Which of the following is the most characteristic neurological complication of diphtheria and typically occurs in the third to fourth week of illness?

Explanation

## Neurological Complications of Diphtheria **Key Point:** Diphtheria toxin causes a demyelinating polyneuropathy that classically presents with **bulbar palsy** (soft palate and pharyngeal paralysis), typically in the 3rd–4th week of illness. This is the most characteristic neurological manifestation. ### Timeline of Neurological Involvement | Week | Neurological Finding | Mechanism | |------|----------------------|----------| | 1–2 | Soft palate paralysis (bulbar) | Toxin-mediated demyelination of CN IX, X | | 2–3 | Pharyngeal and laryngeal weakness | Extension to CN X branches | | 3–4 | Ciliary paralysis (accommodation loss) | CN III involvement | | 4–6 | Limb weakness, sensorimotor neuropathy | Peripheral nerve demyelination | | Later | Respiratory paralysis (if severe) | Diaphragmatic involvement | **High-Yield:** Bulbar palsy in diphtheria is **non-inflammatory** demyelination caused by diphtheria toxin, not direct viral invasion. Recovery is slow (weeks to months) but complete in most survivors. ### Clinical Features of Bulbar Involvement 1. **Soft palate paralysis** → nasal voice, nasal regurgitation 2. **Pharyngeal weakness** → dysphagia, aspiration risk 3. **Laryngeal paralysis** → hoarseness, stridor (if bilateral) 4. **Cranial nerve progression** → CN III (accommodation), CN VI (diplopia), CN XII (tongue) **Clinical Pearl:** The **"bull neck" appearance** (massive cervical lymphadenopathy and edema) is a sign of severe local disease and correlates with higher toxin production and risk of systemic complications, including neurological involvement. **Mnemonic:** **BULBAR PALSY in Diphtheria = BPD** — *Bulbar (CN IX, X), Palsy (demyelination), Delayed (3–4 weeks)* ### Differential from Other Causes of Bulbar Palsy | Condition | Onset | Mechanism | Other Features | |-----------|-------|-----------|----------------| | Diphtheria | 3–4 weeks | Toxin demyelination | Pseudomembrane, myocarditis | | Guillain-Barré syndrome | Acute (1–2 weeks) | Autoimmune demyelination | Ascending paralysis, areflexia | | Poliomyelitis | Acute (days) | Viral motor neuron destruction | Lower motor neuron signs, fever | | Botulism | Acute (12–48 hrs) | Acetylcholine release blockade | Descending paralysis, autonomic signs | [cite:Harrison 21e Ch 139; Park 26e Ch 8]