What is the next step in management for a patient with a wrist injury, who is experiencing involuntary clonus and significant weakness in wrist movement, despite proper alignment with a sling, and has resolved injection site reactions, with good pinching ability but very weak wrist movement?

Management of Post-Injection Wrist Weakness with Clonus

The immediate priority is urgent neurological evaluation with MRI of the cervical spine to rule out compressive myelopathy, as involuntary clonus combined with significant weakness represents upper motor neuron pathology requiring prompt diagnosis and potential surgical intervention.

Critical Clinical Recognition

The combination of involuntary clonus at the wrist with marked weakness is a red flag for cervical spinal cord pathology, not a simple injection complication 1. This presentation mimics action-induced tremors but represents a fundamentally different and more serious neurological condition 1.

Key Distinguishing Features to Assess:

• Clonus characteristics: Involuntary rhythmic muscle contractions at 5-8 Hz frequency, triggered by wrist extension in an outstretched arm 1, 2
• Associated upper motor neuron signs: Check for spasticity, hyperreflexia, and extensor plantar responses bilaterally 1
• Sensory examination: Assess for sensory level or dermatomal changes suggesting cord compression 1
• Duration and progression: Acute onset versus gradual worsening helps differentiate injection injury from underlying myelopathy 1

Immediate Diagnostic Workup

Order urgent MRI of the cervical spine to evaluate for compressive myelopathy, which is the most likely underlying cause when clonus and corticospinal tract signs are present 1. The resolved injection site erythema and swelling indicate the local injection reaction has cleared, making direct injection nerve injury less likely as the primary pathology 3.

Differential Considerations:

• Cervical compressive myelopathy: Most likely given bilateral upper motor neuron signs 1
• Injection-related radial nerve palsy: Would present with wrist drop and weakness but not with clonus or spasticity 4
• Central cord syndrome: Consider if there was trauma or rapid onset 5

Management Algorithm

If MRI Confirms Cervical Myelopathy:

Urgent neurosurgical consultation for decompression within 48-96 hours is indicated, as early surgery (within 96 hours) improves neurological outcomes in spinal cord injury 5. The presence of clonus indicates established upper motor neuron dysfunction requiring definitive treatment 2.

If Injection-Related Nerve Injury is Confirmed:

• Conservative management initially: Most injection-related radial nerve palsies recover spontaneously (13 of 56 cases in one series) 4
• Neurolysis consideration: If no recovery by 3 months, surgical neurolysis shows good outcomes (5 of 7 patients achieved full recovery within 1 year) 4
• Tendon transfer surgery: Reserved for cases without recovery after neurolysis 4

If Spasticity is the Primary Problem:

Botulinum toxin injection (400 units into spastic flexors) combined with 4 weeks of surface EMG biofeedback therapy has demonstrated both short- and long-term improvement in wrist-hand extensor performance 6. However, this approach is only appropriate after ruling out compressive myelopathy requiring surgical intervention 6.

Critical Pitfalls to Avoid

• Do not mistake clonus for tremor: Clonus is rhythmic, involuntary, and indicates upper motor neuron pathology requiring urgent workup 1, 2
• Do not attribute all symptoms to the injection: The resolved injection site inflammation suggests the current neurological findings represent underlying pathology rather than direct injection injury 3
• Do not delay imaging: Cervical myelopathy requires prompt diagnosis, as surgical timing affects outcomes 5, 1
• Do not initiate spasticity treatment before diagnosis: Botulinum toxin or other spasticity treatments should only follow definitive diagnosis and exclusion of surgically correctable lesions 6

Symptomatic Management Pending Workup

While awaiting MRI and specialist evaluation:

• Maintain wrist support: Continue sling use to prevent contracture development 7
• Avoid aggressive passive stretching: This can worsen spasticity and trigger more clonus 2
• Consider baclofen or cold application: These can temporarily reduce clonus severity if causing significant discomfort 2
• Monitor for progression: Any worsening weakness, development of bowel/bladder dysfunction, or gait disturbance requires emergency evaluation 5

The preserved pinching ability suggests some hand intrinsic function remains intact, but the severe wrist weakness with clonus mandates urgent investigation for treatable spinal cord pathology before considering any other interventions 1.