Introduction

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Vitamin B₁₂ deficiency may be the consequence of several pathological conditions (Richard Lee, 1993). The most frequent neurological manifestation is the subacute combined degeneration (SCD) of the spinal cord and polyneuropathy, rarely dementia and damage of the optic nerve occur. Numbness of the limbs and trunk is an early symptom; weakness, clumsiness and spasticity, abnormal reflexes, gait ataxia develop later. The diagnosis is based on the symptoms, the Schilling test, and decreased serum vitamin B₁₂ level. A few publications report characteristic magnetic resonance (MR) changes in SCD (Murata et al., 1993; Tajima et al., 1994; Beltramello et al., 1998; Giron et al., 1998; Youstry et al., 1998).

The SCD may rarely develop as the complication of nitrous oxide (N₂O) anaesthesia (Schilling, 1986; Metz, 1992; Filippo and Holder, 1993). In our two patients the MR imaging (MRI) findings of the spinal cord supported the diagnosis of SCD and were in good correlation with the clinical course.

Case reports

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Case 1

A 57-year-old male, underwent an extra-intracranial artery bypass (sta-mca) operation under N₂O anaesthesia. Two months after the operation clumsiness and weakness of both hands, tingling of the fingers, arm and the chest developed. This was followed by unsteadiness, gait imbalance and paraesthesia of both lower limbs, gradually ascending to the trunk.

Physical examination revealed total alopecia, a smooth, inflammated tongue, fissures in both angles of the mouth. Neurological examination found decreased muscle power on both legs and arms, weak, symmetric tendon reflexes, no Babinski sign. Severe limb and trunk ataxia, worsened during blind-walking. Vibration and position-sense were disturbed on the trunk and the limbs, whereas heat and pain sensation were preserved. Lhermitte sign was present.

The MRI of the cervical and thoracic spinal cord showed symmetrical hyperintense signal changes on T2, and hypointensity on T1-weighted images of the posterior columns with predominance in the Burdach tracts (Fig. 1). No contrast enhancement was noted. MRI of the skull revealed some small lacunas. Somatosensory evoked potential (SEP) investigation proved severe somatosensory conduction disturbance in the spinal cord.

Electromyography (EMG) and Nerve Conduction Study found mixed type polyneuropathy, critical fusion frequency was normal (43/40 Hz).

The laboratory tests found borderline anaemia with low red blood cell (RBC) and white blood cell count, high mean corpuscular volume (MCV) and mean corpuscular haemoglobin content. Serum B₁₂ level was 135 pmol/l (normal value: 146-518 pmol/l), serum and RBC folic acid level were normal. Schilling test was abnormal (2.1% excretion).

Vitamin B₁₂ was administered (1000 g/day for five consecutive days, intramuscularly). Within 3 weeks the gait improved, ataxia, weakness and numbness of the limbs decreased. Four weeks after treatment control cervical MR showed almost total regression of the signal abnormalities in the cervical posterior columns (Fig. 2). His neurological condition remained satisfactory on 3-year follow-up.

Case 2

A 52-year-old male, underwent a cholecystectomy because of perforation of the gallbladder under N₂O anaesthesia. One week after the operation a symmetrical paraesthesia developed on the feet, ascending to the trunk, chest and both arms. This was followed by weakness and clumsiness of all limbs. From the laboratory findings mild anaemia (PCV: 0.35), and the low RBC (2.68 T/l) was remarkable. The MRI showed high intensity signals on T2 scans in the posterior columns bilaterally, along the entire cervical segment of the cord (Fig. 3). Cerebrospinal fluid examination was normal.

General physical examination found livedo reticularis, fissures in the angles of the mouth. The abdomen was swollen and very sensitive to palpation. Neurologically, Lhermitte's sign was present. Tendon reflexes on the upper limbs were absent, on the lower limbs they were weak. There was no Babinski sign. Weakness of the limb muscles with distal preponderance was found. His gait was broad based with spinal ataxia. Decreased graphaesthesia was found on the lower limbs, the lower part of the trunk and on the upper limbs.

Laboratory findings are macrocytic, hyperchrome anaemia, elevated urobilinogen excretion. Serum vitamin B₁₂ level was 166 pmol/l (moderate low), serum and the RBC folic acid content was decreased (8.2 and 58 nmol/l). Blood and bone marrow smear proved megaloblastic haemopoesis. Schilling test gave a value of 0% absorbtion. EMG and Nerve Conduction Study proved mild, mainly demyelinative type polyneuropathy. Visual evoked potentials (VEP) was normal. After administration of 5000 g vitamin B₁₂ intramuscularly, for five consecutive days, his muscle power improved considerably within 1 week, but the numbness and stiffness of the limbs subsided much slower. Abdominal distress remained. Three weeks after treatment, MR of the cord showed the almost total regression of the posterior column changes (Fig. 4). The follow-up showed a satisfactory condition of the patient.

Discussion

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The SCD may be the only manifestation of cobalamine deficiency in one-third of the patients (Hensing, 1981; Lindenbaum et al., 1988). Patients with mild haematological findings are often misdiagnosed. This is illustrated in Case 1, with borderline anaemia and the slightly elevated MCV. Although the second patient presented the typical haematological findings, the link with the neurological symptoms was simply overlooked. Surgical anaesthesia, as a pathogenetic factor was not considered.

Nitrous oxide may provoke symptoms of vitamin B₁₂ deficiency because of the impairment of the methylcobalamin-dependent methionine synthetase reaction (Beltramello et al., 1998; Giron et al., 1998). It seems likely that patients having subclinical vitamin B₁₂ deficiency are candidates for the N₂O provoked SCD (Schilling, 1986; Filippo and Holder, 1993). Some patients respond poorly to vitamin B₁₂ supplementation, but methionin therapy may be successful (Stacy et al., 1992).

High intensity signals in the posterior columns of cervical or thoracic spinal cord on T2 weighted MR scans in SCD were already described in five cases (Berger and Quencer, 1991; Murata et al., 1993; Wolansky et al., 1995). In one of these cases SCD also developed after N₂O anaesthesia (Timms et al., 1993).

Inspite of the clinical evidences of corticospinal tract involvement hyperintense changes in the lateral columns could not be seen, but it may be because of the resolution limits of the MRI (Timms et al., 1993). The MR changes after N₂O narcosis may indicate demyelination, which is proved to be reversible after B₁₂ vitamin substitution. In our cases - with regards to the short latencies after the N₂O exposure - surgical anaesthesia may only have been a provoking factor of the manifestation of subclinical B₁₂ vitamin deficiency.