Pure Apraxia of Speech After Resection Based in the Posterior Middle Frontal Gyrus

Abstract

Background and importance: Apraxia of speech is a disorder of articulatory coordination and planning in speech sound production. Its diagnosis is based on deficits in articulation, prosody, and fluency. It is often described concurrent with aphasia or dysarthria, while pure apraxia of speech is a rare entity.

Clinical presentation: A right-handed man underwent focal surgical resection of a recurrent grade III astrocytoma in the left hemisphere dorsal premotor cortex located in the posterior middle frontal gyrus. After the procedure, he experienced significant long-term speech production difficulties. A battery of standard and custom language and articulatory assessments were administered, revealing intact comprehension and naming abilities, and preserved strength in orofacial articulators, but considerable deficits in articulatory coordination, fluency, and prosody-consistent with diagnosis of pure apraxia of speech. Tractography and resection volumes compared with publicly available imaging data from the Human Connectome Project suggest possible overlap with area 55b, an under-recognized language area in the dorsal premotor cortex and has white matter connectivity with the superior longitudinal fasciculus.

Conclusion: The case reported here details a rare clinical entity, pure apraxia of speech resulting from resection of posterior middle frontal gyrus. While not a classical language area, emerging literature supports the role of this area in the production of fluent speech, and has implications for surgical planning and the general neurobiology of language.

Keywords: AOS; Apraxia of speech; Area 55b; Language deficit; Premotor cortex.

FIGURE 1.

Timeline.

FIGURE 2.

A, Area 55b projected onto axial MRI of the patient prior to the resection causing his AOS. Asterix marks the first resection cavity sparing Area 55b (corresponding to the panel of Figure 1 labelled “initial resection”), which did not cause language deficits. B, Area 55b projected onto the cavity after the resection, which led to AOS deficits. C, Axial, D, coronal, and E, sagittal cuts of the MNI brain co-registered with probability maps of Area 55b (Red) and the defect prior to AOS-causing resection (black). F, 3D reconstruction with the probability map of Area 55b depicted in red and the defect prior to AOS-causing resection (black). SFG = Superior frontal gyrus; 55b = Area 55b; PreC = Pre-central gyrus; * = original resection cavity prior to AOS-causing resection.

FIGURE 3.

A and B, Diffusion tensor imaging (DTI) of a healthy control, showing Frontal aslant tracts (FAT) in pink and tracts arising from area 55b in coronal A and saggital B cuts. C and D, DTI of the patient prior to onset of AOS, showing FAT tracts in red and area 55b tracts in yellow/orange, in coronal C and saggital D cuts. E and F, T1-weighted MRI of the patient after the resection leading to AOS in coronal E and saggital F cuts.