These reactions constitute an autonomic failure in a panic attack. A study done by Fumi Hayano found that the right amygdala was smaller in patients who suffered from panic disorders. The reduction of size occurred in a structure known as the corticomedial nuclear group which the CE nucleus belongs to. This causes interference, which in turn causes abnormal reactions to aversive stimuli in those with panic disorders. In claustrophobic people, this translates as panicking or overreacting to a situation in which the person finds themselves physically confined. Classical conditioning
Claustrophobia results as the mind comes to connect confinement with danger. It often comes as a consequence of a traumatic childhood experience, although the onset can come at any point in an individuals life. Such an experience can occur multiple times, or only once, to make a permanent impression on the mind. The majority of claustrophobic participants in an experiment done by Lars-Göran Öst reported that their phobia had been "acquired as a result of a conditioning experience". In most cases, claustrophobia seems to be the result of past experiences. Conditioning experiences
A few examples of common experiences that could result in the onset of claustrophobia in children (or adults) are as follows:

A child (or, less commonly, an adult) is shut into a pitch-black room and cannot find the door or the light-switch. A child gets shut into a box. A child is locked in a closet. A child falls into a deep pool and cannot swim.
The acquisition of claustrophobia may be part of a vestigial evolutionary survival mechanism, a dormant fear of entrapment and/or suffocation that was once important for the survival of humanity and could be easily awakened at any time. Hostile environments in the past would have made this kind of pre-programmed fear necessary, and so the human mind developed the capacity for "efficient fear conditioning to certain classes of dangerous stimuli".Rachman provides an argument for this theory in his article: "Phobias". He agrees with the statement that phobias generally concern objects that constitute a direct threat to human survival, and that many of these phobias are quickly acquired because of an "inherited biological preparedness". This brings about a prepared phobia, which is not quite innate, but is widely and easily learned. As Rachman explains in the article: "The main features of prepared phobias are that they are very easily acquired, selective, stable, biologically significant, and probably [non-cognitive]." Selective and biologically significant mean that they only relate to things that directly threaten the health, safety, or survival of an individual. Non-cognitive suggests that these fears are acquired unconsciously. Both factors point to the theory that claustrophobia is a prepared phobia that is already pre-programmed into the mind of a human being. Diagnosis
Claustrophobia is the fear of being closed into a small space. It is typically classified as an anxiety disorder and often results in a rather severe panic attack.