Central vs Peripheral Hypotonia: A Bedside Exam Framework
A Systematic Physical Examination Approach to Localize the Floppy Infant and Guide Targeted Workup
- Define hypotonia and distinguish it from weakness, differentiating reduced tone from reduced strength at the bedside
- Perform five key bedside maneuvers — pull-to-sit, ventral suspension, vertical suspension, scarf sign, and popliteal angle — to systematically assess tone
- Identify the clinical features that localize hypotonia to central (upper motor neuron) versus peripheral (lower motor neuron) etiologies
- Apply a diagnostic algorithm using deep tendon reflexes, strength assessment, and associated features to narrow the differential
- Describe the most common central and peripheral causes of hypotonia by age group and their distinguishing clinical features
- Construct a tiered laboratory and imaging workup based on localization, prioritizing high-yield investigations
Central vs Peripheral Hypotonia
A Bedside Exam Framework
Medaptly · Pediatric Neurology Series · April 2026
Agenda
Defining Hypotonia
Tone versus strength — the critical distinction
Tone vs Strength: Precise Definitions
- Definition: Resistance to passive stretch at rest
- Assessment: Passive limb movement by examiner
- Patient role: Relaxed, NOT actively contracting
- Low tone = “Floppy,” rag-doll feel, increased joint range
- Definition: Force generated by active muscle contraction
- Assessment: Observe antigravity movement, resistance to push
- Patient role: Active participation required
- Weakness = Reduced antigravity movement, cannot resist force
Key Rule: Hypotonia + NORMAL strength → Central cause likely. Hypotonia + WEAKNESS → Peripheral cause likely. This single observation drives the entire workup.
Hypotonia by the Numbers
Clinical Pearl: Central causes are FAR more common than peripheral causes. The floppy infant is much more likely to have a brain-based problem than a muscle disease. Don’t jump to the neuromuscular workup before ruling out central causes.
Neuroanatomical Localization
Where along the neuraxis is the lesion?
Localization Along the Neuraxis
Central (UMN)
- Cerebral cortex
- Basal ganglia
- Cerebellum
- Brainstem
- Spinal cord (upper)
Hallmark: Hypotonia + normal/brisk reflexes + no atrophy
Peripheral (LMN)
- Anterior horn cell
- Peripheral nerve
- Neuromuscular junction
- Muscle
Hallmark: Hypotonia + absent reflexes + weakness + atrophy
The Bedside Examination
Five key maneuvers for systematic tone assessment
Maneuver 1: Pull-to-Sit (Traction Response)
Technique
Infant supine. Grasp hands. Pull slowly to sitting position. Observe arm traction, head control, and sitting posture.
| Component | Normal | Central | Peripheral |
|---|---|---|---|
| Arm traction | Slight flexion | Reduced | No resistance |
| Head lag | Minimal | Present, some effort | Complete, no effort |
| Sitting | Head upright briefly | Flops but tries | Flops completely |
Key Distinction: In central hypotonia, the infant may have head lag but intermittently attempts to right the head (effort without success). In peripheral hypotonia, there is NO effort — the head hangs like dead weight.
Maneuver 2: Ventral Suspension (Landau)
Technique
Hold infant prone, one hand under chest/abdomen. Observe head, trunk, and limb posture in suspension.
Central Pattern
- “Inverted U” draping over hand
- Head hangs but MAY briefly lift
- Limbs dangle but show intermittent flexion
- Key: Some antigravity effort present
Peripheral Pattern
- Complete “rag doll” draping
- Head hangs with NO effort to lift
- Limbs hang vertically, motionless
- Key: No antigravity effort at all
Maneuver 3: Vertical Suspension
Technique
Hold infant upright under the arms. Assess shoulder girdle tone (slip-through) and lower limb posture.
What to Observe
- Slip-through sign: Infant slides through hands = reduced shoulder girdle tone
- Leg posture: Flexed (normal), scissoring (central), dangling (peripheral)
- Scissoring: Legs cross due to adductor spasticity — strongly suggests cerebral palsy
Central Clue: Scissoring of legs during vertical suspension in an otherwise hypotonic infant = evolving spastic cerebral palsy. This is the earliest sign of the transition from hypotonic to spastic phenotype.
Maneuver 4: Scarf Sign
Technique
Infant supine. Take one hand and draw the arm across the chest toward the opposite shoulder. Note elbow position relative to midline.
| Finding | Interpretation | Grade |
|---|---|---|
| Elbow does not reach midline | Normal passive tone | Normal |
| Elbow reaches midline | Mildly reduced passive tone | Mild hypotonia |
| Elbow passes midline | Moderately reduced tone | Moderate |
| Arm wraps around neck | Severely reduced passive tone | Severe hypotonia |
Note: The scarf sign assesses PASSIVE tone only. It confirms hypotonia and grades severity but does not differentiate central from peripheral by itself. Combine with strength assessment for localization.
Maneuver 5: Popliteal Angle
Technique
Infant supine. Flex hip to 90°. Extend knee until resistance. Measure angle between thigh and lower leg.
| Popliteal Angle | Interpretation |
|---|---|
| 80–100° | Normal (term neonate) |
| 110–130° | Mild hypotonia |
| 140–160° | Moderate hypotonia |
| >160° | Severe hypotonia |
Gestational Age Matters: Premature infants have physiologically wider popliteal angles. Always interpret against corrected gestational age norms. A 28-week preterm at 40 weeks corrected may still have wider angles than a term-born infant.
Deep Tendon Reflexes — The Master Discriminator
The Single Most Important Localizing Feature: DTRs preserved or brisk = CENTRAL. DTRs absent or diminished = PERIPHERAL.
Central Hypotonia
DTRs preserved or hyperactive. Clonus may be present. Plantar responses may be extensor (Babinski positive).
Peripheral Hypotonia
DTRs absent or diminished. No clonus. Plantar responses may be absent.
Exception: NMJ disorders (myasthenia) may have preserved reflexes early. But NMJ disease is rare in neonates and is typically associated with fluctuating weakness and fatigability rather than constant hypotonia.
Central vs Peripheral
The master comparison and associated features
Master Comparison Table
| Feature | Central Hypotonia | Peripheral Hypotonia |
|---|---|---|
| Tone | Reduced | Reduced |
| Strength | Relatively preserved | Reduced (weak) |
| DTRs | Normal / hyperactive | Absent / diminished |
| Mental status | Often altered / encephalopathic | Usually alert |
| Seizures | Common | Uncommon |
| Dysmorphic features | Often present | Usually absent |
| Fasciculations | Absent | Present (AHC disease) |
| Muscle bulk | Normal | Wasted / atrophied |
Central Hypotonia — Associated Clues
- Encephalopathy: Altered consciousness, poor visual attention, lethargy or irritability. The infant seems “not there.”
- Seizures: Neonatal seizures strongly suggest a central cause. Peripheral causes rarely present with seizures.
- Dysmorphic features: Suggest chromosomal/genetic syndrome (Down, Prader-Willi, Smith-Lemli-Opitz).
- Cortical thumb / fisting: Persistent thumb adduction or hand clenching beyond 3 months = UMN sign.
- Abnormal head size: Microcephaly (brain malformation) or macrocephaly (hydrocephalus, storage disease).
Peripheral Hypotonia — Associated Clues
- “Bright-eyed floppy baby”: Alert, visually engaged, socially interactive but profoundly weak. Hallmark of peripheral pathology.
- Tongue fasciculations: Visible twitching at rest. Pathognomonic for anterior horn cell disease (SMA). Observe with tongue relaxed in mouth.
- Paradoxical breathing: Belly rises while chest collapses (bell-shaped chest). Indicates diaphragmatic weakness (SMA type 1).
- Feeding difficulty: Weak suck, poor latch, nasal regurgitation. Suggests bulbar weakness.
- Arthrogryposis: Congenital joint contractures indicate in-utero neuromuscular disease.
Differential Diagnosis
Common causes organized by localization
Central Causes of Hypotonia
HIE / Perinatal Injury
Most common neonatal cause. History of birth asphyxia, low Apgar, encephalopathy.
Chromosomal Disorders
Down syndrome, Prader-Willi, Williams, Smith-Lemli-Opitz. Dysmorphic features present.
Brain Malformations
Lissencephaly, holoprosencephaly, corpus callosum agenesis. Often with seizures.
Metabolic Disorders
Aminoacidopathies, organic acidurias, peroxisomal, mitochondrial. Progressive.
Peripheral Causes by Motor Unit Level
| Level | Key Diagnoses | Distinguishing Feature |
|---|---|---|
| Anterior horn cell | SMA (types 0–3) | Fasciculations, areflexia, alert, bell chest |
| Peripheral nerve | CMT, hypomyelinating neuropathy | Distal > proximal weakness, sensory loss |
| NMJ | Transient neonatal myasthenia, CMS | Fluctuating weakness, ptosis, fatigability |
| Muscle | Congenital myopathy, CMD, Pompe | Proximal weakness, elevated CK (variable), facial weakness |
2026 Update: Newborn screening for SMA is now universal in the US and expanding globally. SMA should be identified presymptomatically in most cases. If a floppy infant has NOT been screened, SMA genetic testing is the first peripheral workup step.
Prader-Willi Syndrome — The Must-Not-Miss
Neonatal Presentation
- Severe hypotonia at birth
- Poor feeding, weak cry (gavage often needed)
- Genital hypoplasia (cryptorchidism in males)
- Characteristic facies (may be subtle in newborn)
- DTRs present but may be reduced
Diagnosis
- Test: DNA methylation analysis (chromosome 15q11-q13)
- Sensitivity: >99% detection rate
- When to order: ANY hypotonic neonate with feeding difficulty and no clear alternative diagnosis
Low threshold: Order methylation studies early. PWS is treatable (growth hormone) and early diagnosis improves outcomes.
SMA — The Most Common Peripheral Cause
Clinical Recognition
- Alert, bright eyes — normal cognitive engagement despite profound weakness
- Tongue fasciculations — pathognomonic for AHC disease
- Paradoxical breathing — bell-shaped chest, diaphragmatic breathing
- Areflexia — absent DTRs throughout
Treatment Emergency: SMA is now treatable. If NBS not done or pending, order SMN1 deletion testing STAT. Every day before treatment = motor neuron loss.
Tiered Diagnostic Workup
From bedside localization to targeted investigations
Central Hypotonia Workup
- Brain MRI (highest yield single test)
- Chromosomal microarray (CMA)
- DNA methylation (Prader-Willi)
- Metabolic screen: amino acids, organic acids, lactate, ammonia
- Thyroid function (treatable!)
- Whole exome/genome sequencing
- EEG (if seizures suspected)
- VLCFA (peroxisomal disorders)
- Lysosomal enzymes
- Mitochondrial DNA testing
Peripheral Hypotonia Workup
- SMA genetic testing (SMN1 deletion — URGENT)
- Creatine kinase (CK)
- Acid alpha-glucosidase (Pompe disease)
- NBS result review (SMA, Pompe screening)
- Thyroid function
- EMG / NCS (localization within motor unit)
- Neuromuscular gene panel
- Muscle biopsy (if gene panel negative)
- Repetitive nerve stimulation (NMJ)
- Whole exome sequencing
CK Level Interpretation Guide
| CK Level | Suggests | Key Diagnoses |
|---|---|---|
| Normal | AHC, nerve, or NMJ pathology | SMA, neuropathy, CMS, myasthenia |
| Mildly elevated (2–10x) | Structural myopathy | Congenital myopathy, some CMDs |
| Markedly elevated (>10x) | Dystrophic process or Pompe | DMD, severe CMD, Pompe disease |
Pompe Disease Alert: Massively elevated CK + hypotonia + cardiomegaly in an infant = Pompe disease until proven otherwise. Check acid alpha-glucosidase STAT. Enzyme replacement therapy is available and time-sensitive.
Complete Diagnostic Algorithm
Confirm Hypotonia
5 maneuvers: pull-to-sit, ventral, vertical, scarf, popliteal
Assess Strength
Preserved → Central
Weak → Peripheral
Check DTRs
Brisk → Central
Absent → Peripheral
Tiered Workup
Central or peripheral pathway based on localization
At EVERY stage, check for treatable diagnoses first: SMA (gene therapy available), Pompe disease (ERT available), congenital hypothyroidism (thyroid hormone), pyridoxine-dependent epilepsy (B6 trial). Time-sensitive treatments must not be delayed by the workup.
Benign Congenital Hypotonia
Diagnostic Criteria
- Low tone with NORMAL strength
- Normal or brisk DTRs
- No dysmorphic features
- Normal brain MRI
- Normal metabolic and genetic screening
Safety Requirements
- Diagnosis of EXCLUSION only
- Serial developmental follow-up mandatory
- Revise diagnosis if regression occurs
- Milestones at late-normal range (not absent)
- Tone gradually improves over 2–3 years
Key Takeaways
Tone ≠ Strength
Hypotonia + preserved strength = central. Hypotonia + weakness = peripheral. This distinction drives the entire workup.
DTRs Are the Master Discriminator
Brisk reflexes = central. Absent reflexes = peripheral. Test them carefully in every hypotonic infant.
Central Causes Dominate (60–80%)
Don’t jump to the neuromuscular workup. Brain MRI and chromosomal microarray are often the highest-yield first tests.
Identify Treatable Diagnoses First
SMA, Pompe disease, hypothyroidism, and Prader-Willi are treatable and time-sensitive. Screen early.
Five Maneuvers, Five Minutes, Accurate Localization
Pull-to-sit, ventral suspension, vertical suspension, scarf sign, and popliteal angle provide localization at the bedside.
References
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- Harris SR. Congenital hypotonia: clinical and developmental assessment. Dev Med Child Neurol. 2008;50(12):889-892. DOI
- Sparks SE. Neonatal hypotonia. Clin Perinatol. 2015;42(2):363-371. DOI
- Bönnemann CG, et al. Diagnostic approach to the congenital muscular dystrophies. Neuromuscul Disord. 2014;24(4):289-311. DOI
- Leyenaar J, et al. Schematic approach to hypotonia in infancy. CMAJ. 2005;172(4):457-459. DOI
- Paro-Panjan D, Neubauer D. Congenital hypotonia: is there an algorithm? J Child Neurol. 2004;19(6):439-442. DOI
- Schroth MK, et al. Spinal muscular atrophy update in best practices. Neurol Clin Pract. 2025;15:e200374. DOI
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