With shrubby plants like spirea left to grow in natural form, it is recommended to remove to the base about 1/3 of the oldest branches selectively every post bloom pruning session (yeary, every two years, whatever) If you just lop them off like a haircut, you tend to get a lot of weak floppy growth. Spireas can indeed accumulate a lot of dead twiggy growth that is crowded and shaded by the new sprouts.

google: rejuvenation pruning of flowering shrubs..

and throw away your electric shears .. this is a plant.. not a mop of hair ...

on spirea .. you could literally cut the whole thing to 3 inches.. and by july.. have a pretty 3 foot plant.. you just can not kill them [unless you keep shearing them]...

many of our southern friends actually cut them down every year ...

finally.. most if not all shrubs .. are nudey inside .. that is normal ... no sunlight in there to maintain a leaf ...

ken