Myelin oligodendrocyte glycoprotein antibody-associated disease myelitis and the H sign: Correlating radiological patterns with gray matter involvement

A 15-year-old boy with no prior health issues developed pain in both lower limbs associated with paraesthesia and muscle weakness for 5 days along with urinary retention after a 2-week febrile illness. The neurological evaluation showed intact cranial nerve and brain stem functions, normal cognition, lower limb weakness, and exaggerated deep tendon reflexes (DTRs) at the knees and ankles. No significant findings were noted in his family or past medical history. Cerebrospinal fluid (CSF) examination revealed pleocytosis (5 white blood cells/mm³) and increased protein (60 mg/dL). Magnetic resonance imaging (MRI) spine showed longitudinally extensive transverse myelitis (LETM) extending across the cervical and thoracic spinal segments, along with a central T2 hyperintense signal in the cervical cord featuring an “H” sign [Figure 1]. There was no contrast enhancement, MRI brain and optic nerve was normal. Myelin oligodendrocyte glycoprotein (MOG) antibodies with a titer of 1:160 were found in serum which was suggestive of MOG-associated disease (MOGAD). Serum aquaporin 4 and CSF oligoclonal bands were negative. MOGAD diagnosis was confirmed, and treatment included a 5-day course of intravenous methylprednisolone (30 mg/kg/day), followed by a 12-week tapering regimen of oral corticosteroids (2 mg/kg/day). Three months following the treatment, a significant improvement in his condition was noted with no residual symptoms, and repeat neuroimaging was normal.

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Figure 1:

(a) Short tau inversion recovery sagittal image of the cervico-dorsal spine shows longitudinally extensive hyperintense signal involving the cervical and dorsal spinal cord (white arrow). (b) T2-weighted axial image of the involved part of the cervical spine shows central T2 hyperintense signal involving the cervical cord with “H” shape, reminiscent of the “H-sign” (white arrow).

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LETM is a distinctive spinal cord MRI finding observed in a range of neurological conditions under the pediatric acquired demyelinating disease spectrum. These include neuromyelitis optica spectrum disorder (NMOSD), MOGAD, and multiple sclerosis (MS).^[1] LETM has similarly been reported in cases of autoimmune glial fibrillary acidic protein (GFAP) astrocytopathy.^[2]

LETM refers to inflammation-related damage in the spinal cord spanning three or more contiguous vertebral segments. While it is most commonly associated with NMOSD, it is also often found in MOGAD. In most cases of MOGAD and some cases of GFAP, LETM involves the thoracic and cervical spinal cord.^[1] In contrast, MS presents with shorter spinal segments, rather than the longer segments typical of LETM^[3] [Table 1].

On MRI, LETM linked to MOGAD presents as a hyperintense lesion on T2-weighted sagittal images and often displays the characteristic “H-sign” in axial views, reflecting the involvement of central gray matter. The horizontal bar of the “H” sign is a representation of inflammation centered around the gray matter’s central canal. H sign has been described in 29.4–83.3% of cases overall, and in approximately 66% of pediatric cases.^[4] As per the latest international MOGAD panel proposed criteria,^[5] it is included as a supportive diagnostic feature for MOG-associated myelitis.

The lesions in MOGAD show variable enhancement, are poorly demarcated, and lack destructive features such as necrosis or cavitation.^[1] This benign pattern on neuroimaging is consistent with a favorable treatment response and lack of permanent deficits. The abovementioned radiological features are important in differentiating MOGAD from other entities along the demyelinating spectrum. Recognizing these specific radiological features is crucial for accurate diagnosis and timely management of MOGAD.