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Inflammatory demyelinating polyneuropathies exist along a clinical spectrum, with acute (AIDP) and chronic (CIDP) inflammatory demyelinating polyneuropathy representing well-defined endpoints. Subacute inflammatory demyelinating polyneuropathy (SIDP) represents an intermediate entity characterized by symptom progression over four to eight weeks. Due to the absence of standardized diagnostic criteria and overlapping clinical and electrodiagnostic features with both AIDP and CIDP, SIDP remains underrecognized and diagnostically challenging.

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A 51-year-old woman with hypertension, deep vein thrombosis, and pulmonary embolism presented with five weeks of progressive weakness culminating in respiratory failure. Her initial presentation at an outside facility was misattributed to gastroenteritis with hypokalemia. She subsequently developed ascending lower extremity weakness, bulbar symptoms including dysphonia, dysphagia, and diplopia, and autonomic dysfunction including urinary retention and persistent tachycardia. Examination revealed symmetric weakness, generalized areflexia, and decreased sensation in bilateral lower extremities. Respiratory compromise necessitated intubation. CSF analysis demonstrated albuminocytologic dissociation with an elevated IgG index. Electrodiagnostic studies showed absent F-waves across multiple motor nerves, consistent with predominantly demyelinating polyneuropathy with secondary axonal involvement. Infectious, autoimmune, and neuromuscular junction evaluations were unrevealing. Following five sessions of therapeutic plasma exchange, the patient demonstrated meaningful strength improvement and was successfully extubated and discharged to rehabilitation.

The five-week subacute progression beyond AIDP's typical four-week nadir yet short of CIDP's eight-week threshold supported classification within the SIDP spectrum. Prominent bulbar involvement, autonomic dysfunction, and respiratory failure further complicated diagnostic categorization. SIDP occupies a poorly defined intermediate position that risks misclassification, potentially leading to premature cessation or delayed initiation of appropriate therapy. This case highlights SIDP as a distinct and underrecognized entity within the demyelinating neuropathy spectrum. The favorable response to plasma exchange supports a shared immune-mediated pathophysiology across this spectrum.  Early electrodiagnostic testing, CSF analysis, and awareness of the subacute timeframe are critical for timely diagnosis and optimal management