Differential Diagnosis in Neurology

15.4. The Parietal Lobe

The parietal lobe is central to somatosensory function, awareness of personal and extrapersonal space, awareness of the body parts in space and their ability to function. Recently, it has been shown that there is a gravitational awareness in which the patient is aware of the relation between his body position to the vertical plane. This is known as the subjective visual vertical. Area of interest visual cells in the posterior parietal regions ( BA 7a) prime the frontal eye fields to focus on specific objects of interest. There is a "loathness" to move of the contralateral extremities following parietal lesions. The left parietal lobe is vital for reading, writing and receptive speech.

Lesions of BA 3b of SI (the primary parietal cortex or postcentral gyrus) cause the sensation of heaviness in the extremity and are the receptive areas for muscle spindles. Areas BA 3b and BA1 information from cutaneous afferents that signal texture of a palpated object. Area BA2 is for "shape" information and receives afferents from deep tissues (joints, fascia, tendons). Posterior thalamic areas as well as reticular nuclear afferents project to posterior insular cortex as well as SI and SII.

General Parietal Lobe Sensory Function (Similar in Each Hemisphere)

• Two-point discrimination
• Localization of extremities and objects in near and far space (outside of the examiner's grasp)
• Graphesthesia
• Stereognosis
• Point localization
• Perceptual rivalry of double simultaneous stimuli. Non-perception of the damaged side
• Posterior parietal cortex
  • Reaching BA 7a
  • Grasping BA 7b
• Anterior superior portion of the posterior parietal cortex
  • Manipulation or active touch
• Posterior superior parietal cortex
  • Inability to utilize stereotopic vision for hand movements
  • Optic apraxia (BA 5)
• Gesture representations:
  • Bilateral ventral medial parietal lobe
  • Inability to recognize objects (BA 7)
• Inferior parietal lobe
  • Memory or recognition of spatial position
• Medial parietal lobe-sense of direction (R > L)
• Intraoral sensation (parietal operculum)

Right Parietal Lobe

• Visuospatial processing
• Asomatognosia (denial of a body part)
• Denial of illness-assosoganosia
• Asimultagnosia:
  • Inability to synthesize specific parts of an object into a whole (eyes, ears and nose, but patient cannot synthesize this into a face)
• Left visual inattention (neglect)
• Aprosody; flat speech without the proper intonations appropriate for the circumstances
• Constructional apraxia
• Dressing apraxia
• Pressure of speech
• Inability to judge the directional orientation of lines (right posterior parietal areas)
• Benton and Von Allen's facial recognition test:
  • Post rolandic areas
  • Ability to discriminate unfamiliar faces
• A response to a question posed to an adjacent patient
• Right superior parietal cortex:
  • Visual manual coordination
  • Optic ataxia
  • Landmark test; ability to judge a previously bisected line
• Gourmand Syndrome
  • Eating disorder
• Decreased short term memory
• SII: asymbolia for pain:
  • The whole body is represented (head dorsal and feet ventral)
  • Sensation of heat that may envelop the entire body
• Topographic disorientation-right medial parietal lobe
• Topographic amnesia (inability to remember shape and locations)
• Alloesthesia:
  • Touch on the abnormal side is transferred to a homologous position on the normal side
  • Impulse control disorder

Left Parietal Lobe Function

• Von Gerstmann syndrome:
  • Right left confusion
  • Inability to cross the midline
  • Finger agnosia
  • Conduction aphasia
  • Dyslexia with or without agraphia
• Bilateral deficits in point localization
• Alexia without agraphia (BA 39, 40, 41)
• Conduction aphasia:
  • Disconnection of Wernicke's and Broca's area
  • Damage to the arcuate fasciculus
• Inferior left posterior parietal lobule:
  • Deficits in auditory verbal short term memory
• Superior parietal lobule-appreciation of detail
• Parieto-occipital junction – Balint's Syndrome
  • Deficit in visual scanning
  • Optic ataxia (undershooting an object)
  • Deficits of visual fixation (unable to break fixation)
• Color anomia
• Anomic aphasia (supramarginal gyrus)
• Posterior alien hand (posterior parietal BA 5 and 7)
• Ideational apraxia:
  • Supramarginal gyrus
  • Left superior lobule