Fine motor disability is an inability or impairment of an individual to perform tasks that require a degree of manual dexterity.[1][2] Fine motor disability is a symptom of an underlying disease process rather than a disease in its own right. To perform even simple fine motor movements requires communication between the premotor and motor cortex, cerebellum, basal ganglia, corticospinal tracts, and peripheral nerves, not to mention visuospatial, sensory, and executive function processing as well.[3][4] 

Additionally, injuries to musculoskeletal components may also affect fine motor abilities. Fine motor ability is usually synonymous within the literature concerning the ability of an individual to make precise, voluntary, and coordinated movements with their hands. However, foot dexterity is also described concerning athletic ability and the precise lower limb coordination required to perform subtle movements required in sport.[5] Strikingly, individuals with a congenital absence of hands with professions requiring fine motor ability such as painters have cortical 'toe maps' in place of hand representation.[6]

Despite these examples, the manual dexterity of the hand plays a much larger role in evaluating a fine motor disability. For example, all fine motor development milestones in children relate to the ability of a child to use their hands in more and more refined and purposeful movements.[7] For example, at 2 months of age, a child may be expected to hold a rattle, and at 12 months, the ability to hold something with a ‘pincer grip’ between 2 fingers on the same hand.[7] This focus on hand dexterity reflects the importance of an individual’s hands to perform tasks required in day to day life. A defining feature of anthropoids is the ability to perform skillful maneuvers with the hands and has been postulated both in common parlance and the scientific community that pincer ability associated with the opposable thumb garners humans the unique capacity for complex, subtle motor tasks.[8]

The selection pressure of our evolutionary ancestors regarding hand morphology, joint flexibility, and survival advantage in utilizing and fashioning tools created a strong symbiosis between the hands and brain's combined ability to survive and thrive in the environment.[9] This interdependence is reflected in gross neuroanatomy. Dr. Penfield vividly represented the overrepresented hand in the motor and sensory cortex in his 3D homunculus, where the hands represent a vastly greater proportion of the cortex compared with the actual anatomical size of the hands.[10]

Fine motor disability can affect anyone at any age but is especially important during a child’s development with fine motor ability closely monitored by clinicians as significant delays in recognized milestones can indicate a range of underlying neurological disorders.[7] Fine motor skills have been found to independently predict social and cognitive ability in pre-kindergarten children. This further reflects the linked development of problem-solving to the physical manipulation of the environment and the importance of fine motor skills in social play.[11] Three major components of fine motor control are thought to be grip force scaling, speed of movement, and motor coordination.[12]

The intact fine motor function involves complex coordination between numerous central and peripheral nervous system structures; the underlying etiology of subsequent impairment is therefore vast. Below is a brief synopsis of the etiology of fine motor disability with regards to neuroanatomical areas and more generalized causes escaping precise anatomical causation. The following neuroanatomical areas play crucial roles in fine motor control, and therefore any lesion can cause fine motor disability. Causes of lesions/damage include a space-occupying lesion, infection, stroke, toxins, autoimmune inflammation, metabolic, trauma, and congenital absence or abnormality.

Pre-motor and motor cortex- These are higher cortical centers that initiate movement. Specific damage to these areas can lead to complete paralysis of the specific muscle groups and, therefore, the inability to perform fine or gross motor tasks.[13]

Cerebellum- Vital for coordination of movement, especially in the initiation of complex pre-planned fine motor movements. A cerebellar hemisphere lesion is more likely to lead to limb ataxia and, therefore, fine motor disability compared with lesions of the vermis that lead to truncal ataxia.[14]

Basal ganglia- Essential for coordinated movement and acts as a gatekeeper for voluntary movement. Examples of diseases that specifically affect the basal ganglia are Parkinson's and Huntington's disease. The intention tremor associated with these conditions, as well as rigidity, impedes flowing fine movements required for fine motor tasks.[15]

Descending corticospinal tracts- These carry integrated motor information via the brainstem to synapse with lower motor neurons at the relevant spinal nerve level. Lesions will therefore carry upper motor neuron signs, with spasticity, increased reflexes, and varying levels of gross and fine motor dysfunction dependent on lesion severity. An example of dysfunction may arise in the form of cervical myelopathy secondary to pressure from cervical disc prolapse.[16]

Peripheral nerves- These carry integrated motor information from the spinal cord to the target muscle groups. Depending on which nerve is affected will determine the severity of the disability. An example of dysfunction includes carpal tunnel syndrome with worsening fine motor control in the affected hand due to median nerve compression under the flexor retinaculum.[17]

Visuospatial- Visual feedback is an important part of developing fine motor control but can be developed without any visual feedback. There are numerous examples of professional musicians that were born blind yet have a remarkable fine motor ability in playing the piano, for example.[18]

Sensory system- The hand is one of the most highly concentrated areas of sensory nerves and receptors in the human body. Direct sensory feedback with regards to proprioception, tactile sensation, vibration, and avoidance of noxious stimuli is a key component of coordinated goal-directed fine motor behavior concerning one's environment. Any interruption in this sensory feedback can cause debilitating loss of fine motor control. One example of this is diabetic peripheral neuropathy.[19]

Musculoskeletal- The hand contains over 30 muscles, 20 major joints, and hundreds of ligaments. Fine motor control is dependent on precise joint movement; therefore, any disease process that causes joint inflammation/damage/sarcopenia can cause fine motor disability.[20] One example of this is rheumatoid arthritis.[21]

Psychosocial- Any process that interferes with the relay of coordinated motor sequences can limit fine motor control even in well-trained experts. 'Choking' describes the situation of an athlete or musician performing well below their usual level of motor control due to performance-related anxiety. The initiation of the fight or flight response is thought to disrupt the subconscious access to pre-programmed complex coordinated pathways with a more conscious hyper-aware state resulting in poor or clumsy performance.[22]

Neuropsychiatric- Autism spectrum disorder and attentional deficit disorder have both been strongly correlated to developmental delays in fine motor control and has been postulated to be secondary to cognitive development delay.[23][24]

Owing to the enormous variety and causes of fine motor disability, precise epidemiology remains challenging, and fine motor disability is infrequently independently researched, especially with regards to population-based epidemiology. Below are the prevalences of etiologies of fine motor disability; an important caveat being data may include individuals without a significant fine motor disability but a condition strongly associated with fine motor disability.

Estimated developmental disability in children in the US- 17.8%. Further broken down: Attention deficit hyperactivity disorder 9.5%, autism spectrum disorder 2.5%, intellectual disability 1.2% and other causes 4.1%[25]

In adults: 

• Stroke- 2.8% of the US population, 50% of which had a hemiparesis[26]
• Rheumatoid arthritis- 2% of North America, 30% require assistance for personal care[26]
• Traumatic brain injury- 1.1% of the US population, 43% of patients have a lasting disability[26]
• Self-care- 3.6% of the US population, have a disability[27]

As fine motor disability should be thought of as a symptom of an underlying disease process rather than a disease in and of itself, there is no single unifying pathophysiology. As described, the etiology is vast, and the underlying pathophysiology similarly broad.