Limb Apraxias and Related Disorders

Apraxia is an inability to correctly perform learned skilled movements. In limb apraxias, there is an inability to correctly execute these movements in an arm or hand owing to neurological dysfunction. Apraxia is essentially a cognitive deficit in motor programming and results in errors either of the spatiotemporal processing of the movements or in the content of the actions. During the course of an apraxia examination, these errors can help distinguish the major types of limb apraxias.

A first step in recognizing the limb apraxias is distinguishing them from other causes of impaired movement. First, apraxia is distinct from elementary motor deficits such as weakness, hemiparesis, spasticity, ataxia, or extrapyramidal disturbances. Second, apraxia is distinguishable from impaired movements due to primary sensory deficits, hemispatial neglect, spatial or object agnosia, or other sensory or spatial disorders. Third, apraxia is distinct from abnormal movements or postures such as tremor, myoclonus, choreoathetosis, or dystonic posturing. Finally, it is not apraxia if the impaired movements result from other cognitive disorders involving attention, memory, language comprehension, or executive functions ( ).

Limb apraxia is not rare or insignificant. Apraxia occurs in about 50%–80% of patients with left hemisphere lesions and can persist as a chronic deficit in 40%–50% of these. It occurs in a variety of disorders, including stroke ( ), multiple sclerosis ( ), tumors such as parietal gliomas ( ), corticobasal syndrome ( ), Alzheimer disease ( ), some forms of primary progressive aphasia ( ), Parkinson disease, dementia with Lewy bodies ( ), Huntington disease ( ), Creutzfeldt-Jakob disease ( ), and even some patients with schizophrenia ( ). A careful examination for limb apraxia can lead to the differentiation of diseases such as Alzheimer disease from frontotemporal dementia, or dementia with Lewy bodies from other disorders ( ).

Limb apraxia often results in major functional impairment, even when subtle, as it affects critical movements of the arms, hands, and fingers. Limb apraxia correlates with greater caregiver dependence and need for help with activities of daily living (ADLs) ( ), and it can also interfere with rehabilitation therapy and the use of gestural communication.

Despite its importance, clinicians often fail to recognize limb apraxia. In many left hemisphere strokes, right hemiparesis masks the presence of right limb apraxia, and the assumption of normal clumsiness of the nondominant hand may mask the presence of left limb apraxia. Patients with limb apraxia from left hemisphere strokes can have a reduced awareness of limb apraxia ( ), making recovery more difficult. Even when there are no masking factors, the presence of limb apraxia may still go undetected. Many examiners do not evaluate patients for limb apraxia, do not know how to test for apraxia, or cannot recognize the spatiotemporal or content errors produced by this condition.

This chapter is about the limb apraxias. The term apraxia occurs broadly in neurology and is usually interchangeable with dyspraxia . Clinicians use apraxia to describe nonlearned motor dysfunctions including oculomotor movements, gait initiation (magnetic apraxia), and eyelid opening. They also use apraxia to describe skilled motor tasks that are dependent on visuospatial processing, including optic, constructional, and dressing apraxia. Apraxia correctly applies to conditions that are more clearly consistent with the definition of disturbances in learned skilled movements but involve body parts other than the limbs, including oro-buccal-facial and speech apraxias. These clinical entities are not included in this chapter, because they are either not limb apraxias or not true disorders of “praxis” in the sense of disturbances in learned skilled movements ( ). The focus of this chapter is on the seven major limb apraxias of the upper extremities. They include ideomotor apraxia, parietal variant; ideomotor apraxia, disconnection variant; dissociation apraxia; ideational apraxia; and conceptual apraxia. Also included is limb-kinetic apraxia, a disorder that some argue is not a true apraxia, but instead a more basic disturbance in fine motor movements. Callosal apraxias comprise a separate category because of their unique unilateral and varied manifestations.

Historical Perspective

Many clinicians and investigators helped develop the current concept of limb apraxia. In 1866, John Hughlings Jackson probably recognized limb apraxia when he observed that the patient had “power in his muscles and in the centres for coordination of muscular groups, but he—the whole man, or the ‘will’—cannot set them agoing” ( ). In 1870, Carl Maria Finkelnburg used “asymbolia” to describe the clumsy and incomprehensible communicative gestures in aphasics, and in 1890, Meynert distinguished motor asymbolia from decreased motor “images” for movement. In 1899, D. De Buck used “parakinesia” to describe a patient who “though retaining the concepts for her actions, did not succeed in awakening the corresponding kinetic image.” By this time, the stage was set for Hugo Karl Liepmann’s seminal model of the limb apraxias.

In the early 1900s, Liepmann published a series of papers that led to the contemporary concept of limb apraxias. He proposed that the execution of purposeful movements could be divided into three steps ( ). First is the retrieval of the spatial and temporal representation or “movement formulas” of the intended action from the left hemisphere. Second is the transfer and association of these movement formulas via cortical connections with the “innervatory patterns” or motor programs located in the left “sensomotorium” (which includes premotor and supplementary motor areas [SMAs]). Third is the transmission of the information to the left primary motor cortex for performance of the intended actions in the right limb. Finally, in order for the left limb to perform the movements, the information traverses the corpus callosum to the right sensomotorium to activate the right primary motor cortex. Using Heymann Steinthal’s term of “apraxia,” Liepmann classified disturbances in these connections as “ideational, ideo-kinetic (melokinetic), and limb-kinetic apraxia.” Over the years, this classification nomenclature has evolved and the application of these terms has shifted, but Liepmann’s basic formulation of apraxia has persisted to the present day.

A Model for Praxis

Models of apraxia emerge from this historical perspective. Most models of praxis include a left parietal hub with connections to anterior motor areas with a central role of learning and converting mental images of intended action into motor execution ( ) ( Fig. 11.1 ). These spatiotemporal movement formulas, also known as praxicons or visuokinesthetic motor engrams, are necessary for learned skilled movements. Multiple input modalities including visual, verbal-auditory, and tactile can activate these movement formulas. Cells in the inferior parietal lobule fire selectively in response to hand movements, visually presented information about object size and shape, or the actual manipulation of objects, and functional neuroimaging studies show activity of this region in response to recognition of actions associated with object or tool use (transitive actions) ( ). Functional neuroimaging studies have also shown activity in left parietal sub-regions with privileged connectivity to premotor and sensory areas ( ). In addition to movement formulas, the left parietal region appears to contain action semantics and conceptual systems such as tool action, tool–object association information, and general principles of tool use ( ).

If a movement involves the use of a tool or object, action semantics specify knowledge of tool action (turning, pounding, etc.) and the knowledge of which tool or object to use for a task ( ).

Beyond movement formulas and action semantics, a third important element of models of praxis is the motor programs themselves. In the premotor region, the SMA translates the movement formulas into motor programs before sending them on to primary motor cortex ( ). The SMA, which is involved in sequential movements and bimanual coordination of the upper extremities, receives projections from parietal neurons and in turn projects axons to motor neurons in the primary motor cortex. The SMA translates the parietal time-space movement formulas to specific motor programs that activate the motor neurons such that the contralateral extremity moves in the proscribed spatial trajectory and timing. For movements in the ipsilateral extremity, the brain further conveys these programs across the corpus callosum to the opposite premotor cortex.

Beyond this traditional model for praxis, apraxia may result from damage in other regions including the prefrontal cortex, right hemisphere, basal ganglia (putamen and globus pallidus), thalamus, and their white-matter connections.

The prefrontal region participates in sequencing multiple arm, hand, and finger movements. Functional magnetic resonance imaging studies suggest that proximal limb control representations are associated with bilateral inferior frontal gyri for both limbs, while distal limb control areas generally left lateralized for both limbs ( ). Both parietal regions participate in the integration of visual information and upper-extremity movement, and in performing nonpurposeful movements—a necessary aspect for learning new purposeful movements. Although the left inferior parietal lobule is more active than the right during action imagery and actual discrimination of nonpurposeful gestures, the right parietal region is more active during imitation and when these gestures consist of finger postures ( ).

The role of basal ganglia and thalamus is less clear, but they function as part of cortical–subcortical motor loops. Apraxia could, theoretically, result from damage to any of these areas outside the traditional model of praxis.

Newer models of praxis have focused on network activation as opposed to isolated regional activation. The posterior left parietal and temporal cortices as well as the dorsolateral prefrontal cortex are activated when hand gestures are planned and executed. This left parieto-fronto-temporal network, or “praxis representation network” ( ), includes two main streams: a ventral stream that processes semantic knowledge (the “what” pathway) and a dorsal stream that processes spatiotemporal information (the “where” pathway) ( ). Abnormalities of the ventral stream may impair action semantics such as the retrieval of tool-action concepts ( ), and abnormalities of the dorsal pathway may result in spatiotemporal errors in executing movement formulas ( ). The left supramarginal gyrus and left caudal middle temporal gyrus contribute to the integration of concepts with motor representations into actions ( ). Consistent with a left parieto-fronto-temporal network, damage in the inferior frontal cortex reaching to the temporal pole is associated with an increased frequency of Body-Part-as-Object errors on praxis testing ( ).

This left parieto-fronto-temporal praxis network significantly overlaps with the mirror neuron network for understanding the intentional actions of others ( ). Left hemisphere stroke patients with apraxia with deficits in gesture comprehension have had lesions in more anterior parts of the mirror neuron system, whereas those with gesture imitation deficits have had lesions in more posterior parts ( ).

Classification of Limb Apraxias

Beginning with Liepmann, there have been multiple attempts to classify and define the limb apraxias ( ). The classification presented here is based on the seminal work of Heilman and associates, who have significantly contributed to the understanding of the limb apraxias ( ). Depending on the location of the lesion, the patient has different patterns of ability to imitate and recognize gestures, perform sequential movements, and do fine motor activities ( Fig. 11.2 ). The presence of production and content errors further characterizes the subtypes of limb apraxia.

Ideomotor Apraxia, Parietal Variant

The parietal variant of ideomotor apraxia may be the most common and prototypical limb apraxia. Disruption of the movement formulas in the inferior parietal lobule impairs skilled movements on command and to imitation, as well as the recognition of gestures (see Fig. 11.2, A ). Patients make spatial and temporal errors while producing movements. There is a failure to adopt the correct posture or orientation of the arm and hand or to move the limb correctly in space and at the correct speeds. Spatial errors involve the configuration of the hand and fingers, the proper orientation of the limb to the tool or object, and the spatial trajectory of the motion. A major distinguishing feature of the parietal variant of ideomotor apraxia is difficulty recognizing or identifying gestures, implicating damage to the praxicons, visuokinesthetic motor engrams, or movement formulas themselves.

Ideomotor Apraxia, Disconnection Variant

This form of ideomotor apraxia is a disconnection of an intact parietal region from the pathways to primary motor cortices. The disconnection variant of ideomotor apraxia results from disruptions of motor programs in the SMA or in their intra- and interhemispheric connections ( ). These lesions result in impaired pantomime to verbal commands, impaired imitation of gestures, and the presence of spatiotemporal production errors. The movement formulas themselves are preserved and, in contrast to the parietal variant of ideomotor apraxia, these patients can recognize and identify gestures. The lesions lie along the route from the left inferior parietal cortex to primary motor cortices (see Fig. 11.2, B ). Although SMA lesions tend to affect both upper extremities, if the SMA lesion is limited to the right, apraxia may be limited to the left upper extremity.

Dissociation Apraxia

Patients with dissociation apraxia only exhibit errors when the movement is evoked by stimuli in one specific modality, usually the verbal or language modality. Dissociation apraxia is usually a specific disconnection between language areas and movement formulas in the inferior parietal lobule. However, information can reach the inferior parietal lobe via input modalities other than language. Patients with dissociation apraxia may be impaired when attempting to perform skilled movements in response to verbal commands, but they are able to imitate gestures and to indicate or use actual objects correctly. An important distinguishing feature of dissociation apraxia is errors that are often unrecognizable movements, rather than the spatiotemporal or content errors of other apraxia syndromes. In addition to verbal dissociation apraxia (see Fig. 11.2, C ), there can be visual (see Fig. 11.2, D ) and tactile dissociation apraxias as well.

Ideational Apraxia

Ideational apraxia is the inability to correctly order or sequence a series of movements to achieve a goal. It is a disturbance in an overall ideational action plan. When these patients are given components necessary to complete a multistep task, they have trouble carrying out the steps in the proper order, such as preparing, addressing, and then mailing a letter. However, the individual steps are performed accurately. The lesions responsible for ideational apraxias are not clear; the deficits usually occur in patients with diffuse cerebral processes such as dementia, delirium, or extensive lesions in the left hemisphere that involve the frontal lobe and SMA. Unfortunately, use of the term ideational apraxia has been confusing, with the term erroneously applied to conceptual apraxia and other disorders. Ideational apraxia is not a conceptual problem in the proper application or use of tools or objects, but rather a problem in sequencing of actions in multistep behaviors.

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