lateral sclerosis (PLS) and limited electromyography (EMG) motor unit denervation changes evolve into amyotrophic lateral sclerosis (ALS) with prolonged follow-up. Methods: We initially ascertained all PLS patients diagnosed at Mayo Clinic-Rochester (1990-2016). Of 64 total cases, 43 had normal EMGs ("pure" PLS) during the first 4 years after symptom onset and were the focus of a prior publication, documenting absence of evolution to ALS. The remaining 21 patients had limited motor unit changes on EMG needle examination (denervation and most with fibrillation or fasciculation potentials) but insufficient to raise a strong suspicion of ALS; these 21 patients were followed to determine whether they evolved into ALS. Results: Of these 21 patients, the median follow-up was 7 years' disease duration (range: 4-27 years; IQR 5-8.5). They included 11 females (52%) with median onset-age of 57 years (range: 42-72 years). Two patients (10%) subsequently met revised El Escorial criteria for ALS after 7 and 13 years, respectively. The remainder had stable EMG changes with a persistent PLS phenotype. Among these remaining 19 patients, the PLS course was somewhat more aggressive than our previously reported series of 43 patients devoid of EMG denervation. The paraparetic variant was more common than the hemiparetic and bulbar variants, similar to "pure" PLS. Conclusions: Among PLS patients with definite but limited EMG denervation, 2/21 (10%) later developed ALS. The patients in this series had a more progressive clinical course compared to our previously reported pure PLS cases.

The clinical spectrum of primary lateral sclerosis

Primary lateral sclerosis is a distinct entity that has recently been classified as a "restricted phenotype" of ALS. It is characterized by a pattern of isolated upper motor neuron involvement that often begins in the legs and spreads diffusely. Distinction from other conditions requires careful consideration of clinical presentation and time course of disease. Mills' Syndrome is a rare unilateral variant of primary lateral sclerosis. Cognitive and behavioral involvement may occur.

Primary Lateral Sclerosis: Clinical, radiological and molecular features

Primary Lateral Sclerosis (PLS) is an uncommon motor neuron disorder. Despite the well-recognisable constellation of clinical manifestations, the initial diagnosis can be challenging and therapeutic options are currently limited. There have been no recent clinical trials of disease-modifying therapies dedicated to this patient cohort and awareness of recent research developments is limited. The recent consensus diagnostic criteria introduced the category 'probable' PLS which is likely to curtail the diagnostic journey of patients. Extra-motor clinical manifestations are increasingly recognised, challenging the view of PLS as a 'pure' upper motor neuron condition. The post mortem literature of PLS has been expanded by seminal TDP-43 reports and recent PLS studies increasingly avail of meticulous genetic profiling. Research in PLS has gained unprecedented momentum in recent years generating novel academic insights, which may have important clinical ramifications.

Upper Motor Neuron Disorders: Primary Lateral Sclerosis, Upper Motor Neuron Dominant Amyotrophic Lateral Sclerosis, and Hereditary Spastic Paraplegia

Following the exclusion of potentially reversible causes, the differential for those patients presenting with a predominant upper motor neuron syndrome includes primary lateral sclerosis (PLS), hereditary spastic paraplegia (HSP), or upper motor neuron dominant ALS (UMNdALS). Differentiation of these disorders in the early phases of disease remains challenging. While no single clinical or diagnostic tests is specific, there are several developing biomarkers and neuroimaging technologies which may help distinguish PLS from HSP and UMNdALS. Recent consensus diagnostic criteria and use of evolving technologies will allow more precise delineation of PLS from other upper motor neuron disorders and aid in the targeting of potentially disease-modifying therapeutics.

Natural History of "Pure" Primary Lateral Sclerosis

OBJECTIVE

To assess whether primary lateral sclerosis (PLS), classified as pure when the EMG is normal, converts to amyotrophic lateral sclerosis (ALS) after longitudinal follow-up.

METHODS

Retrospective chart review was performed of patients with pure PLS at Mayo Clinic in Rochester, MN (1990-2016). Inclusion criteria required a normal EMG during the first 4 years of symptoms.

RESULTS

Forty-three patients had pure PLS (25 female, 58%) with a median onset age of 50 years (range 38-78 years) and median follow-up at 9 years' disease duration (range 4-36 years). The ascending paraparesis phenotype (n = 30, 70%) was most common, followed by hemiparetic onset (n = 9, 21%) and bulbar onset (n = 4, 9%). Among the 30 paraparetic-onset cases, bladder symptoms (n = 18, 60%) and dysarthria (n = 15, 50%) were more common than pseudobulbar affect (n = 9, 30%) and dysphagia (n = 8, 27%). By the last follow-up, 17 of 30 (56%) used a cane and 6 (20%) required a wheelchair. The paraparetic variant, compared with hemiparetic and bulbar onset, had the youngest onset (48 vs 56 vs 60 years, respectively; p = 0.02). Five patients died; 1 patient required a feeding tube; and none required permanent noninvasive ventilation. Two patients developed an idiopathic multisystem neurodegenerative disorder, which surfaced after 19 and 20 years. Two patients developed minor EMG abnormalities. The remainder 39 had persistently normal EMGs.

CONCLUSIONS

Pure PLS did not convert to ALS after a median of 9 years' disease duration follow-up in our study population. The ascending paraparetic phenotype was most common, with earlier onset and frequent bladder involvement. After years of pure PLS, <5% develop a more pervasive neurodegenerative disorder.

Better understanding the neurobiology of primary lateral sclerosis

Primary lateral sclerosis (PLS) is a rare neurodegenerative disease characterized by progressive degeneration of upper motor neurons (UMNs). Recent studies shed new light onto the cellular events that are particularly important for UMN maintenance including intracellular trafficking, mitochondrial energy homeostasis and lipid metabolism. This review summarizes these advances including the role of Alsin as a gene linked to atypical forms of juvenile PLS, and discusses wider aspects of cellular pathology that have been observed in adult forms of PLS. The review further discusses the prospects of new transgenic upper motor neuron reporter mice, human stem cell-derived UMN cultures, cerebral organoids and non-human primates as future model systems to better understand and ultimately treat PLS.

Evolving diagnostic criteria in primary lateral sclerosis: The clinical and radiological basis of "probable PLS"

INTRODUCTION

Primary lateral sclerosis is a rare neurodegenerative disorder of the upper motor neurons. Diagnostic criteria have changed considerably over the years, and the recent consensus criteria introduced 'probable PLS' for patients with a symptom duration of 2-4 years. The objective of this study is the systematic evaluation of clinical and neuroimaging characteristics in early PLS by studying a group of 'probable PLS patients' in comparison to a cohort of established PLS patients.

METHODS

In a prospective neuroimaging study, thirty-nine patients were stratified by the new consensus criteria into 'probable' (symptom duration 2-4 years) or 'definite' PLS (symptom duration >4 years). Patients were evaluated with a standardised battery of clinical instruments (ALSFRS-r, Penn upper motor neuron score, the modified Ashworth spasticity scale), whole genome sequencing, and underwent structural and diffusion MRI. The imaging profile of the two PLS cohorts were contrasted to a dataset of 100 healthy controls. All 'probable PLS' patients subsequently fulfilled criteria for 'definite' PLS on longitudinal follow-up and none transitioned to develop ALS.

RESULTS

PLS patients tested negative for known ALS- or HSP-associated mutations on whole genome sequencing. Despite their shorter symptom duration, 'probable PLS' patients already exhibited considerable functional disability, upper motor neuron disease burden and the majority of them required walking aids for safe ambulation. Their ALSFRS-r, UMN and modified Ashworth score means were 83%, 98% and 85% of the 'definite' group respectively. Motor cortex thickness was significantly reduced in both PLS groups in comparison to controls, but cortical changes were less widespread in 'probable' PLS on morphometric analyses. Corticospinal tract and corpus callosum metrics were relatively well preserved in the 'probable' group in contrast to the widespread white matter degeneration observed in the 'definite' group.

CONCLUSIONS

Our clinical and radiological analyses support the recent introduction of the 'probable' PLS category, as this cohort already exhibits considerable disability and cerebral changes consistent with established PLS. Before the publication of the new consensus criteria, these patients would have not been diagnosed with PLS on the basis of their symptom duration despite their significant functional impairment and motor cortex atrophy. The introduction of this new category will facilitate earlier recruitment into clinical trials, and shorten the protracted diagnostic uncertainty the majority of PLS patients face.

Primary lateral sclerosis: consensus diagnostic criteria

Primary lateral sclerosis (PLS) is a neurodegenerative disorder of the adult motor system. Characterised by a slowly progressive upper motor neuron syndrome, the diagnosis is clinical, after exclusion of structural, neurodegenerative and metabolic mimics. Differentiation of PLS from upper motor neuron-predominant forms of amyotrophic lateral sclerosis remains a significant challenge in the early symptomatic phase of both disorders, with ongoing debate as to whether they form a clinical and histopathological continuum. Current diagnostic criteria for PLS may be a barrier to therapeutic development, requiring long delays between symptom onset and formal diagnosis. While new technologies sensitive to both upper and lower motor neuron involvement may ultimately resolve controversies in the diagnosis of PLS, we present updated consensus diagnostic criteria with the aim of reducing diagnostic delay, optimising therapeutic trial design and catalysing the development of disease-modifying therapy.

Primary lateral sclerosis: a distinct entity or part of the ALS spectrum?

Primary lateral sclerosis (PLS) has been traditionally viewed as a distinct upper motor neuron condition (UMN) but is increasingly regarded as a sub-phenotype within the amyotrophic lateral sclerosis (ALS) spectrum. Despite established diagnostic criteria, formal diagnosis can be challenging and the protracted diagnostic journey and uncertainty about longer-term prognosis cause considerable distress to patients and caregivers. PLS patients are invariably excluded from ALS clinical trials, while PLS pharmacological trials are lacking. There remains an unmet need for diagnostic biomarkers for upper motor neuron predominant conditions and prognostic indicators regarding prognosis, survival, and risk of conversion to ALS. Validated biomarkers will not only have implications for individualized patient care but also serve as outcome measures in pharmaceutical trials. Given the paucity of post-mortem studies in PLS, novel pathological insights are generally inferred from state-of-the-art imaging studies. Computational neuroimaging has already contributed significantly to the characterization of PLS-associated pathology in vivo and has underscored the role of neuro-inflammation, the presence of extra-motor changes, and confirmed pathological patterns similar to ALS. This systematic review assesses the current state of PLS research across clinical, neuroimaging and neuropathological domains from a combined clinical and academic perspective. We discuss patterns of pathological overlap with other ALS phenotypes, examine if the biological processes of PLS warrant therapeutic strategies distinct from ALS, and evaluate the evidence that classes PLS as a distinct clinico-pathological entity.

The concept and diagnostic criteria of primary lateral sclerosis

OBJECTIVES

Primary lateral sclerosis (PLS) is commonly considered as a motor neuron disease (MND) variant which almost exclusively affects upper motor neurons (UMN). There is still no consensus whether PLS should be regarded as an independent disease entity separate from amyotrophic lateral sclerosis (ALS) or as a comparatively slowly progressive variant of ALS. Given these different views, clinical diagnosis of PLS is a challenge. In this multicenter study, we analyzed clinical features of patients diagnosed with PLS in four specialized MND centers.

MATERIAL AND METHODS

We retrospectively analyzed clinical, laboratory, imaging, and electrophysiological data of 76 patients with PLS diagnosed in four specialized ALS centers. We analyzed the concept of the disease based on our findings and an extensive review of the literature.

RESULTS

We found that 79% of patients showed asymmetrical symptoms, 60% showed clinical or electrophysiological signs of lower motor neuron (LMN) involvement after a mean of 8.4 ± 5.0 years, and extrapyramidal and/or non-motoric symptoms were frequently observed. Interestingly, none of the patients diagnosed with PLS fulfilled the diagnostic criteria proposed by Pringle et al. in 1992.

CONCLUSIONS

Our data show that PLS as a disease entity is still not well enough defined and that there are different concepts about its clinical presentation. We believe that further prospective longitudinal studies are needed in order to refine diagnostic criteria to reflect current clinical practice. Furthermore, neuropathological and neuroimaging approaches might help to arrange PLS in the MND spectrum and its classification.