THE BRAIN GUIDE

The human brain has many parts and functions. This is a brief look only at two structures known to be abnormal in opsoclonus-myoclonus.

Cerebellum

Located at the back of the brain and beneath the cerebral hemispheres, the cerebellum is a small but important structure. It helps us sit, stand, walk, and talk. It also exerts some control over the eyes. The main motor functions of the cerebellum are:

• Coordination
• Speed and smoothness of movements
• Walking or gait

The cerebellum consists of two halves called cerebellar hemispheres. Each controls one side of the body.
Between the hemispheres is a thin central part (vermis) that controls our trunk (posterior vermis) and legs (anterior vermis).

In opsoclonus-myoclonus, cerebellar dysfunction causes many problems including:

• Incoordination (ataxia)
• Inability to walk
• Instability of the trunk (titubation)
• Slurred speech (dysarthria)

We also know the cerebellum has more than just motor functions. It is involved in learning and behavior. These are crucial issues in opsoclonus-myoclonus.

A puzzle: Children with opsoclonus-myoclonus usually have a normal looking cerebellum on imaging studies such as the CT or MRI scan. Many parents ask how that could be possible when symptoms of cerebellar dysfunction are so clear. Appearances can be deceiving. Cerebellar function or metabolism may still be impaired. Problems with synapses or with small populations of cells do not show up on these scans. We have been using other techniques to evaluate cerebellar dysfunction.

Brainstem

The brainstem connects the higher brain with the spinal cord. It contains many tracts going between the two and vital cells that help control the eyes, swallowing, breathing, blood pressure, pupil size, alertness, and sleep.

In opsoclonus-myoclonus, the brainstem and cerebellum probably interact to produce:

• Opsoclonus
• Myoclonus

The brain center that allows us to quickly look at something (saccades) is in the pons, which lies just below the cerebellum. Opsoclonus is an excess of saccades. It is more erratic and sudden than nystagmus, with which it is sometimes confused. Unlike nystagmus, the movements are in all directions.

One brain center for myoclonus is the medulla, the part of the brainstem just above the spinal cord. Muscle jerks during rapid eye movement (REM) sleep come from the lower brainstem. In children with opsoclonus myoclonus, these jerks may be increased and disrupt sleep.

Brainstem dysfunction in opsoclonus-myoclonus also may contribute to:

• Sleep disturbances (insomnia and fragmented sleep)
• Swallowing problems (drooling)

A puzzle: Do the brainstem and cerebellum account for all the abnormal brain function in opsoclonus-myoclonus? Parents are often most concerned about learning and behavior problems. We are investigating other brain areas that may be dysfunctional in children with opsoclonus-myoclonus. We must keep these in mind because both the brainstem and cerebellum have so many connections to other brain areas. These research studies should help define the extent of brain injury.