To the purist, this ought not even be a category. Know any purists? Tell them we said hello. Like it or not , hypotonia is a bookmarker for more specific diagnoses which may not be forthcoming for months or even years.
What it isn't. Although floppyness can be called hypotonia - a description - we distinguish hypotonia when used as an effigy of disease. Mere loose jointedness isn't hypotonia. When the cause of floppyness is obvious and especially when that cause is an external factor such as a mother drugged senseless during delivery and the baby emerges equally sedated, although the child may be hypotonic, that isn't a condition called hypotonia. That is a condition called drugged senseless.
Even calcium shifts and other stuff of well known predisposition get omitted as those things have names of better specificity and clarity and do not need to be differentiated as they already are. Hypotonia is what's left. Hypotonia and medical head scratching go together. So it could be called "beats me - sure is loose" but hypotonia is shorter though not much clearer.
There are some criteria, minimum requirements, to use this term. Self support of the head is the most important. Different tests are used to assess how much hip or trunk or arm tone there is - tone meaning postural maintenance against gravity or examiner's manipulations. Again purists would distinguish hypotonia from weakness, but they seem to not be around when the label gets tacked on.
Let's review the logic involved in finding out what is the diagnosis (cause) of a particular instance of hypotonia. Firstly, the thing called hypotonia is a FEEL ... feels limp or feels as if there is no motor power. It is also a LOOK ... looks like nothing holding that part up or trying to hold the correct posture.
So at a minimum, is that look or feel
Focal hypotonia (say, just one arm or perhaps just the hips) demands that we be sure that thinks are mechanically linked. Are there intact joints. Muscles without a fulcrum cannot exert full leverage. We have seen children treated for "Hypotonia" who actually had dislocated hips and nothing else. Normal facial expression, arm usage, neck control, and knee and feet movements suggest that a part specific diagnosis needs to be pursued. Take an x-ray.
If an entire limb seems to be the region, then what is common to that limb? A plexus (brachial plexus in for the arm and sacral plexus for the leg) may be an issue. Spinal cord pathology >> might << just manifest on one side and in just one limb. But more likely would not be so specific. See? This is how we narrow down the scope of the search. What does what? What is common to the scope of what we see?
If more generalized - and generalized can be spotty - then think systemic. Systemic can be generally systemic - chemistry - or tissue specific generalized - nerve or muscle based.
Under the chemistry column is a variety of metabolic disorders which indirectly make muscle and nerve function depressed. Also we find genetic disorders which produce those chemistries in a measurable way.
But remember, the brain itself is a major organ of bodily energy and activation. When brain function is depressed, bodily activities fall away. When you peel away the obvious direct causes such as maternal drugs or drug effects associated with labor and delivery (by maybe just waiting them out), and the obvious genetic ones indicated by other gross findings, then the single most associated diagnosis with profound and generalized hypotonia is mental retardation (which becomes evident later).
Direct tissue defects can produce generalized patterns of hypotonia. Often they are generalized but with certain suggestive zones of greater or lesser involvement. Muscle disorders include the congenital muscular dystrophies and myotonic dystrophies. There are muscle metabolic disorders such as maltase or cytochrome oxidase or phosphorylase deficiency. There are muscle substructure disorders which show actual structural anomaly of the muscle such as central core or nemaline rod or myotubular myopathies. Those are really set apart by the fact that their specific alterations can be seen microscopically.
Then we have neuro specific anomalies. Those are the ones which - more or less - spare the muscles. More or less means that some genetic defects which disturb nerve chemistry in a big way may also disturb muscle chemistry to a lesser degree. The reverse is true as well. Some muscular dystrophies have domonstable disorder of nerve function.
Either way, one of the early differentiations is between a muscle mediated or a neurologic mediated disorder. Neurologic can be peripheral or central.
Central means brain and spinal cord. Peripheral means nerves. The central hypotonias include a large span of disorders. Down syndrome, Prader-Willi, familial dysautonomia, and many of the metabolic disorders listed in this section manifest in this category. What is important in all this is that diagnosis can be suggested by certain features, but there is methodology to be followed in seeking the exact cause in the fewest possible steps and with the least turmoil.