What is the likely diagnosis of a child presenting with truncal paralysis, brisk lower limb reflexes, and plantar flexor response?

Likely Diagnosis: Spastic Cerebral Palsy

The clinical presentation of truncal paralysis (hypotonia) with brisk lower limb reflexes and plantar flexor responses in a child is most consistent with spastic cerebral palsy, particularly affecting the trunk and lower extremities. 1

Key Diagnostic Features

Upper Motor Neuron Pattern with Atypical Plantar Response

• Brisk (hyperactive) lower limb reflexes indicate upper motor neuron dysfunction, which is the hallmark of spastic cerebral palsy 1
• Plantar flexor responses (downgoing toes) do NOT exclude upper motor neuron lesions in children - this is a critical clinical pitfall 2
• In a landmark study of 57 children with spastic cerebral palsy, 80.8% demonstrated flexor plantar responses despite confirmed upper motor neuron damage, challenging the traditional expectation of extensor responses 2
• This paradoxical finding occurs because damage to the immature nervous system before complete myelination and connection development alters the typical reflex patterns 2

Truncal Involvement Pattern

• Truncal paralysis (hypotonia or weakness) combined with spastic lower extremities suggests bilateral involvement, likely representing spastic diplegia or quadriplegia 3, 4
• Truncal ataxia specifically indicates cerebellar vermian pathology when coordination is impaired, but truncal weakness with spasticity points to cerebral palsy 1
• The combination of truncal hypotonia with limb spasticity is well-documented in cerebral palsy, where different body regions may show varying tone abnormalities 1, 5

Immediate Diagnostic Workup

Essential Neuroimaging

• MRI of the brain is mandatory to identify characteristic patterns: white matter injury (56% of cases), cortical/deep gray matter lesions affecting basal ganglia or thalamus (18%), or brain maldevelopments (9%) 1, 3
• MRI findings combined with clinical examination achieve >95% diagnostic accuracy for cerebral palsy 3

Motor Assessment Tools

• Hammersmith Infant Neurological Examination (HINE) should be performed - scores <57 at 3 months have 96% predictive value for cerebral palsy 3, 4
• For infants 3-5 months corrected age, General Movements Assessment (Prechtl method) showing absent fidgety movements has 95-98% sensitivity 3

Clinical History Requirements

• Document pregnancy complications, prematurity, birth asphyxia, neonatal encephalopathy, intrauterine growth restriction, or genetic abnormalities 1, 4
• Assess developmental milestones - inability to sit by 9 months or early hand asymmetry are red flags 1, 3

Critical Differential Considerations

Rule Out Progressive Disorders

• Cerebral palsy is non-progressive by definition - any acute neurological deterioration requires urgent neuroimaging to exclude stroke, hemorrhage, or spinal cord compression 5
• Measure serum creatine phosphokinase (CK) to exclude Duchenne muscular dystrophy, which presents with weakness and hypotonia but typically shows diminished (not brisk) reflexes 1

Other Conditions to Exclude

• Spinal cord pathology can present with truncal weakness and lower limb spasticity - MRI spine may be needed if cranial imaging is normal 1
• Metabolic disorders or mitochondrial disease should be considered if developmental regression occurs 1

Classification and Severity Assessment

Motor Type Documentation

• Spasticity accounts for 85-91% of cerebral palsy cases and is the most likely motor type given brisk reflexes 3, 4
• Document topographical distribution: bilateral diplegia (37% of cases) or quadriplegia (24%) most consistent with truncal and lower limb involvement 5
• Use Gross Motor Function Classification System (GMFCS) to standardize severity from Level I (walks independently) to Level V (requires wheelchair) 5

Mandatory Screening for Comorbidities

• Epilepsy screening (35% prevalence in cerebral palsy) 3, 5
• Vision assessment (11% affected) and hearing evaluation (4% affected) 5
• Intellectual disability screening (49% prevalence) 3, 5
• Hip displacement monitoring (28% develop this complication) 5
• Chronic pain assessment (75% experience chronic pain) 5

Immediate Management Steps

• Refer immediately to cerebral palsy-specific early intervention focusing on postural control, movement training, and parent education 3
• Early intervention before 6 months corrected age optimizes neuroplasticity and functional outcomes 1, 3
• Provide family psychological support and education about the diagnosis 3