parietal lobe

ANATOMY: Boundaries: frontal lobe anteriorly (central sulcus), occipital lobe posteriorly (parieto-occipital sulcus), temporal lobe inferiorly (lateral sulcus/Sylvian fissure). Key regions: (1) Postcentral gyrus = primary somatosensory cortex (Brodmann areas 3, 1, 2), arranged as the sensory homunculus. The lower limb/foot is medial (ACA territory) and the face/hand is lateral (MCA territory). (2) Superior parietal lobule (areas 5, 7) handles spatial awareness and sensory integration, including stereognosis and graphesthesia. (3) Inferior parietal lobule includes the supramarginal gyrus (area 40) and angular gyrus (area 39), which are involved in language, reading/writing, and calculation in the dominant hemisphere. (4) Precuneus (medial) is involved in visuospatial processing and self-referential thought.

BLOOD SUPPLY: Mostly MCA (lateral convexity) and ACA (medial surface). The posterior parietal region is a watershed zone between the MCA, ACA, and PCA. Global hypoperfusion can cause "man-in-a-barrel" syndrome or Balint syndrome.

SENSORY PATHWAY: Dorsal column-medial lemniscus and spinothalamic tracts synapse in the VPL (body) and VPM (face) of the thalamus, then project to the postcentral gyrus via the posterior limb of the internal capsule.

LESION SYNDROMES: - Primary somatosensory cortex: contralateral loss of fine touch, proprioception, vibration, and two-point discrimination. Crude touch and pain are relatively preserved because of thalamic perception. - Cortical sensory loss: astereognosis (can't identify objects by touch), agraphesthesia, and impaired two-point discrimination, with intact primary modalities. - Non-dominant (usually right) parietal lesion: contralateral hemispatial neglect (most often left), anosognosia (denial of deficit), asomatognosia, constructional/dressing apraxia, and impaired visuospatial tasks (e.g., clock drawing with numbers crowded on one side). Seen commonly after right MCA stroke. - Dominant (usually left) parietal lesion: Gerstmann syndrome (angular gyrus), with agraphia, acalculia, finger agnosia, and left-right disorientation. Also ideomotor/ideational apraxia and alexia with agraphia. - Bilateral posterior parietal/parieto-occipital lesions: Balint syndrome (simultanagnosia, optic ataxia, oculomotor apraxia). - Optic radiation (superior fibers, Baum's loop = parietal): contralateral inferior quadrantanopia ("pie on the floor"). Remember "PITS": Parietal-Inferior, Temporal-Superior. - Abnormal optokinetic nystagmus: with a parietal lesion, the target is not tracked toward the lesion side (slow phase impaired).

CLINICAL PEARLS: Hemispatial neglect is tested with line bisection, clock drawing, and double simultaneous stimulation (extinction). The non-dominant hemisphere attends to both sides of space, while the dominant hemisphere attends mainly to the contralateral side. That is why right-sided lesions cause more severe neglect. Parietal lobe seizures can cause contralateral paresthesias or a sensory "march." Alzheimer disease and corticobasal degeneration commonly involve the parietal cortex (asymmetric apraxia, alien limb phenomenon in CBD). Posterior cortical atrophy is a visual variant of AD with Balint-like features.

HIGH-YIELD MNEMONIC: Dominant parietal = Gerstmann ("Can't Write, Can't Add, Can't tell Left from right, Can't name Fingers"). Non-dominant parietal = neglect.