What are RMS (Rheumatoid Myositis Syndrome) and PPMs (Parts Per Million) in a medical context?

Understanding RMS and PPMs in Multiple Sclerosis

In the context of multiple sclerosis (MS), RMS refers to Relapsing-Remitting Multiple Sclerosis, which is characterized by clearly defined relapses with full or partial recovery, and represents approximately 85% of MS cases at onset.

Types of Multiple Sclerosis

Relapsing-Remitting MS (RMS)

• Characterized by relapses (episodes of new or worsening neurological symptoms) followed by periods of remission with stable neurological disability 1
• Represents approximately 85% of MS cases at diagnosis 1, 2
• Features:
  • Active inflammation evidenced by contrast-enhancing lesions on MRI
  • Relapses at onset with stable neurologic disability between episodes
  • May eventually progress to Secondary Progressive MS (SPMS)

Primary Progressive MS (PPMS)

• Characterized by progressive neurological damage from disease onset rather than distinct relapses and remissions 1
• Represents approximately 15% of MS cases 1, 2
• Features:
  • Progressive course from disease onset
  • Less inflammatory activity on MRI compared to RMS
  • Lower incidence of focal white matter lesions 3

Diagnostic Criteria for MS

The McDonald criteria and MAGNIMS consensus guidelines are used for MS diagnosis, requiring evidence of:

1. Dissemination in Space (DIS): Evidence of damage in different parts of the nervous system

   • Presence of at least one typical MS lesion in at least two characteristic regions 1
   • Characteristic regions include periventricular, juxtacortical, infratentorial, and spinal cord areas 4

2. Dissemination in Time (DIT): Evidence of damage occurring at different times

   • Can be demonstrated by new T2 lesions on follow-up MRI or simultaneous presence of enhancing and non-enhancing lesions 4

3. No better explanation for the clinical presentation 1

Diagnostic Differences Between RMS and PPMS

According to the 2017 McDonald criteria 4:

MS Subtype	Dissemination in Space	Dissemination in Time
RMS	One or more lesions in at least two characteristic locations*	New T2/gadolinium-enhancing lesions on follow-up MRI OR simultaneous presence of enhancing and non-enhancing lesions
PPMS	Two of the following: ≥1 lesion in characteristic MS regions, ≥2 spinal cord lesions, or CSF evidence of oligoclonal bands	1 year of disease progression (retrospectively or prospectively determined)

*Characteristic locations: periventricular, juxtacortical, infratentorial, and spinal cord

Recent research suggests that a unified set of diagnostic criteria for both RMS and PPMS may be feasible, as the 2017 McDonald RMS criteria performed well in PPMS diagnosis when properly applied 5.

MRI in MS Diagnosis

MRI is crucial for MS diagnosis and should be performed to:

• Support clinical diagnosis
• Rule out other disorders
• Replace certain clinical criteria in many patients 4

Key MRI findings include:

• Periventricular lesions ("Dawson's fingers")
• Juxtacortical lesions
• Infratentorial lesions
• Spinal cord lesions
• Gadolinium-enhancing lesions indicating active inflammation 1

PPMs in MS Context

While "PPMs" typically stands for "Parts Per Million" in scientific contexts, this abbreviation is not commonly used in MS literature. In MS clinical practice, relevant abbreviations might include:

• PPMS - Primary Progressive Multiple Sclerosis
• PMBCs - Peripheral Mononuclear Blood Cells (relevant in MS research)
• PML - Progressive Multifocal Leukoencephalopathy (a serious complication of some MS treatments)

Pathophysiological Differences

Recent research suggests that while RMS and PPMS have distinct clinical presentations, their underlying pathophysiology may have more similarities than previously thought:

• Both RMS and PPMS show evidence of inflammation, though the pattern differs 3
• Soluble biomarkers and post-mortem pathology do not clearly differentiate between RMS and PPMS diagnoses 6
• Both forms involve T-cell and B-cell mediated inflammation, though PPMS may have less prominent blood-brain barrier disruption 3

Clinical Implications

Understanding the distinction between RMS and PPMS is critical for:

• Appropriate treatment selection
• Prognosis discussions
• Clinical trial eligibility

However, emerging evidence suggests that the traditional separation between these forms may be more of a clinical spectrum rather than distinct pathophysiological entities 6, 5.