Neuropathology
Listed below are some characteristic findings in ASD brains on molecular and cellular levels regardless of the specific genetic variation or mutation contributing to autism in a particular individual:

Limbic system with smaller neurons that are more densely packed together. Given that the limbic system is the main center of emotions and memory in the human brain, this observation may explain social impairment in ASD. Fewer and smaller Purkinje neurons in the cerebellum. New research suggest a role of the cerebellum in emotional processing and language. Increased number of astrocytes and microglia in the cerebral cortex. These cells provide metabolic and functional support to neurons and act as immune cells in the nervous system, respectively. Increased brain size in early childhood causing macrocephaly in 15–20% of ASD individuals. The brain size however normalizes by mid-childhood. This variation in brain size in not uniform in the ASD brain with some parts like the frontal and temporal lobes being larger, some like the parietal and occipital lobes being normal sized, and some like cerebellar vermis, corpus callosum, and basal ganglia being smaller than neurotypical individuals. Cell adhesion molecules that are essential to formation and maintenance of connections between neurons, neuroligins found on postsynaptic neurons that bind presynaptic cell adhesion molecules, and proteins that anchor cell adhesion molecules to neurons are all found to be mutated in ASD. Gut-immune-brain axis
46% to 84% of autistic individuals have GI-related problems like reflux, diarrhea, constipation, inflammatory bowel disease, and food allergies.
Parkinsonism is a clinical syndrome characterized by tremor, bradykinesia (slowed movements), rigidity, and postural instability. These are the four motor symptoms found in Parkinsons disease (PD), after which it is named, dementia with Lewy bodies (DLB), Parkinsons disease dementia (PDD), and many other conditions. This set of symptoms occurs in a wide range of conditions and may have many causes, including neurodegenerative conditions, drugs, toxins, metabolic diseases, and neurological conditions other than PD. Signs and symptoms
Parkinsonism is a clinical syndrome characterized by the four motor symptoms found in Parkinsons disease: tremor, bradykinesia (slowed movements), rigidity, and postural instability.Parkinsonism gait problems can lead to falls and serious physical injuries. Other common symptoms include:

Tremors when resting (mostly in the hands)
Short, shuffling gait
Slow movements (bradykinesia)
Loss of sound perception leading to low, soft speech
Difficulty sleeping
Dry skin
Apathy
Lack of facial expressions
Balance problems
Frequent falls
Very small handwriting
Rigid, stiff muscles
Cogwheeling (jerky feeling in arm or leg)

Conditions
Parkinsonism occurs in many conditions. Neurological
Neurodegenerative conditions and Parkinson plus syndrome that can cause parkinsonism include:
Corticobasal degeneration
Dementia with Lewy bodies
The relationship (if any) with essential tremor is not clear.