Hirayama disease, also known as benign focal amyotrophy, is characterized by initially progressive muscular weakness and distal upper limb wasting that predominantly affects young men in their 20s and 30s, with spontaneous arrest of disease progression occurring within a few years [1]. Hirayama disease is a self-limiting disorder, but an early diagnosis is needed to prevent deterioration. Although the pathophysiology of Hirayama disease is unknown, circulatory disorders in the spinal cord are hypothesized. Neck flexion tightens the dura and damages the intramedullary microcirculation, causing nerve damage [2]. Therefore, other causes of circulatory and neurological problems might directly or indirectly affect Hirayama disease, and medical staff have noted the coronavirus disease 2019 (COVID-19) vaccine as a possible cause. Its neurological side effects include headache, Guillain-Barré syndrome (GBS), venous sinus thrombosis (VST), transverse myelitis, facial nerve palsy, small-fiber neuropathy, newly developing multiple sclerosis, and others that have been reported only in a few patients [3]. Although the mechanism by which these diseases occur is unknown, hypotheses include antibodies to the COVID-19 vaccine forming a complex causing thrombosis or hyperviscosity of the patient's blood [4].

Here we report a case of Hirayama disease observed on spine magnetic resonance imaging (MRI) in a 15-year-old girl who complained of unilateral muscle weakness after COVID-19 mRNA vaccination.

On December 27, 2021, a 15-year-old girl visited a pediatric outpatient clinic. She had received a Pfizer-BioNTech COVID-19 vaccine 3 weeks prior. She was asymptomatic at that time, but her right leg began to lose strength the night before the hospital visit. Her symptoms persisted until the following morning, when she visited the hospital. At the outpatient visit, the left upper and lower extremities were asymptomatic, but the right upper and lower extremities showed a manual muscle test grade of fair to good, and the distal muscles were weaker than the proximal muscles. The light touch sensation on the right half of her body, excluding the face, was decreased. The patient was admitted to the Department of Pediatrics for a differential diagnosis of cervical myelopathy, GBS, and a brain tumor. After hospitalization, brain MRI, nerve conduction studies (Table 1), and somatosensory evoked potential studies (Fig. 1) were performed in collaboration with the neurologists at the hospital. Most of the test results were within the normal range. However, contrast-enhanced spinal MRI showed enlargement of the posterior epidural space with epidural venous plexus engorgement and enhancement at the cervicothoracic junction to the T8 level. The prominent posterior epidural vessels appeared consistent with the imaging findings of Hirayama disease (Fig. 2). It is known that Hirayama disease does not usually invade the lower extremities. Therefore, this patient had an atypical presentation. Regarding the reason for the involvement of the lower extremities, the medical staff kept in mind the possibility that it was accompanied by conversion disorder. The MRI showed typical Hirayama disease findings, and the final diagnosis was Hirayama disease.

On December 30, the patient was referred to the Department of Rehabilitation Medicine, where an evaluation revealed the following: Berg Balance Scale score, 45 points; K-Modified Barthel Index score, 61 points; a full range of motion; motor assessment scale scores, 52 points on the right side and 54 points on the left side; Motricity Index: upper extremity, 59.0 points and lower extremity, 56.0 points; functional ability assessment, 18 points; Fugl-Meyer Assessment (FMA) of the upper body, 116 points on the right side and 126 points on the left side; FMA of the lower body, 73 points on the right side and 86 points on the left side; and weakness of right finger extension and adduction, a “fair plus” result in manual muscle tests. In addition, in a blood test conducted on December 31 to differentiate other diseases that cause muscle weakness, such as GBS, Miller Fisher syndrome, and neuromyelitis optica, negative results were obtained for anti-GM1 immunoglobulin G (IgG) isotype, anti-GD1B IgG isotype, anti-GQ1b IgG, and anti-aquaporin 4 IgG. In the COVID-19 antibody assays, the Roche total antibody value was 201 U/mL and the Abbott IgG value was 4,621 AU/mL, 7.5 times higher than the maximum Roche antibody value (26.7 U/mL) measured 1 month after vaccination and 6.3 times higher than the maximum Abbott IgG antibody value (732.8 AU/mL) [5]. In short, the increase in the antibody load after the COVID-19 vaccination was abnormally high. Although the exact mechanism was difficult to determine, the medical staff assumed that the elevated antibody load may have affected the development of Hirayama disease in some way.

This is the first report of Hirayama disease after COVID-19 mRNA vaccination in a female adolescent. Hirayama disease occurs in young people in their 20s and 30s and much more often in male than in female patients at a ratio of 20:1 [1]. It shows an insidious onset and slow progression of muscular atrophy in the distal upper limbs, including the thenar, hypothenar, interossei muscles, and wrist flexors and extensors, but it spares the brachioradialis muscles [6]. The amyotrophy is unilateral in most patients, but it can be bilateral. Most patients show “cold paresis” symptoms, in which the finger muscles weaken in cold environments. Paresthesia is uncommon; however, in some patients, local sensation is lost in both hands. Most (73%) patients stop developing symptoms within 5 years [1]. Amyotrophic lateral sclerosis, spinal muscle atrophy, C8-T1 radiculopathy, compressive myelopathy, cervical spondylotic myelopathy, syringomyelia, post-polio syndrome, multifocal motor neuropathy, and toxic neuropathy should be considered in the differential diagnosis [7]. To differentiate Hirayama disease from the above-mentioned diseases, nerve conduction velocity, electromyography (EMG), motor-evoked potentials (MEPs), and cerebrospinal fluid (CSF) tapping can be performed, and MRI yields the most characteristic findings. In previous research, EMG showed acute and chronic denervation of the atrophied muscles [8], while the findings of nerve conduction velocity and CSF tapping analyses were almost normal [2]. If MEPs are measured after transcranial magnetic stimulation is performed with neck flexion, the latency increases and the amplitude decreases, suggesting the pathogenic role of neck flexion in this disease [9]. Neck flexion on MRI is diagnostic for Hirayama disease, with characteristic findings of focal hyperintensities on T2-weighted images of the anterior caudal segments of the cervical spinal cord [10]. Our patient had no symptoms of muscle atrophy, but MRI showed a typical pattern of Hirayama disease, including an enlarged posterior epidural space with epidural venous plexus engorgement, enhancement at the cervicothoracic junction to the T8 level, and prominent posterior epidural vessels in the flexion view.

In response to the current COVID-19 pandemic, vaccines are being administered to minimize complications. However, neurological complications can occur from the vaccination itself, including headache, GBS, VST, transverse osteomyelitis, facial neuropathy, small nerve neuropathy, and multiple sclerosis, as mentioned earlier [3]. In our patient, spinal cord damage occurred due to the immune response to the vaccination; the diagnosis of Hirayama disease was made based on the MRI findings, despite the lack of full consistency with the clinical features. Given the observation of an excessively increased COVID-19 antibody load, this overreaction may have affected the blood vessels of the spine via thrombosis, hyperviscosity of the blood, or other unknown mechanisms by forming an immune complex.

Further research is needed, but considering the findings of the current case, attention should be paid to the possibility of Hirayama disease in the current circumstances of widespread Pfizer-BioNTech COVID-19 vaccination among Asian adolescents. Although no cases have been reported, if the above symptoms occur after vaccination, the possibility of Hirayama disease should be considered.