Lentiform fork sign: a magnetic resonance finding in a case of acute metabolic acidosis

Abstract

We report a 33 year-old woman addicted to chronic unspecified solvents abuse with stupor, respiratory disorders, tetraplegia and severe metabolic acidosis. On admission an unenhanced cranial CT scan showed symmetrical hypodensities of both lentiform nuclei. MR imaging performed 12 hours after stupor demonstrates bilateral putaminal hemorrhagic necrosis, bilateral external capsule, corona radiata and deep cerebellar hyperintensities with right cingulate cortex involvement. DWI reflected bilateral putaminal hyperintensities with restricted water diffusion as to citotoxic edema and development of vasogenic edema in the external capsule recalling a fork. On day twenty, after specific treatments MRI demonstrated a bilateral putaminal marginal enhancement. Bilateral putaminal necrosis is a characteristic but non-specific radiological finding of methanol poisoning. Lentiform Fork sign is a rare MRI finding reported in literature in 22 patients with various conditions characterized by metabolic acidosis. Vasogenic edema may be due to the differences in metabolic vulnerability between neurons and astrocytes. We postulate that metabolic acidosis could have an important role to generate this sign.

Keywords: MRI; diffusion-weighted imaging; lentiform fork sign; metabolic acidosis; putaminal necrosis.

Figure 1

A) Unenhanced CT scan indicates symmetrical hypodensities of both lentiform nuclei (arrowheads). B-D) MRI FLAIR images reveal bilateral hyperintensities involving the lentiform nuclei (short white arrow), right cingulum cortex (long white arrow) and cerebellum (arrowhead) with a brightly hyperintense rim surrounding both putamina resembling a fork. E) The T2 sequence shows bilateral putaminal hyperintensities with the lentiform fork (arrowheads). F) T1 MRI shows hypointensity over the basal ganglia bilaterally (arrowheads). G) Axial T2 Fast Field Echo (FFE) demonstrates hypointensities on both basal ganglia (arrowheads).

Figure 2

A) DWI shows diffusion restriction in both basal ganglia, in particular the putamen and globus pallidus. B) ADC map shows low signal intensities in the lentiform nucleus bilaterally and high signal intensities of both the forks.

Figure 3

A) Brain CT scan on day 10 shows a soft reduction of bilateral putaminal hypodensities (arrowheads). B,C) Follow-up brain MRI (20 days later) reveals a reduction of lentiform hyperintensities (short white arrows) and post-contrast T1 sequences demonstrate bilateral putaminal lesions with marginal enhancement (long white arrows).