The midline of the body is what you would get if you drew a straight line from the top of your head right down the middle. Crossing the midline is where your right hand performs tasks on the left hand side of your body and vice versa. Crossing the midline is also helpful for doing tasks that require you to use two hands to work together.
Often children who have difficulties crossing the midline also have a retained Asymmetrical Tonic Reflex (ATNR). Now we all know that reflexes are important - you want to have reflexes when you accidentally touch something hot! But having reflexes is just as important as having those reflexes integrate when you no longer need them.
The ATNR reflex is important for helping an infant into the proper position for the birthing process, and once they're born it helps with things like keeping their airway clear when they're on their stomach, and is important for developing hand-eye coordination. But after about six months of age, it isn't so useful anymore and it needs to be replaced by more complex motor patterns and functions. If this doesn't happen, the child can be faced with a whole list of learning difficulties.
Difficulties crossing the midline and using two hands together
Issues with developing a dominant hand. Typically the brain determines which hand is dominant based on skill and mastery of fine motor movements, however when a child can't cross the midline, both hands get used on their respective sides of the body. This means that they both remain relatively unskilled.
Poor handwriting. A lack of hand dominance leads to reduced handwriting skills as the decreased fine motor skill makes it difficult to learn letter formations and accuracy with a pen or pencil. Children with these difficulties often struggle to draw horizontal or diagonal lines, and tend to develop some coping strategies such as to tilt the paper, or adjust it off to one side so they can keep up with their peers.
Difficulty with reading as a result of poor visual tracking skills. Visual tracking relies on the ability to cross the midline so that a child doesn't lose their place as they move their eyes across the page.
Gross motor skill development. Throwing, catching, and even walking can be difficult if the two sides of the brain can't communicate effectively.
But the good news? Reflexes can be worked on and integrated through play, rhythmic movements and exercises!