“Invisible” Brain Stem Infarction at the First Day

Background

In specific stroke cases, serial diffusion-weighted magnetic resonance imaging (DW MRI) on day 1 was unable to show a lesion, whereas that on day 4 and later clearly revealed a lesion. However, clinical features of this phenomenon (“invisible” brain stem infarction [IBI] at the first day) have not been fully delineated.

Methods

We retrospectively recruited 212 stroke patients in the Emergency Unit and Neurology Department. Among these, we studied patients with IBI. Definition of IBI is that acute and clear brain stem symptoms/signs on arrival were ameliorated at discharge and appearance of high signal intensity on serial DW images with low apparent diffusion coefficient (ADC) by 1.5 T MRI with 2-mm slices.

Results

IBI were found in only 6 patients. Day 1 invisible stroke was found only in the brain stem (17%, 6 of 35) but none (0 of 177) in the hemispheric infarction (P < .05). In most patients with IBI, DW MRI turned out visible at the third/fourth day. Before the fourth day, DW/ADC signal changes in patients with IBI were minimal. In IBI, lesion size (mean 2.7 mm²) was smaller than that of visible cases (mean 7.3 mm²). In IBI, lesion location was mostly at the dorsolateral medulla. In IBI, sensory disturbance was significantly more common (67%) than visible cases (24%; P < .05), whereas dysarthria was less common (0%; P < .01) than visible cases (66%; P < .01).

Conclusions

It is likely that patients with smaller stroke volume, sensory disturbance, and medullary location are prone to develop IBI. When evaluating stroke using MRI criteria, recognition of IBI is important to start early management.

Introduction

It is well recognized that diffusion-weighted magnetic resonance imaging (DW MRI) is a sensitive and specific technique for imaging acute hemispheric infarction. In contrast, its utility in the diagnosis of acute brain stem infarction has not been fully established.1, 2 This is because in specific cases, serial DW MRI on day 1 was unable to show a lesion, whereas that on day 4 and later clearly revealed a lesion. This phenomenon (“invisible” brain stem infarction [IBI] at the first day) occurs with a discrete lesion at dorsolateral medulla (presenting isolated vertigo,³ isolated hemiataxia,⁴ Wallenberg syndrome,5, 6, 7, 8, 9 or Avellis syndrome⁸), dorsal pons (isolated internuclear ophthalmoplegia,¹⁰ or Millard–Gubler syndrome⁸), and basal midbrain (Weber syndrome⁸). Recognition of IBI in clinical practice is extremely important because anticoagulation and neuroprotection should start as early as possible. However, it remains unclear whether specific neurologic symptom, lesion location, or lesion size is related with IBI and what is the putative mechanism of IBI. Here, we systematically investigated IBI in our neurologic emergency cases.