Introduction

There are global alterations in the development of individuals with Down syndrome, delaying motor milestones – both fine and gross – and lending to differential motor skill performance. The acquisition of early motor behaviors occurs over a more extensive period, with important motor milestones achieved later than those observed in typically developing children. Motor development is assessed in terms of the Gross Motor Function Measure (GMFM), evaluating functional changes in children with motor disabilities and allowing for comparison with the normative progression.

Though there is considerable variability in the degree of impairment and length of delay in Trisomy 21, motor development has been correlated with pathophysiological processes and atypical neuroanatomy – specifically cerebellar hypoplasia, the underdevelopment of the cerebellum.

Functional Review

Deficiency in the motor functioning – mild, moderate, or severe – of individuals with Down syndrome influences the rate at which functioning improves within the first six years of life but does not noticeably influence the eventual fulfillment of gross motor milestones during early childhood (Palisano, et. al., 2001). Scores on the Gross Motor Function Measure improve the fastest during infancy, with slower improvement in scores as movement complexity increases and functioning requires increased motor control. Standing position and walking ability, for instance, are significantly delayed in children with Down syndrome (Malak, et. al., 2015).

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(Palisano et. al., 2001)

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(Palisano et. al., 2001)
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Volumetric magnetic resonance imaging (MRI) studies of individuals with Down syndrome reveal lower brain weight and disproportionately reduced volumes in the cerebellar regions (Pinter, et. al., p. 1659, 2001; Vicari, p. 361, 2006). Similar studies have revealed a reduction in the granule cell density of both white and grey matter of approximately 70% compared to typically developing children as well as a delay in myelination (Malak, et. al., 2015). By the sixth month of life, significant changes in the volume of the cerebellum are noted together with deficient motor development in the absence of motor milestones (Malak, et. al., 2015). Acquiring motor skills at each level necessitates the involvement of the cerebellum. It is theorized that cerebellar hypoplasia – the incomplete or underdevelopment of the cerebellum – lends to the impairment of motor functioning and coordination difficulties in most children with Down syndrome. Research emphasizes the role of cerebellar hypoplasia in “muscle hypotonia, problems with movement fluency and axial control (axial truncal muscle), and body balance, coordination, and speech disorders” (Malak, et. al., 2015).
Hypotonia – reduced muscle tone – is often evidenced in individuals with Down syndrome and lends to a delay in achieving motor milestones. In order to initiate movement, a muscle must be able to generate a certain amount of force, dependent upon the intended movement. The generation of this force for contraction requires a certain level of muscle tone (Latash, Wood & Ulrich, 2008). Low muscle tone, therefore, disorders movement including the posture and balance required to perform motor tasks such as sitting upright and walking.

(Ferreira-Vasques & Lamonica, 2015)
Anatomical Review

The spinocerebellum, a functional subdivision of the cerebellum, extends through the central portions of both the anterior and posterior lobes, receives input from the spinocerebellar tract, and projects to rubrospinal, vestibulospinal, and reticulospinal tracts. Encompassing the vermis and intermediate zones of the cerebellum, the spinocerebellum are involved in motor coordination and execution through by integrating sensory input with motor commands (Knierim, 1997). The vermis – formed after the hemispheres – is more likely to be absent or underdeveloped than other parts of the cerebellum (Fredericks, p. 460, 1996).

Input Pathways

Spinocerebellar and cuneocerebellar tracts communicate proprioceptive and cutaneous information from the spinal cord to the spinocerebellum. The cerebellar vermis receives input from the visual, vestibular, and auditory systems as well as motor cortical input from the inferior cerebellar peduncle. Somatosensory information travels along the spinocerebellar pathway from the trunk and head to the vermis. There are also direct projections to the vermis from the primary sensory neurons of the vestibular system. Additionally, the cerebellar vermis receives visual and auditory input relayed from brainstem nuclei.

Output Pathways

The vermis projects to the fastigial nuclei, which give rise to multiplicity of descending pathways – the brainstem reticular formation and lateral vestibular nuclei – and ascending pathways going to the thalamus and primary motor cortex. Projections to the reticular formation from the fastigial nuclei form the medial descending tracts for motor execution, and overall control of movement is coordinated through projections to the cortex and the ventral lateral portion of the thalamus. The vermis is involved in establishing and maintaining balance through projections to the lateral vestibular nuclei.

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Figure 5.5, Neuroscience Online (Knierim, 1997)
Conclusion

Though the cerebellum does not function to induct movement, it provides information to other brain regions that are involved in motor initiation. Stimulating synchronous and accurate movements necessary for purposeful motor activity, the cerebellum is involved in the moderation and management of activities in the production of skilled voluntary movement (Fredericks, p. 445, 1996). Cerebellar hypoplasia in individuals with Down syndrome has been suggested to be a causal factor in the developmental delay of motor functioning and alteration in motor skill performance – delaying the initiation of movement and altering the form of this movement. Knowledge of the psychomotor development of individuals with Down syndrome, especially children, has practical implications for interventions in physical and occupational therapy in successfully achieving motor goals.

Glossary

Down syndrome: a genetic disorder caused when abnormal cell division results in extra genetic material from chromosome 21; varies in severity, causes lifelong intellectual disability and developmental delays, and in some people it causes health problems
Hypoplasia: incomplete development or underdevelopment of an organ or tissue
Hypotonia: low muscle tone
Vermis: Latin word for “worm,” located along the midsagittal plane of the cerebellum
Cerebellum: the “little brain,” a structure that is located at the back of the brain, underlying the occipital and temporal lobes of the cerebral cortex, accounts for 10% of the brain’s volume and contains over 50% of the total number of neurons in the brain; involved in maintenance of balance and posture, coordination of voluntary movements, motor learning, and cognitive functions
Gross Motor Function Measure (GMFM): a clinical tool designed to evaluate change in gross motor function in children, items span the spectrum from activities in lying and rolling up to walking, running and jumping skills

Suggested Readings & Relevant Sources

Close Interrelation of Motor Development and Cognitive Development and of the Cerebellum and Prefrontal Cortex by Adele Diamond – Diamond discusses the role of the cerebellum in the development of cognitive and motor faculties, accentuating the role of cerebellar abnormalities in the symptomology of developmental disorders.
Gross Motor Skills in Children with Down Syndrome: A Guide for Parents and Professionals (Topics in Down Syndrome) [Second Edition] by Patricia C. Winders – Presents a physiological explanation for gross motor developmental delay in children with Down syndrome, including a comprehensive physical therapy treatment plan.
Delays in Motor Development in Children with Down Syndrome by Roksana Malak, Anna Kostiukow, Agnieszka Krawczyk-Wasielewska, Ewa Mojs, and Wlodzimierz Samborski – Studies on the neurological and physiological basis for the motor developmental delays in Down syndrome and the effectiveness of physical therapy interventions in achieving motor milestones.

Quiz questions & answers (T-F & 3-5 Short Answer/Essay)

True/False
1. The deficiencies in motor functioning of individuals with Down syndrome influences the rate at which functioning improves within the first four years of life. (T/F)
2. Deficits do not noticeably influence the eventual fulfillment of gross motor milestones during early childhood. (T/F)
3. There is little variability in the degree of impairment and length of delay among individuals with Down syndrome. (T/F)
4. Granule cell density of both white and grey matter is reduced by approximately 30% in children with Down syndrome compared to typically developing children. (T/F)
5. In addition to hypotonia, cerebellar hypoplasia leads to problems in balance and coordination. (T/F)

Short Answer
6. What is hypotonia?
7. What findings resulted from the magnetic resonance imaging (MRI) studies conducted by Pinter and colleagues?
8. List four problems for which cerebellar hypoplasia is responsible.
9. Why is the underdevelopment of the vermis significant in the presentation of cerebellar hypoplasia?

References
Ferreira-Vasques AT, Lamonica DA. Motor, linguistic, personal and social aspects of children with Down syndrome [Online]. Department of Speech-Language Pathology and Audiology, Bauru School of Dentistry, University of São Paulo, Brazil. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4560504/ [23 Nov. 2015].
Fredericks CM. Disorders of the cerebellum and its connections. In: Pathophysiology of the Motor Systems: Principles and Clinical Presentations. Philadelphia, PA: FA Davis Company, 1996.
Knierim J. Cerebellum. In: Neuroscience Online. Houston, TX: UT Health, 1997.
Latash, M, Wood L, Ulrich D. What is currently known about hypotonia, motor skill development, and physical activity in Down syndrome [Online]. Down Syndrome Education International. http://www.down-syndrome.org/reviews/2074/?page=1 [11 Nov. 2008].
Malak R, Kostiukow A, Krawczyk-Wasielewska A, Mojs E, Samborski W. Delays in motor development in children with Down syndrome. Medical Science Monitor : International Medical Journal of Experimental and Clinical Research 21: 1904–1910, 2015.
Palisano RJ, Walter SD, Russell DJ, Rosenbaum PL, Gemus M, Galuppi BE, Cunningham L. Gross motor function of children with Down syndrome: Creation of motor growth curves. Arch Phys Med Rehabil 82: 494-500, 2001.
Pinter JD, Eliez S, Schmitt, JE, Capone GT, Reiss AL. Neuroanatomy of Down’s syndrome: A high resolution MRI study. Am J Psychiatry 158: 1659-1665, 2001.
Vicari S. Motor development and neuropsychological patterns in persons with Down syndrome. Behavior Genetics 36: 355-364, 2006.

Answers to Quiz Questions

1. False
2. True
3. False
4. False
5. True
6. Hypotonia is reduced muscle tone.
7. The volumetric magnetic resonance imaging (MRI) studies revealed that individuals with Down syndrome have a lower brain weight and disproportionately reduced volumes in the cerebellar regions.
8. Cerebellar hypoplasia is responsible for difficulties in movement fluency, axial control, balance, and coordination.
9. Formed after the lateral and intermediate zones of the cerebellum, the vermis is more likely to be absent or underdeveloped than these other parts. The vermis receives input from the visual, vestibular, and auditory systems as well as motor cortical input. This zone is involved in establishing and maintaining balance through projections to the lateral vestibular nuclei in addition to its involvement in motor execution through the spinocerebellum. Cerebellar hypoplasia presents with delays in motor development associated with vermal underdevelopment.
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