A previously healthy, fully immunized, 7-year-old boy presented with left-sided neck pain, leftward head tilt, and left arm weakness. Two days prior to his visit, he developed a high fever and symptoms resembling a common cold. The following day, he experienced headaches, vomiting, and neck pain severe enough to cause his head to tilt to the left. On the day of presentation, he exhibited weakness in the left arm, rendering him unable to lift it. He was alert and oriented to time, person, and place. His cranial nerve examination was normal. Motor grades for the left shoulder and elbow were grade II, while the motor grades for the fingers were grade IV. The deep tendon reflexes (DTR) of the left biceps and triceps were 1+. Muscle strength and reflexes on the right side were entirely normal. Magnetic resonance imaging (MRI) of the brain was normal, and sonography of the neck revealed several enlarged lymph nodes bilaterally at neck level 2. Cerebrospinal fluid (CSF) analysis showed an elevated white blood cell count (156/mm³), with all other findings unremarkable. Nerve conduction studies (NCS) indicated decreased compound muscle action potential (CMAP) amplitudes across the left musculocutaneous nerve (Fig. 1A). MRI of the spine demonstrated a long segmental signal change and swelling in the spinal cord extending from C2 to T1, primarily involving the gray matter, particularly the anterior horn, without evidence of enhancement (Fig. 1B and C). He was diagnosed with AFM based on his clinical history, pleocytosis, spine MRI findings, and NCS results. He was treated with intravenous cefotaxime (2 g/dose every 8 hours for 3 days) until bacterial meningitis could be excluded. Following the AFM diagnosis and in line with reports suggesting efficacy, intravenous immunoglobulin (IVIG) was administered on day 6 after symptom onset, at a dosage of 2 g/kg for 1 day. Tests for viruses and bacteria in CSF, a respiratory swab, and stool samples were negative. Aquaporin-4 antibody and myelin oligodendrocyte glycoprotein (MOG) antibody tests were negative. By day 7, the patient developed pain in both knees and weakness in the proximal muscles of the right leg, leaving him unable to stand or walk. Two weeks after symptom onset, headaches and neck pain had resolved, though slight head tilting persisted. Muscle weakness had slightly improved; he could walk well without pain and move the left arm laterally. About 8 weeks following symptom onset, muscle strength in his left shoulder and elbow had improved from grade 2 to nearly grade 3. Neck pain, leftward tilt, and right proximal leg weakness had completely resolved. Subsequently, the patient was lost to follow-up, and further progress could not be determined.

AFM is an inflammatory disorder primarily affecting the gray matter of the spinal cord. Recognized in the United States during the early 2010s as a polio-like disease, AFM was formally described in 2014 after an outbreak in Colorado, likely associated with recent enterovirus D68 infections [1,2]. It should be noted that other viruses, such as enterovirus 71 and West Nile virus, can also cause acute flaccid paralysis [2]. In addition to the United States, hundreds of cases have been reported across Europe, Asia, Australia, and Africa. In Korea, AFM was reported in an adult in 2019 [3]. No definitive AFM cases have been reported among Korean children. In 2012, one case of acute flaccid paralysis associated with enterovirus 71 was reported; however, due to the absence of spine MRI and NCS findings, it is uncertain whether this case met the diagnostic criteria for AFM [4].

AFM typically affects young children and presents mainly with the sudden onset of limb weakness [1]. Symptoms such as fever, cough, and runny nose typically precede the onset; gastrointestinal symptoms may also occur beforehand. AFM should be suspected when these initial symptoms are followed by severe acute limb weakness. Pain in the extremities, neck, or back may also occur. In the present case, the patient experienced not only sudden-onset left arm weakness but also severe neck pain on the same side. Ultrasound results did not reveal other infectious or inflammatory diseases, suggesting the neck pain was related to AFM. Bulbar weakness, respiratory muscle involvement, weakness of bladder or bowel muscles, and oculomotor weakness may also occur. Physical examination typically reveals proximal muscle weakness more prominently than distal weakness, usually limited to one or two limbs. Additionally, DTR are typically diminished or absent [5].

AFM may mimic other disorders, such as Guillain-Barré syndrome (GBS) or acute transverse myelitis (TM), in its clinical presentation (Table 1) [2,5]. However, there are key differences; GBS typically presents with symmetrical, ascending paralysis, whereas in our patient with AFM, weakness initially appeared in the upper limb, followed by proximal weakness in the right leg 7 days later [6]. Conversely, in TM, the legs are more commonly affected, and symptoms typically include sensory deficits along with bladder and/or bowel dysfunction [7].

T2 hyperintensity of the spinal cord gray matter on MRI is the most characteristic diagnostic feature of AFM [2]. MRI lesions in AFM are typically longitudinally extensive, selectively involve the anterior horn, and appear edematous. The cervical region is the most commonly affected area. Additionally, a T2-hyperintense lesion may occasionally be seen in brain imaging, particularly in the dorsal pons [2,5]. In contrast, spine MRI findings in TM typically show well-defined lesions with white matter enhancement rather than gray matter involvement, and brain MRI often reveals supratentorial abnormalities [2,7]. Despite these distinctions, overlaps in clinical symptoms and MRI findings exist, making tests for serum MOG-immunoglobulin G (IgG), aquaporin-4-IgG, and CSF oligoclonal bands advisable [7]. CSF examination and NCS also aid in diagnosing AFM. Pleocytosis in CSF is found in almost all AFM patients [2]; however, pleocytosis is uncommon in GBS, whereas protein elevation is typically observed [1,2,5].

Electromyography and NCS are not mandatory for diagnosing AFM but are useful for distinguishing AFM from other conditions. Diminished or absent CMAP can serve as an early diagnostic indicator, sometimes observed within a few days of symptom onset. Decreased or absent F waves and normal sensory NCS findings are also key diagnostic indicators [8]. In our patient, NCS were conducted 5 days after symptom onset, demonstrating almost no CMAP from the left musculocutaneous nerve, while sensory nerve function was normal. Through NCS, we confirmed neuropathy and excluded alternative diagnoses, such as GBS, which typically has sensory conduction defects, and TM, which is characterized by normal NCS findings. A decrease in CMAP can be seen in acute motor axonal neuropathy, a subtype of GBS.

Moreover, confirming the presence of enterovirus D68 or A71 infection is important. In this case, viral workups of CSF, nasopharyngeal swab, and stool samples were performed, but results were negative. In summary, the primary indicators of AFM include acute-onset limb weakness affecting one or more limbs with lower motor neuron features, characteristic MRI findings, and CSF pleocytosis.

Currently, there is no established treatment for AFM; however, IVIG is considered potentially effective due to its antiviral and immunomodulatory properties [2]. Furthermore, an experimental mouse model of enterovirus D68-associated AFM showed reduced paralysis and spinal cord viral loads with human IVIG containing neutralizing antibodies to enterovirus D68, while dexamethasone treatment worsened motor impairment and increased mortality [9]. Although its efficacy remains uncertain, plasmapheresis might be considered, as it removes autoantibodies and could diminish the viral immune response [10]. Prognosis varies among patients; however, fewer than 10% achieve full recovery. Recovery is often incomplete, leaving severe residual neurological deficits that require long-term rehabilitation. In cases of poor outcomes, nerve and tendon transfer surgeries are sometimes performed [1,2].

To the best of our knowledge, this is the first pediatric AFM case reported in South Korea. Thus, although it may have previously been overlooked, AFM should be considered in South Korea whenever acute weakness is observed.

The present study was approved by the Institutional Review Board of Hallym University Health System (IRB No. 2024-12-008). Due to the retrospective nature of this study, written informed consent was waived.