Stenophobia is the irrational and persistent fear of narrow things or places and is fairly uncommon and unheard of.
The fear of narrow places might have been caused by for example hiking in a narrow trail and falling off or getting off-track, or having to cross a narrow bridge which caused one to be extremely scared of the experience. For others, a narrow path or a narrow and enclosed room can cause one to feel smothered or trapped, especially in a claustrophobic person thus this phobia is triggered.
One might also be big physically and they fear that if they have to go into a narrow room or pass a narrow path, they will get stuck and will not be able to move.
People with this phobia may also fear that something that is narrow will not be able to contain whatever things that they have. Example, one might choose a wide-mouthed glass to drink from instead of one with a narrow-rim. People with this fear usually prefer to stay in wide and open spaces and may display symptoms of anxiety such as palpitations, tremors, nausea, vomiting, breathlessness and the feeling of being trapped when put in a narrow place.
Treatment includes behavioral therapy. Desensitization may also help. Avoidance of anything narrow is usually sufficient to relieve symptoms of anxiety.