During a routine well-child visit, a 3-year-old boy is assessed for gross motor milestones. The child can walk up stairs with alternating feet and jump with both feet together. Which is the most common cause of failure to achieve these milestones by age 3 years?

Explanation

## Most Common Cause of Gross Motor Delay in Early Childhood **Key Point:** Cerebral palsy (CP) is the most common motor disorder of childhood and the leading cause of gross motor developmental delay, affecting approximately 2–3 per 1000 live births. ### Definition and Epidemiology - **Cerebral palsy:** Non-progressive disorder of movement and posture due to lesion/dysfunction of the immature brain - **Incidence:** 2–3 per 1000 live births globally; higher in low-income settings - **Timing of injury:** Prenatal (80%), perinatal (10%), postnatal (10%) - **Most common type:** Spastic diplegia (lower limbs > upper limbs) ### Pathophysiology 1. Antenatal or perinatal brain injury (hypoxia, infection, prematurity, intracranial hemorrhage) 2. Disruption of motor pathways in basal ganglia, cerebellum, or corticospinal tracts 3. Results in abnormal tone, reflexes, and motor control 4. Non-progressive but effects become more apparent with age as motor demands increase ### Clinical Presentation by Age - **0–6 months:** Abnormal tone (hypertonia or hypotonia), asymmetrical movements, poor head control - **6–12 months:** Delayed rolling, sitting, or standing; persistent primitive reflexes - **12–24 months:** Delayed walking, stiffness, toe-walking, asymmetrical gait - **2–3 years:** Inability to climb stairs with alternating feet, poor balance, scissor gait **Clinical Pearl:** Scissor gait (hip adduction, internal rotation, knee flexion) and toe-walking are hallmark signs of spastic diplegia in toddlers and are often the presenting complaint at age 2–3 years. ### Why CP Is Most Common **High-Yield:** CP is 10–50 times more common than muscular dystrophy, 5–10 times more common than developmental dysplasia of the hip (DDH) as a cause of motor delay, and much more common than rickets. It is the leading cause of physical disability in children. ### Comparison with Other Causes | Condition | Prevalence | Onset | Key Feature | Progression | |-----------|-----------|-------|-------------|-------------| | **Cerebral palsy** | 2–3/1000 | Birth/early infancy | Non-progressive motor disorder, abnormal tone | Static; functional limitations increase with age | | Developmental dysplasia of hip | 1–3/1000 | Birth | Unilateral/bilateral hip instability | Progressive if untreated; treatable early | | Rickets | Variable (endemic in some regions) | 6–24 months | Bone deformity, hypocalcemia, muscle weakness | Progressive; reversible with treatment | | Muscular dystrophy | 1–3/10,000 | 2–5 years typically | Progressive weakness, elevated CK, muscle wasting | Progressive and degenerative | ### Diagnostic Approach - **Clinical assessment:** Tone, reflexes, gait pattern, asymmetry - **Imaging:** MRI brain to identify lesion (periventricular leukomalacia, intraventricular hemorrhage, cortical malformation) - **Exclusion:** CK level (normal in CP, elevated in muscular dystrophy), imaging of hips (normal in CP) [cite:Nelson Pediatrics 21e Ch 612]