Subacute combined degeneration of the cord secondary to nitrous oxide abuse

A 25-year-old male presented with progressive paraesthesia in bilateral upper and lower limbs. On clinical examination the patient had impaired sensation to vibration and reduced proprioception. A gait abnormality was also noted. The patient provided a history of recent illicit drug use.

The patient proceeded to undergo MRI whole spine. This demonstrated marked symmetrical T2/STIR hyperintense signal within the bilateral dorsal columns in a classic ‘inverted V’ appearance (Figure 1). The confluent signal abnormality extended over a relatively long segment of the cervical spine from the approximate level of C2-C6 (Figure 2). The findings confirm the main clinical differential diagnosis of subacute combined degeneration of the cord secondary to nitrous oxide abuse.

The lateral spinothalamic tracts, which can also be involved in subacute combined degeneration of the cord, are spared in this case.

Subacute combined degeneration (SCD) of the spinal cord is a neurological disease characterized by degeneration of the dorsal and lateral columns of the spinal cord due to demyelination [1]. It is a vitamin B-12 deficiency myelopathy with varied causes including nutritional deficiencies, gastric illness (bariatric surgery, pernicious anemia, gastritis), and drug-induced causes [2]. The severity and reversibility of the disease is primarily dependent on etiology with drug induced disease secondary to illicit nitrous oxide use being typically reversible [3]. Disease course is typified by distal paresthesia involving the legs, and sensory ataxia followed by spastic paraparesis progressing to neuropathy without intervention [4]. The pathology underlying this is impaired methylation of myelin sheath by B12 inactivation, and patchy spongy vacuolization.

The most common imaging finding in SCD is linear hyperintense T2 signal extending along the bilateral dorsal columns. On axial T2 images through the cord, the appearance is termed the inverted V sign. The lateral spinothalamic tracts is rarely involved with the anterior column almost always spared [5, 6]. Very few disease processes present like this on MR appearance in isolation; although acquired immune deficiency syndrome-associated vacuolar myelopathy and copper deficiency are known to be radiologically identical to SCD [7, 8]. Demyelination from multiple sclerosis is typically asymmetrical, dorsolaterally within the cord, and less than two vertebral body segments in length. Infections and autoimmune processes are more centrally positioned and involve more than two-thirds of the cross-sectional area of the cord while cord neoplasms are typically expansile [7, 8].

MRI sequences to prioritise are sagittal T2 imaging of the cord (lesions most commonly occur in the cervical/upper thoracic cord) with targeted axial imaging as appropriate. Good axial slice selection allows accurate anatomical localisation of the signal abnormality within the cord and is crucial for making the diagnosis confidently. Post IV contrast T1 imaging can be used however enhancement is uncommon; in equivocal cases, the lack of contrast enhancement can be an important relevant negative.

Patients are treated using high-dose hydroxy-cobalamin. MRI can be useful to assess for signal-change in lesions as part of monitoring treatment response, particularly in cases where the initial diagnosis is uncertain. After 8-12 months of treatment, the hyperintense lesions of the cord will have resolved in the majority of the patients [9]. Patients can see symptom improvement up to three years later while on treatment [9].

In this case we describe the typical features of SCD secondary to illicit Nitric Oxide use and the importance of early diagnosis to ensure prompt treatment. The extent of resolution of symptoms in SCD is inversely proportional to their duration and severity which underscores the importance of early diagnosis [10]. Radiologists should be aware of the typical imaging appearance (dorsal column T2 hyperintense signal) and the importance of good axial imaging.

Written informed patient consent for publication has been obtained.