- Top of page
Motor deficits in cerebral palsy (CP) have been well documented; however, associated sensory impairment in CP remains poorly understood. We examined tactile object recognition in the hands using geometric shapes, common objects, and capital letters. Discrimination of tactile roughness was tested using paired horizontal gratings of varied groove widths passively translated across the index finger. We tested 17 individuals with hemiplegia (mean 13y 9mo [SD 5y 2mo]; 6 males, 11 females), 21 with diplegia (mean 14y 10mo [SD 7y]; 10 males, 11 females), and 21 without disabilities (mean 14y 10mo [SD 5y 1mo]; 11 males, 10 females). All participants with CP fell within level I or II of the Gross Motor Function Classification System and level I or II of the Manual Abilities Classification System. Individuals with CP were significantly less accurate compared with those without disabilities on all tactile object-recognition tasks using their non-dominant hand. Both groups of patients also had significantly higher thresholds for groove-width differences with both hands compared with those without disabilities. Within the group with diplegia, only roughness discrimination differed between hands, whereas within the group with hemiplegia, significant between-limb differences were present for all tasks. Despite mild motor deficits compared with the entire population of individuals with CP, this sample demonstrated ubiquitous tactile deficits.
Cerebral palsy (CP) encompasses non-progressive heterogeneous disorders of the developing central nervous system and is the most prevalent childhood physical disability, affecting 2 to 3 per 1000 live births.1 The etiologies of diplegic and hemiplegic CP commonly involve pathology of the central nervous system that alters normal development of the somatosensory system. For example, recent diffusion tensor imaging in individuals with diplegic CP showed prominent damage to the thalamocortical projections to the somatosensory cortex, with less frequent severe damage to the corticospinal tracts, despite a history and clinical presentation consistent with motor tract injury.2
Tactile input is used to localize and characterize the various qualities of touch. Cutaneous input also contributes to proprioceptive information for coordinated motor action.3 Bolanos et al.4 proposed that somatosensory testing be an important part of rehabilitation assessments because tactile discrimination and tactile object recognition are necessary for finger dexterity. Decreased somatosensory functioning of the hand (two-point discrimination, stereognosis, and pressure sensitivity) correlates with diminished dexterity of the affected hand in hemiplegic CP.5 A series of object manipulation studies found that children with CP inaccurately plan and scale the rate of grip, and that load force increases according to the object being grasped.6 The authors related this deficiency to impaired somatosensory perceptions.7 Therefore, numerous reports associate motor impairments to somatosensory deficits in CP, providing a rationale for the importance of assessing tactile sensitivity in this population.
With the exception of the studies on quantitative grip force6,7 and that by Krumlinde-Sundholm and Eliasson,8 which used established and novel measures to assess tactile sensibility and dexterity in children with hemiplegia compared with those without disabilities, most previous studies in CP examined tactile abilities using imprecise, non-standardized, or inadequately parameterized measures, and often failed to include normative data. Additionally, several studies combined data from various subtypes and severities of CP, ignoring the unique brain lesions associated with different CP subtypes. Furthermore, when the sample was narrowed to a single subtype, most studies focused on those with hemiplegia rather than diplegia, despite similar prevalence of both clinical conditions.1
Previous studies found stereognosis and spatial acuity the most often and significantly impaired tactile modalities in CP.4,9 Stereognosis deficits have been identified in CP by using familiar objects and shapes, and spatial acuity deficits have been assessed with two-point discrimination.4,5,7–12 In hemiplegia, Semmes-Weinstein monofilaments and the Manual Form Perception Test13 have also revealed punctate tactile and stereognostic deficits respectively.7,8,12 However, evidence of tactile deficits in CP is largely contradictory, especially between studies that reported data from different types of assessment, but even among studies that used identical assessment tools. For example, Cooper and colleagues12 reported no significant differences in stereognosis, proprioception, and light touch between hands in hemiplegia, whereas others showed significant differences between hands in these somatosensory modalities.5 Furthermore, contradictory reports of tactile deficits in the dominant hand in hemiplegia have included findings of deficiencies10 as well as intact tactile function.5,11 Most previous studies reported deficits in stereognosis and/or two-point discrimination for approximately 30 to 50% of people with CP,9 yet others reported much more prevalent tactile deficiencies.5,12 A paucity of standardized or sufficiently sensitive psychophysical assessments plausibly underlies the contradictory findings on impaired touch in CP. Tests of two-point discrimination are the most widely assessed tactile ability in many previous CP studies. However, standard two-point discrimination testing can be unreliable because application force varies across trials and testers,14 non-spatial cues exist from instrument vibration,15 two points (broad stimulus) versus one (narrow stimulus) stimulate different areal extents,16 and stimulation from pressure points can be non-synchronous.14
Johnson, Van Boven, Phillips (JVP) domes assess spatial discrimination with varying grating dimensions, while addressing the above limitations of two-point discrimination.17 Sanger and Kukke17 revealed spatial discrimination abnormalities in individuals with diplegia using JVP domes. However, half of the participants with diplegia in that study were unable to detect the largest grating, suggesting that commercially available JVP domes do not sensitively measure tactile abilities in diplegia. Similarly, our pilot testing showed JVP domes did not have wide enough groove widths to measure tactile abilities accurately in people with CP.
In the present study, we evaluated somatosensory tactile psychophysics in both hands of individuals with diplegia and hemiplegia and compared performance to an age-matched group without disability. We determined somatosensory abilities in individuals with clinically described diplegia or hemiplegia, who were within levels I or II on the Gross Motor Functional Classification System18 and Manual Ability Classification System.19 A mild subgroup was tested to differentiate motor from somatosensory deficits, which becomes more difficult as the degree of motor involvement increases. A larger effort will be needed to characterize somatosensory abilities in a wider range of severities and types of CP. Additionally, we obtained functional neuroimaging data from many of these same participants, which was possible in those able to maintain postural stability during scanning owing to fewer motor and tone abnormalities.
We assessed tactile identification of common objects, geometric shapes, raised letters, and discrimination of roughness. The three object recognition tasks were comparable to previous well-studied evaluations of the tactile shape recognition system20 and were selected because of functional relevance to manual dexterity and everyday experience in CP. Collectively the touched objects varied in size, familiarity, and difficulty. Each task probed a different aspect of tactile processing. For example, tactile identification of common objects allows for familiarity to compensate for diminished sensations because the objects touched in the current study are commonly experienced haptically. Tactile experience is less with embossed geometric shapes and letters. The geometric shape task probed the effects of object size on tactile abilities parametrically with the hypothesis that haptic identification of smaller versus larger shapes would be more difficult. Finally, we hypothesized that the letter identification task would most sensitively reveal group differences because this task is the most novel by touch and requires integration of multiple tactile shape features before correctly naming the letter. Most people have little or no experience with touching embossed letters, where the tactile shape features of each letter must be felt, stored, and integrated before accessing sublexical brain centers to identify the letter.
A previously described sensitive assay of texture perception was a roughness discrimination task based on horizontal gratings of varying dimensions.21,22 This task minimizes the effects of application force variance with repeated applications and non-spatial cues, thereby avoiding the aforementioned limitations of classical two-point discrimination testing. Here, roughness perception is proportional to groove width, with larger groove widths perceived as being rougher.23 Therefore discrimination thresholds are determined by varying felt groove-width differences to assess potential sensitivity differences between limbs and among groups. Groove widths chosen for this study were based on pilot testing in participants with CP and without disabilities and were aimed at approximating 75% performance accuracy for both hands, which is the Weber threshold commonly used for two-alternative forced choice tasks.22
All tactile tests compared performance between dominant and non-dominant hands and among hemiplegia, diplegia, and control groups. We hypothesized that individuals with diplegia would be less proficient in tactile object recognition and exhibit higher thresholds for discrimination of differences in groove width compared with those without disabilities, and that participants with hemiplegia would be less proficient on their non-dominant hand compared with their dominant hand, and on both sides compared with those without disabilities.
- Top of page
Individuals with CP exhibited tactile sensory deficits in both hands. The observed deficit magnitudes were surprising, especially for the dominant hand, given mild motor impairments in all participants with CP. For example, in diplegia, roughness discrimination and tactile object recognition of geometric shapes and embossed letters were deficient bilaterally; and in hemiplegia, haptic identification of embossed letters and roughness perception deficits were present bilaterally. The clinical significance of these findings on the dominant side in hemiplegia and the relation between somatosensory and motor deficits in all groups and limbs warrants further study.
Previous investigations of upper extremity touch in diplegia did not separate analyses by side of limb dominance but instead grouped results from both limbs or strictly by side (left and right), irrespective of predominant use. Examinations based on combined assessment of both upper limbs may overlook possible bilateral asymmetries in diplegia owing to asymmetrical brain injuries or differences in use. In contrast, studies of hemiplegia usually grouped data by limb dominance, typically using the clinical motor symptoms to determine affected side(s); (e.g., see Krumlinde-Sundholm and Eliasson8). We indexed hand dominance using a modified Edinburgh Handedness Inventory,24 which indicates the limb (left or right) predominantly used during 12 common motor tasks (e.g., writing, throwing, and kicking). The Edinburgh Handedness Inventory validity in individuals with CP has not been established. However, the categorization of each participant’s dominant side matched their reported or diagnosed less affected side. Limb dominance, however, does not imply normality, but simply identifies the limb used most successfully on motor tasks.
The non-dominant arm showed greater sensory impairments, but the dominant hand also was affected in diplegia. Observed deficits were consistent with previous reports of problems in identifying shapes, common objects, or in two-point discrimination thresholds.4,10,11 However, there were subtle differences between hands in diplegia. The non-dominant hand in diplegia was significantly less sensitive on all tactile tasks compared with those without disabilities. The dominant hand exhibited significant deficits on the geometric shapes, letters, and roughness discrimination tasks compared with those without disabilities. These findings suggest that the tests used were more sensitive than those used previously.
Individuals with hemiplegia have pervasive touch deficits of the non-dominant hand, as reported previously using clinically oriented tests.4,5,7–12 Previous reports are equivocal on touch deficits with the dominant hand in hemiplegia. Some investigators reported comparable touch sensitivity of the dominant hands in hemiplegia and control groups,8,11 whereas others found tactile perception deficits.10 In the present study, the letter identification and roughness discrimination tasks revealed significant deficits of the dominant hand in hemiplegia. Possibly these findings reflected that both tasks required attending to and remembering multiple tactile features, again emphasizing considerably heightened sensitivity over standard clinical assessments in revealing even bilateral upper limb touch deficits in hemiplegia.
The tasks used in the present study have been extensively vetted with neurologically normal individuals in studies involving recognition of shapes and common objects,20 raised letters,25 and surface roughness.21 Tasks using shapes and common objects probed recognition based on identifying a few contours that linked tactile input to extended prior experience and memory. Differences in object size or shape had no effect on performance on the geometric shapes task. The letter identification task most effectively revealed differences between limbs and among groups. The sensitivity of the letters task possibly reflected less familiarity with touching embossed letters and with its inherent complexity, which involves perceiving, remembering, and integrating multiple shape features (e.g. points, line orientations, spaces) before assembling a composite image that is compared against a sublexical memory. Similarly, using horizontal gratings to examine roughness perception provides an especially sensitive, parametric method for assessing tactile sensory abilities. It is a tactile test in which increasing the grating groove width increases perception of surface roughness.23 Exposure to tactile gratings and judgments about surface roughness are probably atypical experiences with consequent infrequent categorization; therefore discrimination of roughness differences is relatively untainted by everyday lexical categories.
The tactile sensory impairments in both upper limbs of individuals with CP probably impact tactile guidance of the hands, especially in haptics, and possibly contribute to awkward dexterity owing to diminished sensory information when touching objects. Thus a clinical presentation of diminished motor coordination might be considered a combined sensorimotor deficit. The effects of diminished somatosensory input on motor function in CP require further examination. These results recommend that clinical assessments be broadened to include psychophysical somatosensory testing of all limbs in CP. The tasks used here can be adapted for clinical use to detect differences between individuals and between limbs for an individual. Letter identification and roughness discrimination tasks sensitively detected subtle tactile deficits, even in individuals with mild diplegia or hemiplegia. The functional relevance of tactile deficits, particularly for individuals with CP with greater motor involvement who are likely to have even more pronounced sensory impairment, warrants further investigation.