Nitrous oxide (N₂O) is a colorless and nonflammable gas that is used in surgery and dentistry due to its anesthetic effects. N₂O has recently been misused for recreational purposes due to its euphoric effects when inhaled. The recreational use of N₂O is currently prohibited in Korea due to its negative clinical effects, such as inducing vitamin B₁₂ deficiency1 or mechanisms that lead to thrombosis.2 However, N₂O is still available in whipped-cream canisters or small bulbs that are predominantly used for recreation by young people.3 Reported cases of neurological and psychiatric manifestations of N₂O abuse have recently increased among adults in Korea.4

The neurological complications associated with N₂O abuse usually manifest as subacute combined degeneration (SCD) of the spinal cord. N₂O deactivates the enzyme methionine synthase by inactivating methylcobalamin and impairing the methylation of myelin sheath proteins, leading to spinal cord degeneration and myelopathy.4 These processes can also cause peripheral nervous polyneuropathies. Several clinicians in Korea have reported various clinical symptoms and abnormal findings in tests after N₂O inhalation, such as in electrophysiological examinations and magnetic resonance imaging (MRI).5 Additionally, vitamin B₁₂ deficiency tends to the induction of hyperhomocysteinemia, which in turn induces thrombosis.2 There are numerous previously reported cases of hyperhomocysteinemia inducing deep vein thrombosis or pulmonary thromboembolism (PTE).2,6

The use of illegal drugs such as cocaine, heroin, methamphetamine, and cannabis is reportedly lower in Korean than in other countries.7 Illegal drug usage is therefore considered by Korean clinicians less when they are determining symptom etiology. Since it is rare for patients to report their N₂O use, diagnosing patients with atypical clinical manifestations is also difficult. A good understanding of the clinical manifestations of N₂O usage is therefore important. Here we report nine patients with neurological manifestations and abnormal laboratory findings following N₂O abuse.

Nine patients aged between 23 and 35 years visited our hospital from August 2017 to December 2019 (Table 1). All patients had a history of N₂O inhalation during the 6 months prior to admission. The interval between the last N₂O inhalation and symptom onset varied from 2 weeks to 4 months, while the interval between symptom onset and hospital visit varied from 3 days to 1 month, with the majority of patients visiting 1 week after symptom onset. Five patients were female. Sensory change was the initial symptom of all patients. These sensory symptoms varied between patients and included an ascending tingling sensation in both legs (patient no. 1), numbness in both legs (patient no. 3 and 7), and acroparesthesia (patient no. 2, 4, 5, 8, and 9). Five patients experienced muscle weakness, three of which only had lower limb weakness (patient no. 3, 6, and 9), and two had both upper and lower limb weakness (patient no. 1 and 4). Muscle weakness only in the ankles was found in patient no. 5. One patient had dyspepsia. None of the patients were vegetarians or had previously received gastrointestinal surgery.

Neurological examinations indicated that seven of the nine patients had limb paresis. Three had mild weakness in ankle dorsiflexion and the other four had quadriparesis. Sensory changes were detected in patient no. 1, while patient no. 7 and 9 had a loss of vibration sensation in both legs. Among the other patients, six had stocking-glove hypesthesia (patient no. 2, 4, 5, 6, 8, and 9). Hypoactivity of the deep tendon reflex (DTR) occurred in four cases (patient no. 4, 5, 6, and 7), four had normoactive DTR, and one had hyperactive DTR (patient no. 9). Patient no. 2 had dysmetria of the upper limbs and all patients except no. 5 showed abnormal results in the Romberg test. All patients had an impaired tandem gait. Digital rectal examinations indicated that patient no. 4 had decreased anal tone.

Laboratory findings revealed low hemoglobin levels and large mean corpuscular volumes in four and six patients, respectively. Only one patient had a low serum vitamin B₁₂ level (patient no. 7). Elevated serum homocysteine levels were indicated in seven cases (patient no. 1, 2, 3, 5, 6, 7, and 9), and five patients had high blood MMA levels.

NCSs were performed on eight patients (Table 2). Among them, four had axonal neuropathies (patient nos. 3, 4, 5, and 6) and one had longer F-wave and H-reflex latencies in both lower limbs (patient no. 7). Seven patients had a decreased sural nerve conduction velocity or longer H-reflex latency. Most patients had more abnormalities in the lower limbs than in the upper limbs, distal limb regions were affected more than were proximal regions, and sensory nerves were involved more frequently than were motor nerves. Demyelination was not observed in any patient. The reference values and raw data of patients' nerve conduction study are shown respectively on Supplementary Table 1, 2.

Seven patients received spinal MRI, five of whom had signal changes in the dorsal column of the cervical spine, as indicated in T2-weighted images (Fig. 1). Patient no. 9 had concurrent hyperintensity lesions in the anterior, lateral, and posterior spinal column (Fig. 2). Two patients (patient no. 5 and 7) showed normal results in spinal MRI.

All patients received hydroxocobalamin as a vitamin B₁₂ supplement. Three patients received additional steroid therapy. Intravenous immunoglobulin G was administered to one patient (patient no. 5) due to their observed clinical features being similar to those associated with early Guillain-Barré syndrome. Patient no.1 suffered from N₂O addiction and depression, so they received psychiatric consulting.

A literature search identified five articles describing SCD attributed to recreational N₂O abuse, including three male and seven female cases. All of these cases had gait disturbances or sensory changes as initial symptoms accompanied by signal changes in cervical MRI. NCSs indicated neuropathic patterns or early-stage polyneuropathy. One patient presented with incidental PTE and lung infarction. Table 3 summarizes the details of the published cases.5,9,10

N₂O is known to selectively oxidize folate and vitamin B₁₂, rendering them inactive and unable to degrade homocysteine into methionine, which is required for normal myelin production.11 Active vitamin B₁₂ is also required to convert MMA into succinyl-CoA. Serum levels of both homocysteine and MMA are elevated during vitamin B₁₂ deficiency. Previous in vitro studies have demonstrated the cytotoxic effect of homocysteine on cortical astrocytes and that of MMA on primary neuronal cultures.12,13 This interference with vitamin B₁₂ metabolism could lead to demyelination in the central or peripheral nervous system and also to megaloblastic anemia. These pathophysiological mechanisms result in N₂O intoxication being associated with clinical features such as numbness in the extremities, ataxia, and psychomotor symptoms (including impaired memory function and depression).14

There were marked variations in clinical symptoms, laboratory findings, electrophysiological findings, and MRI results associated with N₂O usage among the cases examined in this study. Sensory nerve fibers are generally affected more frequently by toxins.15 Similar to other toxic neuropathies, we found that all of our patients experienced sensory changes in their distal limbs, and that these changes were correlated with NCS abnormalities. Motor weakness was observed in 50% of cases, but motor NCS abnormalities were not found to be correlated with these symptoms. In two cases (patient no. 1 and 7), only F-wave and H-reflex latency changes were identified. These findings resembled the early clinical features of Guillain-Barré syndrome, but our results indicated no demyelinating patterns. Considering the relationships between N₂O, vitamin B₁₂ deficiency, and hypomyelination, these axonal patterns detected through NCSs suggest that an alternative mechanism of N₂O directly contributes to axonal damage.16

Most of the patients had high serum homocysteine or MMA levels despite normal vitamin B₁₂ levels. Due to the illegality of recreational N₂O use in Korea, most patients with neurological symptoms were assumed to have ingested vitamin B₁₂ prior to visiting our hospital. A previous case report indicated that homocysteine and MMA levels could be normalized by vitamin B₁₂ treatment.17

MRI findings in previously reported cases of N₂O intoxication related to SCD have frequently identified symmetric bilateral T2-weighted hyperintensity lesions at the posterior spinal cord. This specific finding is known as the “inverted-V sign.” In the present case series, the MRI findings of about half of the patients were compatible with SCD. However, two patients who had no detected MRI signal changes had abnormal NCS results, which represents the first report of this phenomenon in Korea. One patient also had concurrent involvement in the anterior, lateral, and posterior spinal column.

There have been several reports of the deleterious effects of N₂O abuse in Korea. Kwoun et al.5 reported the first case of myeloneuropathy following chronic N₂O abuse in 2003. In that report, the patient had paresthesia in both distal limbs and ataxia. Spinal MRI indicated high signal intensities from segments C2 to C5 on a T2-weighted image. Furthermore, NCSs indicated a demyelinating neuropathic pattern. Serum vitamin B₁₂ levels were low and those of homocysteine were high. Two other cases of SCD caused by N₂O gas were reported by Kwon et al.18 in 2019. Both patients had sensory changes in their limbs and the inverted-V sign was visible in T2-weighted spinal MRI. NCSs indicated axonal motor polyneuropathy in that case. In all previous cases of N₂O intoxication in Korea, spinal cord MRI indicated signal changes similar to those observed in SCD. Unlike previously reported cases, we found two patients who had abnormal NCS results despite having normal spinal MRI results. We therefore suggest that clinicians need to consider the possibility of N₂O inhalation in patients whose NCSs produce axonal degeneration findings, even if signal changes are not observed in spinal cord MRI.

The present study has revealed that N₂O abuse may be associated with a wide range of clinical features, including psychiatric problems such as addiction and depression, especially among adolescents and young adults. Due to the illegality of recreational N₂O use in Korea, patients tend to hide their history of N₂O use or inaccurately report symptom onset. Addressing this issue requires treatments for neurological deficits, the careful recording of the medical histories, and psychiatric support for patients.

Our study had several limitations. First, its retrospective design made it difficult to precisely evaluate and quantify the data analyzed for each patient. Second, this study included only nine cases from a single center, so it might not be representative of all patients experiencing N₂O intoxication in Korea. Third, all patients had supplemented their vitamin B₁₂ intake before the clinical evaluations, which makes it difficult to accurately interpret the results. Fourth, the patients tended to not reveal their history of N₂O inhalation because it is currently illegal to use N₂O gas for recreational purposes in Korea. It was therefore difficult to evaluate the correlations between clinical features and the exact amount and duration of N₂O inhalation.

The clinical manifestations of N₂O intoxication related to neurological disorders varied between the included patients, but had a general commonality. Several clinical indicators may include young adults with progressive subacute weakness and lower-extremity-dominant sensory changes. Increased serum MMA, homocysteine, axonal-type sensorimotor polyneuropathy patterns in electrophysiological tests, and predominant posterior column involvement observed in MRI may indicate the presence of N₂O intoxication related to neurological disorders. Careful evaluations of the clinical history of N₂O abuse are therefore warranted when these clinical findings are observed.