( ( top MIGRAINE ) ( ( HEADACHE ( ( A ( recurring headache ) ) ( related ( to ( dilation ( of ( ( the ( blood ( vessels outside ) ) ) ( of ( ( the skull ) ( that ( may ( be ( ( ( a result ) ( of ( chemical changes ) ) ) ( that ( cause ( spasms ( of ( vessels ( inside ( the skull ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ top) (NN MIGRAINE)) (VP (VBP HEADACHE) (NP (NP (DT A) (VBG recurring) (NN headache)) (VP (VBN related) (PP (TO to) (NP (NP (NN dilation)) (PP (IN of) (NP (NP (DT the) (NN blood) (NNS vessels) (NN outside)) (PP (IN of) (NP (NP (DT the) (NN skull)) (SBAR (WHNP (WDT that)) (S (VP (MD may) (VP (VB be) (NP (NP (DT a) (NN result)) (PP (IN of) (NP (NN chemical) (NNS changes))) (SBAR (WHNP (WDT that)) (S (VP (VBP cause) (NP (NP (NNS spasms)) (PP (IN of) (NP (NP (NNS vessels)) (PP (IN inside) (NP (DT the) (NN skull))))))))))))))))))))))) (. .)))
top MIGRAINE HEADACHE A recurring headache related to dilation of the blood vessels outside of the skull that may be a result of chemical changes that cause spasms of vessels inside the skull.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head)))) (VP (VBP dilate)))) (, ,) (NP (NP (DT a) (NN migraine)) (, ,) (NP (DT a) (JJ severe) (NN headache)) (, ,)) (VP (MD can) (VP (VB occur))) (. .)))