A 4-year-old child with suspected diphtheria presents with a thick pharyngeal membrane, fever, and cervical lymphadenopathy. The child has also developed bilateral lower limb weakness and difficulty in accommodation 2 weeks after onset of illness. Which investigation would best confirm the diagnosis of diphtheria-related neurological complication?

Explanation

## Confirming Toxigenic Diphtheria in Neurological Complications ### Clinical Context: Diphtheria Neuropathy **Key Point:** The neurological complications of diphtheria (cranial nerve palsies, descending paralysis, autonomic dysfunction) are caused by the **diphtheria toxin**, not by direct bacterial invasion. Therefore, confirming toxin production by the isolated organism is essential to establish the diagnosis. ### Why Elek Test? The **Elek test** (immunodiffusion test) is the gold standard for confirming toxin production: - Uses antitoxin-impregnated filter paper in an agar plate - When the bacterial isolate produces toxin, a characteristic precipitin line forms at the antigen-antibody interface - Distinguishes **toxigenic** strains (cause systemic complications) from **non-toxigenic** strains (local infection only) - Performed on the bacterial isolate cultured from the throat/nasopharynx ### Pathophysiology of Diphtheria Neuropathy **Clinical Pearl:** Diphtheria toxin inhibits protein synthesis by: 1. ADP-ribosylating elongation factor 2 (EF-2) 2. Blocking ribosomal translocation 3. Causing demyelination of peripheral nerves This leads to: - **Early (1–2 weeks):** Cranial nerve palsies (CN III, IV, VI, IX, X, XII) - **Late (3–8 weeks):** Descending paralysis, respiratory muscle involvement, autonomic dysfunction ### Timing of Diagnosis **High-Yield:** By the time neurological symptoms appear (2+ weeks), the organism may no longer be culturable from the throat. However, if a culture was obtained early, the Elek test on that isolate confirms toxin production and explains the delayed neuropathy. ### Why Other Tests Are Wrong | Investigation | Why Not Appropriate | | --- | --- | | **Schick test** | Measures *antitoxin* (immunity), not toxin production. Positive Schick = susceptible (no antitoxin); negative = immune. Does not confirm active toxin production. | | **EMG/NCS** | Confirms the presence and pattern of neuropathy (demyelinating vs. axonal) but does NOT identify the causative organism or toxin. | | **CSF analysis** | Diphtheria is not a meningitis; CSF is typically normal or shows mild protein elevation. Culture of CSF would be negative. | ### Antitoxin Administration **Warning:** Antitoxin should be given **immediately** on clinical suspicion — do NOT wait for Elek test results. The test confirms the diagnosis retrospectively and guides severity assessment. [cite:Park 26e Ch 9; Harrison 21e Ch 145]