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Absent Reflex in One Arm: Neurological Causes and Urgent Evaluation

Clinical Significance

Absent reflex (areflexia) in one upper limb indicates a lesion in the reflex arc: sensory neuron → interneuron → motor neuron → muscle. Localization is key.

Anatomic Localization and Causes

1. Peripheral Nerve (C5–T1 Roots/Plexus/Nerves)

  • C5–C6 (C6 radiculopathy): Biceps, brachioradialis reflexes ↓/absent.
  • C7 (Triceps reflex): Most common radiculopathy.
  • C8–T1: Finger flexors; no standard deep tendon reflex.
  • Brachial plexus injury: Trauma, Parsonage-Turner, thoracic outlet.
  • Peripheral neuropathy: Usually symmetric; asymmetric if mononeuropathy multiplex.

2. Spinal Cord

  • Intramedullary lesion (syrinx, tumor, MS plaque) at cervical level.
  • Compressive myelopathy (spondylosis, disc, tumor).
  • Brown-Séquard syndrome: Ipsilateral reflex loss below lesion.

3. Brainstem / Cerebral Hemisphere

  • Stroke (capsular, corona radiata) → contralateral hyperreflexia (UMN), not areflexia.
  • Acute stroke: Initial flaccidity → later spasticity/hyperreflexia.
  • Brainstem lesion affecting cranial nerve nuclei or descending tracts.

4. Neuromuscular Junction / Muscle

  • Myasthenia gravis (fatigable weakness, reflexes usually preserved).
  • Botulism, Lambert-Eaton.
  • Myopathy (proximal weakness, reflexes preserved until late).

5. Local Structural

  • Severe swelling, hematoma, compartment syndrome compressing nerve.
  • Recent fracture/surgery.

Associated Findings to Guide Localization

  • Weakness pattern: Myotomal vs. peripheral nerve.
  • Sensory loss: Dermatomal vs. peripheral nerve.
  • Other reflexes: Brisk elsewhere = UMN; globally absent = neuropathy/AMS.
  • Fasciculations: LMN (anterior horn, root, plexus, nerve).
  • Atrophy: Chronic denervation.

Urgent Evaluation Needed If

  • Acute onset + weakness (stroke, cord compression, Guillain-Barré).
  • Trauma history.
  • Progressive symptoms.
  • Systemic signs (fever, weight loss).

Diagnostic Workup

  1. Detailed neuro exam.
  2. NCS/EMG (localize, axonal vs. demyelinating).
  3. MRI cervical spine (cord, roots).
  4. Labs: Glucose, B12, TSH, ESR/CRP, immunology, CK.
  5. Lumbar puncture if GBS, MS, infection suspected.

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