Good lord, Luna’s head looks so misshapen it looks like parts of it are like... sliding out of place. Gross.
Her head is so horrifying it makes it easier to overlook the awful contractures in her hands and ankles. This is an indication that something is seriously wrong with her brain. Basically, her brain and muscles don't communicate well, so her limbs are constantly tensed in a spasm. It's a condition called hypertonia. It's also possible that she has some low tone, or hypotonia, as well. This "mixed tone" is, in general, indicative of a more severe neurological pathology than either hyper- or hypotonia alone.
Most people have probably encountered someone with cerebral palsy. Luna is a good example of someone who has the most severe type of CP, spastic quadriplegia. That means that all four of her limbs are affected by abnormal muscle tone, because her brain is so damaged. For a lot of people with CP, the high tone has a major, negative impact on quality of life. Anyone who can imagine a constant, severe charley horse probably has a bit of an idea of why it's so miserable. If Luna has any awareness at all, she is very likely in constant pain. People with mild CP will also describe how frustrating it is to have muscles that won't do what you want them to do. For example, they might have trouble letting go of a glass of water after picking it up, causing them to spill on themselves. Many of them have trouble with speech articulation, meaning they have normal intelligence but cannot physically "get the words out", which is obviously frustrating because it causes other people to treat them like they're retarded.
The high muscle tone can be treated. Some patients get regular Botox injections in the tighter muscles. Botox is botulinum toxin, which is the same thing that causes paralysis in cases of botulism from food. By paralyzing the affected muscles, it allows the other ones to do what they're supposed to do, without the signal from the brain being drowned out by nonsense, basically. There's also a medication called baclofen, which is a muscle relaxant similar to what you might get if you throw out your back. It can be taken by mouth, but if the spasticity is very severe or affects many of the muscles, it can also be given via an intrathecal pump, which is a little reservoir that is implanted in the abdomen. From there, a tiny tube is placed into the spinal column and the pump dispenses the solution into the spinal fluid at a constant rate. The pump does need to have its battery replacemed every 5-10 years, and the medication reservoir has to be refilled several times a year by inserting a long needle through the abdomen.
There are also orthopedic braces and splints that help keep everything in the right place in spite of the pulling of the muscles that are too tight. I thought Luna had some thumb spica wrist splints to help with the contractures in her arms, but I might have been misremembering because I don't see them in recent photos.
The reason you want to do all of these intense interventions as soon as possible is because joint contractures can become permanent, leading to complete loss of function of the joint and skeletal deformity. For example, it's really common for people who have spasticity in their legs to end up with dislocated hips, because of how the muscles pull on the bone, eventually causing the hips to become misshapen. This can be fixed surgically, but it's a major operation, involving a lot of pain and a very long recovery period during which the kid has to be immobilized basically from the chest to the toes, and most surgeons try to wait as long as possible before doing it. It is also strongly advised to make the child bear weight on his or her legs as much as possible, even if he or she won't ever walk. This helps the hips maintain proper anatomical arrangement and promotes bone density. A lot of profoundly disabled people struggle with bone fractures, and it can make it really hard for the kid and the caregiver, both emotionally and physically. Most parents would feel pretty shitty if they broke their kid's femur just by putting him to bed.
Have we ever seen Luna in a stander? I'm wondering if Robyn is using chiropractic in lieu of actual physical therapy.
(Sorry, this response got really long and spergy. I can spoiler it if people would prefer that.)