Paraparesis in Unilateral Medial Medullary Infarction

Dear Editor,

The classical triad of medial medullary infarction (MMI) includes hemiparesis, loss of posterior column sensation on the contralateral side, and ipsilateral palsy of the hypoglossal nerve.¹ Motor weakness caused by involvement of the corticospinal tract at the medullary pyramid is known to be the most common symptom in MMI.2 Paralysis due to MMI is mostly hemiparesis (more commonly contralateral than ipsilateral), and cases with quadriparesis or monoparesis have also been reported in MMI.² However, to the best of our knowledge, paraparesis in MMI has not been reported previously. Here we report the case of a patient with unilateral MMI who presented with paraparesis.

A 56-year-old male was admitted to our emergency room with the sudden onset of right-leg monoparesis that occurred 7 hours prior to admission. He had a history of hypertension and diabetes with poor medical control, and 15 pack-year smoking. His right-leg motor power was grade 4 on the Medical Research Council (MRC) scale. Diffusion-weighted imaging (DWI) revealed a small restricted diffusion in the right medial medulla (Fig. 1A). Four hours later after admission, the patient complained of weakness in the left leg (MRC grade 2). He did not show arm or facial weakness, sensory impairment, or tongue deviation. Follow-up DWI demonstrated upward extension of the initial DWI lesion into the right medial medulla (Fig. 1B). His left-leg weakness improved to MRC grade 4 after the therapeutic-induced hypertension using phenylephrine.

Fig. 1. Imaging of the patient. A: Initial diffusion-weighted imaging (DWI) shows a focal high signal intensity in the right medial medulla (arrow 1). B: Follow-up DWI reveals upward extension of the initial DWI lesion into the right medial medulla (arrow 2). C: Schematic of the mechanism of bilateral weakness in a unilateral medial medullary lesion: ipsilateral weakness due to involvement of the corticospinal tract at or below the pyramidal decussation (1) and contralateral weakness due to involvement of the corticospinal tract above the pyramidal decussation (2).

The ipsilateral leg weakness in this case might have been due to a caudal medullary lesion that disrupted the pyramidal tract at or below the decussation (Fig. 1C). The progression to contralateral leg weakness after admission might have been caused by involvement of the pyramidal tract just above the decussation (Fig. 1C). It was not clear why only the legs (and not the arms) were affected in this case. Because the medullary pyramid is the narrowest area of the descending corticospinal tract, paralysis involving the arm and leg simultaneously is seen in most of the patients with MMI, but a few cases of monoparesis have been reported.²,³ Similarly, only leg somatotopy within the medullary pyramid might have been involved in our patient. Somatotopy of the corticospinal tract has been investigated in many studies, however, relatively little is known about the somatotopy of the corticospinal tract at the lower medullary pyramid.⁴,⁵

In summary, we have presented the first case report of paraparesis in unilateral MMI. In cases of weakness in both legs, the possibility of a unilateral medial medullary lesion adjacent to (above and below) the pyramidal decussation could also be considered along with other differential localization.