Clinical Phenotypes

1. The Clinical Continuum of MS

Modern MS is best understood as a single biological process expressed along a continuum of clinical states. The historical splitting of cases into discrete «courses» remains useful for prognosis, communication and trial design, but does not correspond to fundamentally distinct diseases.

2. Clinically Isolated Syndrome (CIS)

A first clinical episode of CNS demyelination, lasting >24 h, in the absence of fever or infection, in someone not previously known to have MS. CIS is the prototypical «single-attack» presentation; with appropriate MRI and CSF support it now often is diagnostic of MS under McDonald 2017.

• Conversion to MS by 10 years: ~80% if any T2 lesion on baseline MRI; ~20% if MRI normal (Fisniku et al., Brain 2008).
• CHAMPS (Jacobs et al., NEJM 2000) and ETOMS (Comi et al., Lancet 2001) trials demonstrated that early IFN-β reduces conversion to clinically definite MS by ~40%.
• Modern practice: treat CIS with high MRI burden as MS at diagnosis.

3. Radiologically Isolated Syndrome (RIS)

MRI lesions meeting MS dissemination-in-space criteria, found incidentally in someone with no clinical history of demyelinating events. ~30% convert to a clinical first event within 5 years; risk is higher with infratentorial or spinal-cord lesions, gadolinium-enhancing lesions, or positive CSF OCBs.

RIS is not yet formally MS (McDonald 2017), but two recent randomised trials (TERIS with teriflunomide, ARISE with dimethyl fumarate; Lebrun-Frenay et al., JAMA Neurol 2023; Okuda et al., Ann Neurol 2023) showed early DMT delays first clinical event — raising the prospect of treating MS before symptom onset.

4. Relapsing-Remitting MS (RRMS)

The commonest course at onset. Defined by clearly delineated relapses with full or partial recovery, separated by clinical stability. Classical features:

• Relapse — new neurological symptoms or worsening of previous symptoms, lasting >24 h, in the absence of fever or infection (a «true» relapse); pseudo-relapses (Uhthoff phenomenon, infection-driven worsening) must be distinguished.
• Annualised relapse rate (ARR) — modern untreated cohorts ~0.4–0.6; on placebo arms of pivotal trials ~0.3–0.4 (a falling secular trend).
• Recovery — ~85% of relapses recover >75% of deficit within 12 weeks; partial residual is the rule with longer disease duration.
• Activity — relapses + new MRI lesions defines «active» disease in Lublin terminology.

In the pre-DMT era, ~60% of RRMS patients transitioned to SPMS within 20 years; modern long-term registries suggest this fraction is now substantially lower, particularly in patients started early on high-efficacy DMTs (Brown et al., JAMA 2019; Spelman et al., Neurology 2021).

5. Secondary Progressive MS (SPMS)

SPMS is a phase of gradual, irreversible disability accumulation following an initial RRMS course, with or without superimposed relapses or MRI activity. Diagnosis is retrospective: at least 6–12 months of progression, independent of any relapse, in a previously RRMS patient.

• Median time RRMS → SPMS — ~20 years (untreated, London-Ontario cohort); shorter in older-onset patients.
• Pathology — axonal loss, cortical demyelination, meningeal B-cell follicles, slow expansion of chronic active (paramagnetic-rim) lesions.
• DMT efficacy — modest. Siponimod showed a 21% relative reduction in 3-month confirmed disability progression in EXPAND (Kappos et al., Lancet 2018), driven mostly by «active» SPMS.

Modern terminology distinguishes active (relapses or new MRI lesions) versus not-active, and with progression versus without progression, giving four SPMS subtypes that determine eligibility for specific DMTs.

6. Primary Progressive MS (PPMS)

~10–15% of MS cases. Progressive disability accumulation from onset, without preceding relapses. Characteristic features:

• Older onset — mean age ~40, vs ~30 for RRMS.
• Sex ratio — closer to 1:1.
• Typical syndrome — progressive spastic paraparesis with bladder dysfunction, secondary to spinal-cord predominant disease (the «chronic progressive myelopathy» presentation).
• MRI — fewer cerebral T2 lesions than RRMS; cord atrophy prominent.
• CSF — OCBs in ~85–90% (slightly less than RRMS).

ORATORIO (Montalban et al., NEJM 2017) was the first to demonstrate DMT efficacy in PPMS: ocrelizumab reduced 12-week confirmed disability progression by 24% over 120 weeks; the effect was greatest in younger patients with gadolinium-enhancing lesions, suggesting that anti-CD20 works in PPMS via the same mechanism it works in RRMS — suppression of inflammatory activity.

7. The Lublin/Reingold/Kappos 2014 Phenotype Framework

The 2014 revision of MS clinical phenotypes (Lublin et al., Neurology 2014) replaced the older 1996 classification (which included «progressive-relapsing MS») with a two-axis description:

The framework abandons «progressive-relapsing MS» and unifies progressive courses (primary & secondary) by their shared activity/progression axes. It is now the basis of regulatory labelling in major jurisdictions.

8. PIRA — Progression Independent of Relapse Activity

Modern long-term registries have shown that, even in apparently RRMS patients with very few relapses, a slow accumulation of confirmed disability occurs between relapses. This phenomenon — PIRA — has transformed thinking about progression:

• Kappos et al., JAMA Neurol 2020: pooled OPERA-I/II analysis — ~80% of confirmed disability accumulation in RRMS was independent of relapses.
• Implication: progression is not exclusive to SPMS; it is a continuous process underlying RRMS, masked by relapse-driven recovery.
• Mechanism: chronic compartmentalised inflammation, paramagnetic-rim lesions, cortical pathology, slow neurodegeneration.
• Therapeutic target: BTK inhibitors are designed primarily for PIRA / smouldering disease.

9. Paediatric, Late-Onset & Aggressive MS

• Paediatric MS — ~3–5% of cases. Almost always RRMS; relapses ~2–3× more frequent than in adults; cognitive impact disproportionate.
• Late-onset MS (> 50 yr) — ~5%. Higher proportion of PPMS; less responsive to DMT; more comorbidity; differential against vascular small-vessel disease and CADASIL.
• Aggressive MS — rapid disability accrual (≥ EDSS 4 within 5 years). Definition heterogeneous. Trial-based criteria from the European MS Platform inform treatment escalation. Increasingly treated with high-efficacy DMTs from the outset (induction strategy) or autologous haematopoietic stem-cell transplantation (AHSCT, MIST trial).
• Tumefactive MS — large (> 2 cm) lesions with mass effect, ring enhancement, sometimes mimicking glioma. May respond dramatically to corticosteroids; biopsy reveals demyelinating, not neoplastic, pathology.
• Marburg variant — rare, fulminant, often fatal subacute demyelination.
• Balo’s concentric sclerosis — concentric rings of demyelination on MRI; pathologically MS, often aggressive.

10. The Differentials — NMOSD & MOGAD

Two distinct demyelinating diseases were carved out of «MS» in the modern era and must be excluded before MS is diagnosed:

Misdiagnosis of NMOSD or MOGAD as MS exposes patients to ineffective and sometimes harmful therapy. Modern practice is to test AQP4-IgG and MOG-IgG (cell-based assays) at presentation in any «MS-like» optic-neuritis or myelitis with atypical features.