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Can anybody telling me why in the whole English language only 'Asthma' has four constants without any vowels in between/

Even foreign names entering the language must have vowels between any two or three letters.

Thankyou
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eighth (4), offspring (5), hamstring (4).

And have a look at "hymn". It has a vowel sound, that's true; but there are no vowels in the spelling.
Comments  
'Strength' is also in the English language. 'latchstring' has 6 consonants together, although it is really two words conjoined.

'Asthma' was the original greek spelling I think, so that's probably why it has stayed that way. I'm not aware of any barriers against letter combinations entering the language, other than ease of use.
 miriam's reply was promoted to an answer.