Introduction

Methotrexate (MTX) is an anticancer and immunomodulatory drug. It is considered the main agent for the treatment of rheumatoid arthritis (RA) and is the choice for initial treatment of patients with moderately to severely active RA [1]. MTX is a structural analogue of folic acid that acts by inhibiting dihydrofolate reductase (DHFR), thereby depriving cells of tetrahydrofolic acid and inhibiting the purine metabolism necessary for cellular reproduction [2]. MTX is a highly ionised and lipid-insoluble compound that barely penetrates the blood–brain barrier (BBB); thereby, central nervous system (CNS) toxicity is often mild and is characterised by mood disorders, dizziness and headaches [3, 4]. However, leukoencephalopathy is a recognised complication of the drug and is most often reported following intrathecal or intravenous administration [5–8]; infrequently, this syndrome can occur with low oral doses [9–16]. However, the exact relationship between the dose and route of MTX administration, its combination with radiation and the subsequent development of leukoencephalopathy is not clear, and hence, its occurrence is highly unpredictable [12].

The cerebellum is particularly vulnerable to toxicity. The most common cause is alcohol-related; however, several drugs have been associated with cerebellum toxicity such as anticonvulsants, antineoplastics, lithium salts and calcineurin inhibitors [17]. In a few cases, cerebellopathy associated with MTX encephalopathy has been described [16].

We report a case of leukoencephalopathy with severe involvement of the cerebellum following long-term low-dose oral MTX in a patient with polymorphisms related to MTX toxicity.

Case Report

A 68-year-old male diagnosed with RA in 2006 and on treatment with oral MTX 25 mg weekly since 2008 and folate supplementation 10 mg weekly was admitted to the emergency room due to dysarthria. His past medical history includes hypothyroidism, dyslipidaemia, chronic obstructive pulmonary disease (COPD) without domiciliary oxygen and acute myocardial infarction in 2009 that required revascularisation with three stents. No azole derivatives were administered. His dysarthria started 20 days prior to the admission, with an unsteady gait, lack of coordination and difficulty concentrating. The general physical examination showed a blood pressure of 130/80 mmHg. Cardiopulmonary auscultation revealed a cardiac arrhythmia with abolition of the right-base vesicular murmur. A neurological examination showed a punctuation in the Montreal Cognitive Assessment Test (MOCA) score of 25/35 at the expense of remote memory and executive functions, a saccadic gaze with hypometria, dysarthria, bilateral intention tremor and an unsteady gait. In the emergency room, he was diagnosed with cardiac failure due to atrial fibrillation and right pleural effusion that resulted in greater dyspnoea than is typical. Treatment with digoxin and diuretics was started, and he was admitted to the internal medicine department for management with subsequent follow-up by neurologists.

Laboratory studies showed an ESR of 73 mm/h. CBC, coagulation profile and biochemical tests, including fasting blood glucose, electrolytes, and renal, liver and thyroid (TSH 2.78 and T4 1.43) function tests were normal. Positive rheumatoid factor was found, but anti-Ro-, anti-La- and anticardiolipin-dependent β2-glycoprotein I antibodies were negative. The serum VDRL, HIV, herpes virus polymerase chain reaction (PCR), HBsAg, VHC, Brucella and Borrelia burgdorferi were negative, but a positive titre of 1/640 for Rickettsia conorii was found. Although there was no epidemiological antecedent, treatment with doxycycline was started. On lumbar puncture, cerebrospinal fluid (CSF) pressure was found to be normal and showed normal cells (<5 cells/mm3), glucose (0.82 mg/dl) and proteins (50 mg/dl), and only a mild elevation in myelin basic protein (3.1) and IgG oligoclonal bands were detected with a normal Tibbling index (0.4). CSF gram stain, bacterial and fungal cultures, PCR for herpes virus and JC virus, and cytology for malignant cells were all negative. CSF MTX levels were undetectable 2 weeks after stopping the drug.

Magnetic resonance imaging (MRI) of the brain showed symmetrical lesions in the white matter of the cerebellar hemispheres that spread to the left cerebellar peduncle. These lesions were hyperintense on T2-weighted and fluid-attenuated inversion recovery sequences (FLAIR) with a diffusion restriction on diffusion-weighted imaging (DWI) and the apparent diffusion coefficient map (ADC). On postgadolinium T1-weighted images, there were mild enhancements (Fig. 1). MR spectroscopy (MRS) showed mildly decreased creatinine and N-acetyl aspartate (NAA) with increased choline and lactate. There were also hyperintense white matter lesions in both hemispheres and subcortical white matter around occipital horns, with an area of digitate shell morphology in the right centrum semiovale.

Fig. 1
figure 1

MRI of the brain. a T2-weighted. b Diffusion-weighted imaging (DWI). c, d Axial FLAIR. Hyperintense symmetrical lesions in the central white matter of the cerebellar hemispheres with restriction on DWI (arrows)

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Neurosonology studies, including carotid ultrasonography and transcranial and vertebrobasilar Doppler, were normal. Echocardiography showed severe left ventricular systolic dysfunction. The chest X-ray showed right pleural effusion described as moderate to severe by CT scan. A thoracentesis was performed by the pulmonologist, showing an exudate with multiple cholesterol crystals compatible with pseudochylothorax probably related to his rheumatoid arthritis.

The pharmacogenetics determination was made by OpenArray© technology (Applied Biosystems©, Foster City, USA), which included three single-nucleotide polymorphisms (SNPs) related to MTX metabolism and transport: ABCB1 3435T (rs1045642), MTHFR C677T (rs1801133) and SLCO1B1 (rs4149056), showing a heterozygous genotype in MTHFR and ABCB1 genes.

The diagnosis was established as delayed MTX-induced leukoencephalopathy localised on the cerebellum. Medication with antirheumatic drugs was stopped immediately on admission, and methylprednisolone was increased to 40 mg/8 h due to COPD exacerbation. The patient gradually improved both respiratory and neurological status; thus, he was discharged without antirheumatic treatment and tapering off steroids. At the 1-month follow-up visit, the patient was asymptomatic with a MOCA test score of 35/35. The 4-month follow-up MRI showed mild improvement (Fig. 2) and the Rickettsia title was negative. No alternative treatment for RA was attempted.

Fig. 2
figure 2

Control MRI (b, d) and its comparison with the initial MRI (a, c). Mild improvement in the cerebellar lesions (arrows) is observed

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Discussion

MTX-induced leukoencephalopathy is a rare complication in MTX therapy. It ranges from mild reversible leukoencephalopathy to irreversible and even fatal disseminated necrotising leukoencephalopathy (DNL). It has been described more frequently in association with intravenous or intrathecal administration; however, a few cases related to oral administration have been reported. The clinical profiles of the eight published cases are summarised in Table 1.

Table 1 Epidemiological, clinical, radiological and outcome characteristics in oral low-dose MTX therapy
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The basic pathophysiologic mechanisms leading to MTX-induced encephalopathy are unknown but are most likely multifactorial [7]. Several mechanisms have been proposed. These include increased adenosine accumulation [18], homocysteine elevation and its excitatory effect on n-methyl-d-aspartate (NMDA) receptor [19] and alterations in biopterin metabolism [6].

The incidence ranges from 3 to 10 % although this datum is estimated mostly in a paediatric population with intravenous/intrathecal administration [8]; with the oral route, it is an unusual complication. Predisposing factors remain poorly understood. Potential risk factors include the specific therapeutic modality and dosage, an association with cranial irradiation, long-term treatment [12, 13], genetic background and idiosyncratic patient predilections [7, 8]. Therefore, MTX toxicity seems to be the result of a complex relationship between the dose of chemotherapy, exposure time and the patient's genetic basis. In our series, all patients were on long-term treatment, with a mean time of 1,177.8 days (4 months to 7 years), suggesting a cumulative toxic effect on the blood–brain barrier.

Some polymorphisms may regulate the side effects of MTX [20]. The most common polymorphism of MTHFR is a C > T substitution at nucleotide position 677 that causes a substitution of valine for alanine in the functional enzyme. This substitution decreases its activity by 35 % in people who are heterozygous and by 70 % in those who are homozygous [21]. The TT-677 genotype results in an imbalance in the intracellular folate pools, and treatment with antimetabolites such as MTX can increase homocysteinaemia, causing additional toxicity. It has been suggested that homocysteine is at least partly responsible for ischaemic white matter changes, mineralising microangiopathy and focal neurological deficits observed after MTX treatment [22]. There are some reports of MTHFR C677T polymorphisms and MTX toxicity in patients receiving MTX [21, 23–25]. The role of related polymorphisms (such as MTHFR A1298C) [25], transporters associated with MTX clinical effects (such SLCO1B1 [26] and ABCB1 [27] polymorphisms) and the synergies between them need to be clarified.

The most frequently reported symptom was epileptic seizures in up to 88 % of patients, followed by non-specific symptoms such cognitive dysfunction, disorientation, headaches and visual disturbances [15]. Apart from ours, only one patient presented a cerebellar syndrome [12].

The CSF analysis often shows a normal pleocytosis and normal proteins. However, mild elevations of both parameters could be seen. In some cases, elevation of the myelin basic protein, the IgG or its index can be present, possibly as a reflection of the demyelination process [11, 14]. In only one patient, increased levels of MTX in the CSF were found [16].

Neuroimaging is the most useful test for the diagnosis. The typical findings are hyperintense, symmetrical, white matter lesions, located preferably in the parieto-occipital lobes on T2 and FLAIR [28]. However, involvement of other structures such as the frontal lobe, brainstem, basal ganglia, thalamus, cerebellum and even spinal cord has been described. The restricted diffusion on DWI and anisotropic diffusion studies suggest vasogenic oedema and a disruption of the BBB as the pathogenic mechanism. Classically, the lesions are non-enhancing on postgadolinium T1-weighted images; however, in some cases, a solid enhancement has been reported [12]. It is hypothesised that the enhancement and the presence of multiple low-signal foci within the T2WI abnormalities are suggestive of DNL, a severe form of encephalopathy [29]. The limited data from MRS studies showed a reduced NAA and elevated choline with or without a lactate peak [12, 30], which may be associated with an acute demyelinating mechanism [31].

Biopsy is not useful for the diagnosis; in the two reported cases with histopathology, an extensive demyelination with macrophagic infiltration, pericapillary lymphomononuclear infiltrate and fibrinoid changes in the tunica intima was described [11, 12]. This pattern is similar to those reported in intravenous administration encephalopathy [8].

Treatment is based on drug cessation. It appears that prompt cessation of treatment is associated with a better outcome with resolution of the symptoms in most patients within months [15]. In the previous reported cases, three patients died or did not show obvious neurological improvement [9, 11, 12]. In the remaining patients, although they were asymptomatic, the radiological recovery was much slower.

Our patient developed a severe cerebellar syndrome while on long-term, low-dose, oral MTX therapy and had never received intravenous or intrathecal MTX or cranial irradiation. These symptoms were attributable to the predominant involvement of the cerebellum as shown on MRI. High-intensity signals on the ADC map and restriction on DWI with the positive enhancement effect suggested the presence of vasogenic oedema and disruption of the blood–brain barrier. The involvement in rickettsial encephalitis is often due to an invasion of the vascular endothelial cells, resulting in widespread vasculitis of capillaries, arterioles and small arteries [32]. The absence of compatible symptoms of encephalitis, the findings in the MRI, bilateral but asymmetric distributions of MRI abnormalities, and the normocytosis in CSF are incompatible with a clinical picture of cerebrovascular disorders and encephalitis. MRI findings in the present patient were compatible with those of MTX-induced leukoencephalopathy. Rapid improvement of neurological symptoms and MRI findings after high-dose steroid therapy with cessation of MTX also supports the possible central role of the MTX in the pathogenesis of leukoencephalopathy in the present patient.

Conclusion

MTX-induced leukoencephalopathy can occur even with low-dose administration. This complication is unusual but important, as there is the possibility of neurological sequelae or death. Early recognition of the condition and withdrawal of the drug lead to a better prognosis. The nature of the relationship between the polymorphism and CNS toxicity is still unclear, and further studies are warranted.