Primary Lateral Sclerosis

Pathophysiology description Primary lateral sclerosis is a rare, predominantly upper motor neuron (UMN) neurodegenerative disorder marked by selective degeneration of corticospinal (pyramidal) neurons in layer V of the primary motor cortex (Betz cells), with secondary degeneration of the corticospinal tract and relative sparing of lower motor neurons (LMN). Autopsy and review evidence describe diffuse brain atrophy with selective loss of pyramidal neurons in the precentral gyrus, degeneration of descending motor white matter, and variable ubiquitin- and TDP-43–positive cytoplasmic inclusions in cortex, highlighting overlap with ALS/FTD‐linked proteostasis failure while maintaining a consistent resistance to LMN degeneration in most cases (quotes: “selective loss of pyramidal (Betz) neurons in layer V of the precentral gyrus,” “degeneration of the corticospinal tract,” “TDP‑43–positive, ubiquitinated cytoplasmic inclusions” (Vacchiano 2024) (vacchiano2024primarylateralsclerosis pages 2-3, vacchiano2024primarylateralsclerosis pages 1-2). Ultrastructural studies and patient cellular models implicate organellar dysfunction—mitochondrial and Golgi apparatus abnormalities—suggesting impaired ATP production, oxidative stress, and disturbed trafficking/post‑translational processing as contributory mechanisms (“marked mitochondrial and Golgi ultrastructural defects… impaired ATP production… altered post‑translational processing and lipid homeostasis”) (Vacchiano 2024 summary of mechanistic studies) (vacchiano2024primarylateralsclerosis pages 3-5, vacchiano2024primarylateralsclerosis pages 7-9). Multimodal neuroimaging shows the motor cortex and corticospinal system as the core nodes of pathology, but with disease-related connectivity and atrophic changes extending to the corpus callosum and cerebellar systems (notably dorsal dentate nucleus), underscoring that PLS is a network-level disorder beyond the primary motor strip (Kleinerova 2024; Pioro 2020) (kleinerova2024supraandinfratentorial pages 1-2, pioro2020neuroimaginginprimary pages 1-5).

Mechanistically, cortical physiology in PLS differs from ALS: transcranial magnetic stimulation (TMS) frequently shows high cortical thresholds, prolonged central motor conduction times, and frank cortical inexcitability (absent MEPs), in contrast to the early cortical hyperexcitability typical of ALS; these TMS signatures serve as objective biomarkers of UMN dysfunction and aid diagnostic stratification (de Carvalho 2020; Vacchiano 2024) (carvalho2020neurophysiologicalfeaturesof pages 1-6, vacchiano2024primarylateralsclerosis pages 7-9). Together, evidence supports a model where selective vulnerability of corticospinal projection neurons (Betz cells) with impaired axonal maintenance and long-range connectivity results in progressive UMN signs and spasticity, with broader cerebello‑cortical and callosal disconnection contributing to gait, dexterity, bulbar involvement, and pseudobulbar affect (Kleinerova 2024; Vacchiano 2024) (kleinerova2024supraandinfratentorial pages 1-2, vacchiano2024primarylateralsclerosis pages 7-9).

Core Pathophysiology - Primary mechanisms: selective degeneration of upper motor neurons in primary motor cortex; distal axonopathy and Wallerian degeneration of corticospinal tracts; proteostasis disturbance with ubiquitinated/TDP‑43–positive inclusions in some cases; organellar dysfunction (mitochondria, Golgi) and associated bioenergetic/trafficking stress; network-level disconnection including transcallosal and cerebello‑cortical circuits (vacchiano2024primarylateralsclerosis pages 2-3, vacchiano2024primarylateralsclerosis pages 7-9, kleinerova2024supraandinfratentorial pages 1-2, vacchiano2024primarylateralsclerosis pages 1-2, pioro2020neuroimaginginprimary pages 1-5). - Dysregulated molecular pathways: protein homeostasis (ubiquitination/TDP‑43 aggregation); endolysosomal and intracellular trafficking (ALS2/alsin signaling); mitochondrial organization and oxidative stress; Golgi organization; glutamatergic–GABAergic balance alterations inferred from TMS physiology (vacchiano2024primarylateralsclerosis pages 3-5, vacchiano2024primarylateralsclerosis pages 2-3, carvalho2020neurophysiologicalfeaturesof pages 1-6). - Affected cellular processes: axonal maintenance and degeneration (Wallerian changes), synaptic transmission/excitability of corticospinal neurons, astroglial activation/gliosis (reflected by MRS myo‑inositol) (vacchiano2024primarylateralsclerosis pages 7-9, pioro2020neuroimaginginprimary pages 1-5, carvalho2020neurophysiologicalfeaturesof pages 1-6).

Recent developments (2023–2024) - Multimodal longitudinal neuroradiology in 43 PLS cases identified baseline structural and functional disconnection of cerebello‑cortical circuits and atrophy of vermis, cerebellar lobes, and dorsal dentate nucleus, alongside primary motor cortex atrophy and transcallosal degeneration—broadening PLS beyond a purely motor cortex–CST disorder (Journal of Neurology, online March 5, 2024; https://doi.org/10.1007/s00415-024-12261-z) (kleinerova2024supraandinfratentorial pages 1-2). - Contemporary overview synthesizing neuropathology and imaging confirms “knife-edge” precentral atrophy, CST hyperintensities, DTI FA reductions/increased diffusivity in motor tracts, increased functional connectivity (putatively compensatory), and FDG-PET hypometabolism of prefrontal/premotor cortices; emphasizes cortical inexcitability on TMS as a PLS signature and summarizes organellar defects and proteinopathy findings (J Clin Med, Jan 19, 2024; https://doi.org/10.3390/jcm13020578) (vacchiano2024primarylateralsclerosis pages 7-9, vacchiano2024primarylateralsclerosis pages 1-2). - Genetics: An international WGS analysis (120 PLS) reported enrichment of rare variants across MND/HSP genes without a single causal locus; repeat expansions identified in ATXN1 (12.2%) and NIPA1 (7.3%), with no C9orf72 expansion detected, supporting targeted but not universal genetic contributions and advising reconsideration of criteria discouraging genetic testing (medRxiv, posted Jul 23, 2023; https://doi.org/10.1101/2023.07.19.23292817) (kalia2023geneticandphenotype pages 1-3).

Current applications and real‑world implementations - Neurophysiology: TMS is used to quantify UMN dysfunction in PLS, demonstrating high resting motor thresholds, delayed CMCT, and frequent cortical inexcitability; these measures support diagnosis, monitoring, and differentiation from ALS where cortical hyperexcitability predominates (de Carvalho 2020; Vacchiano 2024) (carvalho2020neurophysiologicalfeaturesof pages 1-6, vacchiano2024primarylateralsclerosis pages 7-9). - Neuroimaging: Quantitative MRI (structural, DTI), SWI “motor band sign,” and FDG‑PET are implemented to detect motor cortex atrophy, CST degeneration, and network disconnection; imaging abnormalities correlate with clinical severity and can be present even in earlier “probable PLS” stages (Pioro 2020; Vacchiano 2024; Kleinerova 2024) (pioro2020neuroimaginginprimary pages 1-5, vacchiano2024primarylateralsclerosis pages 7-9, kleinerova2024supraandinfratentorial pages 1-2). - Biomarkers: Blood/CSF neurofilament levels are widely used in MND; most series suggest lower NfL in PLS than ALS and utility for differential diagnosis within UMN syndromes, though disease specificity is imperfect; current reviews stress the need for PLS‑specific biomarkers (Vacchiano 2024 overview) (vacchiano2024primarylateralsclerosis pages 1-2).

Expert opinions and analysis - Expert reviews emphasize that PLS is not confined to the precentral gyrus: consistent callosal and cerebellar network involvement is now demonstrable and likely contributes to gait, bulbar impairment, and pseudobulbar affect; radiological signatures can predate fulfillment of traditional duration‑based criteria (Kleinerova 2024; Pioro 2020) (kleinerova2024supraandinfratentorial pages 1-2, pioro2020neuroimaginginprimary pages 1-5). - Neurophysiology experts underscore cortical inexcitability as a relatively specific physiological signature of PLS (contrasting ALS hyperexcitability) and advocate integrating TMS with clinical and imaging data for diagnosis and longitudinal monitoring (de Carvalho 2020; Vacchiano 2024) (carvalho2020neurophysiologicalfeaturesof pages 1-6, vacchiano2024primarylateralsclerosis pages 7-9).

Relevant statistics and data - Genetics (WGS, 120 PLS): repeat expansions—ATXN1 12.2%, NIPA1 7.3%; overall, PLS harbors fewer clinically actionable MND‑associated variants than ALS, but up to ~11% may benefit from genetic testing (medRxiv 2023; https://doi.org/10.1101/2023.07.19.23292817) (kalia2023geneticandphenotype pages 1-3). - Imaging (43 PLS): baseline cerebello‑cortical structural disconnection and dorsal dentate atrophy; significant primary motor cortex atrophy and transcallosal degeneration; longitudinal follow‑up did not capture statistically significant progressive change over the interval studied (J Neurol 2024; https://doi.org/10.1007/s00415-024-12261-z) (kleinerova2024supraandinfratentorial pages 1-2).

Key Molecular Players - Genes/Proteins (HGNC): ALS2/alsin (endosomal trafficking; loss-of-function linked to corticospinal neuron vulnerability), TBK1, FIG4, UBQLN2, OPTN, NEFL, SPG11, SPG7 (genetic overlaps reported in PLS cohorts/case series); TDP‑43 (TARDBP) proteinopathy present in some PLS autopsies (Vacchiano 2024; Kalia 2023) (vacchiano2024primarylateralsclerosis pages 1-2, kalia2023geneticandphenotype pages 1-3, vacchiano2024primarylateralsclerosis pages 2-3). - Chemical entities (CHEBI): neurofilament proteins as fluid biomarkers; myo‑inositol increases on MRS consistent with gliosis (Vacchiano 2024) (vacchiano2024primarylateralsclerosis pages 7-9). - Cell Types (CL): corticospinal (UMN) pyramidal neurons including Betz cells; astrocytes (gliosis) (Vacchiano 2024; de Carvalho 2020) (vacchiano2024primarylateralsclerosis pages 2-3, carvalho2020neurophysiologicalfeaturesof pages 1-6). - Anatomical locations (UBERON): primary motor cortex (precentral gyrus), corticospinal tract, corpus callosum (mid‑body motor fibers), cerebellar cortex and dorsal dentate nucleus (Kleinerova 2024; Vacchiano 2024; Pioro 2020) (kleinerova2024supraandinfratentorial pages 1-2, vacchiano2024primarylateralsclerosis pages 7-9, pioro2020neuroimaginginprimary pages 1-5).

Biological Processes (GO annotations) - Axon degeneration (Wallerian degeneration) in corticospinal pathways (vacchiano2024primarylateralsclerosis pages 2-3). - Mitochondrial organization/ATP production and oxidative stress responses implicated by ultrastructural defects and patient cell metabolism (vacchiano2024primarylateralsclerosis pages 3-5, vacchiano2024primarylateralsclerosis pages 7-9). - Golgi organization/protein trafficking and post‑translational processing (vacchiano2024primarylateralsclerosis pages 3-5). - Protein aggregation/ubiquitination (TDP‑43–positive inclusions) (vacchiano2024primarylateralsclerosis pages 2-3). - Glutamatergic synaptic transmission/inhibitory‑excitatory balance alterations inferred from TMS physiology (carvalho2020neurophysiologicalfeaturesof pages 1-6, vacchiano2024primarylateralsclerosis pages 7-9). - Astroglial activation/gliosis (MRS myo‑inositol increases) (vacchiano2024primarylateralsclerosis pages 7-9).

Cellular Components - Cytoplasm (site of ubiquitinated/TDP‑43–positive inclusions) (vacchiano2024primarylateralsclerosis pages 2-3). - Mitochondria and Golgi apparatus (ultrastructural pathology) (vacchiano2024primarylateralsclerosis pages 3-5). - Axons of corticospinal neurons and interhemispheric callosal fibers (white matter microstructure changes on DTI) (kleinerova2024supraandinfratentorial pages 1-2, pioro2020neuroimaginginprimary pages 1-5). - Extracellular space and astroglial milieu (gliosis reflected by myo‑inositol) (vacchiano2024primarylateralsclerosis pages 7-9).

Disease Progression - Typical sequence: insidious onset of UMN signs in lower limbs (most common), progressing to involve upper limbs and bulbar regions; clinically characterized by spasticity, hyperreflexia, slow gait, pseudobulbar affect, with rare respiratory failure and overall slower course than ALS (Vacchiano 2024) (vacchiano2024primarylateralsclerosis pages 1-2). - Pathoanatomical cascade: Betz cell and layer‑V corticospinal neuron degeneration → transsynaptic/anterograde CST axon degeneration (Wallerian) → motor cortical atrophy and CST tractopathy; progressive network disconnection extends to callosal motor fibers and cerebro‑cerebellar pathways, compounding motor and affective dysfunctions (Kleinerova 2024; Pioro 2020) (kleinerova2024supraandinfratentorial pages 1-2, pioro2020neuroimaginginprimary pages 1-5).

Phenotypic Manifestations (selected HP terms and links to mechanisms) - Spasticity, hyperreflexia, brisk jaw jerk, Babinski sign (UMN dysfunction; motor cortex/CST degeneration) (Vacchiano 2024) (vacchiano2024primarylateralsclerosis pages 1-2). - Gait impairment and balance issues (CST and cerebello‑cortical disconnection; dentate atrophy) (Kleinerova 2024) (kleinerova2024supraandinfratentorial pages 1-2). - Bulbar symptoms and pseudobulbar affect (cortico‑bulbar and cerebellar network involvement) (Kleinerova 2024; Vacchiano 2024) (kleinerova2024supraandinfratentorial pages 1-2, vacchiano2024primarylateralsclerosis pages 1-2). - Cognitive/behavioral changes in a subset (frontotemporal network involvement; PET/MRI hypometabolism/atrophy) (Vacchiano 2024; Pioro 2020) (vacchiano2024primarylateralsclerosis pages 7-9, pioro2020neuroimaginginprimary pages 1-5).

Differentiation from ALS and HSP - Compared with ALS: PLS shows predominant UMN pathology with absent or minimal progressive LMN signs for years; TMS more often reveals cortical inexcitability (vs. ALS hyperexcitability); NfL generally lower in PLS than ALS; imaging shows pronounced precentral atrophy and CST tractopathy, but survival is longer and respiratory failure is uncommon (Vacchiano 2024; de Carvalho 2020; Pioro 2020) (vacchiano2024primarylateralsclerosis pages 7-9, carvalho2020neurophysiologicalfeaturesof pages 1-6, pioro2020neuroimaginginprimary pages 1-5). - Compared with HSP: HSP also presents with spastic paraparesis, but is typically earlier-onset, genetically defined (numerous SPG genes), and shows more selective long tract degeneration of the spinal cord with less prominent cortical atrophy; PLS tends to have more evident motor cortex changes, callosal involvement, cerebello‑cortical disconnection, and a broader cortical network footprint on advanced imaging (Vacchiano 2024) (vacchiano2024primarylateralsclerosis pages 1-2).

Evidence items with URLs and dates (selected quotes) - Neuropathology and inclusions: “diffuse brain atrophy with selective loss of pyramidal (Betz) neurons in layer V of the precentral gyrus… degeneration of the corticospinal tract… many cases display TDP‑43–positive, ubiquitinated cytoplasmic inclusions” (J Clin Med, Jan 19, 2024; https://doi.org/10.3390/jcm13020578) (vacchiano2024primarylateralsclerosis pages 2-3, vacchiano2024primarylateralsclerosis pages 1-2). - Organellar pathology: “Alsin/ALS2… apical dendrite disruption with vacuoles, and marked mitochondrial and Golgi ultrastructural defects, implying impaired ATP production, altered post‑translational processing and lipid homeostasis” (J Clin Med, Jan 19, 2024; https://doi.org/10.3390/jcm13020578) (vacchiano2024primarylateralsclerosis pages 3-5, vacchiano2024primarylateralsclerosis pages 7-9). - Neuroimaging network involvement: “Cerebello‑frontal, ‑temporal, ‑parietal, ‑occipital and cerebello‑thalamic structural disconnection… dorsal dentate was atrophic… significant primary motor cortex atrophy and inter‑hemispheric transcallosal degeneration” (J Neurol, Mar 5, 2024; https://doi.org/10.1007/s00415-024-12261-z) (kleinerova2024supraandinfratentorial pages 1-2). - TMS signature: “high cortical threshold and delayed central conduction times… frequent motor cortex inexcitability… sensitive to identify cortical dysfunction in PLS” (ALS & FTD, Nov 2020; https://doi.org/10.1080/21678421.2020.1837174) (carvalho2020neurophysiologicalfeaturesof pages 1-6); overview reaffirms cortical inexcitability as “a specific signature of PLS” (J Clin Med, 2024; https://doi.org/10.3390/jcm13020578) (vacchiano2024primarylateralsclerosis pages 7-9). - Genetics: “We identified variants in… FIG4, FUS, SPG7, SPG11, SQSTM1… repeat expansions in ATXN1 (12.2%) and NIPA1 (7.3%), but none in the C9orf72 and ATXN2 genes… up to 11%… might benefit from genetic testing” (medRxiv, Jul 23, 2023; https://doi.org/10.1101/2023.07.19.23292817) (kalia2023geneticandphenotype pages 1-3).

Gene/protein annotations with ontology terms (HGNC, GO) and structures (CL, UBERON) are summarized below.

Table: Compact ontology-grounded table mapping key entities/processes in Primary Lateral Sclerosis to ontologies and concise roles, with primary supporting evidence citations (context IDs) for use in a disease knowledge base.

Notes on biomarker landscape - Neurofilament (NfL) is an established axonal injury marker across MND; in UMN‑predominant syndromes like PLS, NfL tends to be lower than in classic ALS and may assist differential diagnosis, but specificity is limited and PLS‑targeted biomarker panels remain an unmet need (Vacchiano 2024; overview) (vacchiano2024primarylateralsclerosis pages 1-2).

Limitations and open questions - Despite accumulating imaging and physiology evidence, causal molecular drivers of selective UMN vulnerability in sporadic PLS remain unresolved. Longitudinal imaging trajectories can be slow, and TMS measures require standardized protocols across centers (Pioro 2020; Kleinerova 2024; de Carvalho 2020) (pioro2020neuroimaginginprimary pages 1-5, kleinerova2024supraandinfratentorial pages 1-2, carvalho2020neurophysiologicalfeaturesof pages 1-6).

References

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