Isolated ring finger palsy due to cortical infarction of the primary motor cortex hand area

Introduction: Isolated hand paralysis caused by small cortical infarction is rarely observed and may be misdiagnosed as peripheral nerve lesion when presenting with predominant involvement of the ulnar- or radial-sided fingers. We report a case of isolated palsy of the ring finger due to cortical infarction.

Case report: A 68-year-old right handed man suddenly developed numbness in his right hand and right facial side followed by weakness of his right ring finger. Examination showed weakness particular in extension of the ring finger (MRC grade 3/5) whereas flexion, abduction and adduction were only mildly impaired (MRC grade 4+/5). The strength of the other fingers, wrist, elbow and shoulder and leg were normal. Facial weakness was absent. Sensibility for aesthesia, algesia, vibration, temperature and joint position were normal in all limbs including in his right ring finger. There was no Dupuytren contraction present. Brain MRI with diffusion-weighted imaging showed a small ischemic infarction located at the epsilon-shaped precentral knob on the contralateral precentral gyrus. Somato-sensory evoked potentials were normal to both hands with no evidence for a peripheral or central sensory affection. Vascular risk factors were hypertension and hypercholesterolemia. Transthoracic and transesophageal echocardiograms revealed a PFO without relevant shunt function and grade II insufficiency of the mitral valve. The electrocardiogram showed sinus rhythm with ventricular extrasystoles, no signs of atrial fibrillation. Extra- and transcranial ultrasond showed atherosclerotic changes but no stenosis or occlusion of the arteries. The stroke etiology was embolic, more likely artery-to-artery embolism than cardiogenic embolism.

Discussion: The patient reported here presented isolated weakness of his right ring finger due to infarction in the hand area of the primary motor cortex (Brodman area 4). Recent MRI and functional MRI studys located the cortical motor hand area at the precentral knob with its characteristic shape of an inverted omega or an epsilon on the precentral gyrus in the axial plane, sagittal MRI indicated that the hand area is located in the middle of the lower portion of the anterior wall of the central sulcus. In accordance with previous studies, as lesions related to predominant involvement of the ulnar fingers were located more medially than those associated with paralysis of the radial fingers, the MRI lesion in this case was placed in the medial part of the precentral knob. A recent study revealed that infarction in the medial portion of the precentral knob topographically representing ulnar-sided fingers was frequently associated with stenosis or occlusion of internal carotid artery (ICA) or middle cerebral artery (MCA), considering a pathogenetic hemodynamic mechanism in the border zone between MCA and the anterior cerebral artery (ACA) territories. However, there was no evidence for hemodynamic mechanism in our case.