It is not always the small space that triggers these emotions, but it is more the fear of the possibilities of what could happen while confined to that area. When anxiety levels start to reach a certain ppo level, the person may start to experience:

sweating and/or chills
accelerated heart rate and a rise in blood pressure
dizziness, fainting spells, lightheadedness and frozen in fear
dry mouth
hyperventilation
hot flashes
shaking or trembling and a sense of "butterflies" in the stomach
nausea
headache
numbness
a choking sensation
tightness in the chest/chest pain and difficulty breathing
an urge to use the bathroom
confusion or disorientation
fear of harm or illness

Causes
The fear of enclosed spaces is an irrational fear. Most claustrophobic people who find themselves in a room without windows consciously know that they are not in danger, yet these same people will be afraid, possibly terrified to the point of incapacitation, and many do not know why. Amygdala
The amygdala is one of the smallest structures in the brain, but also one of the most powerful. The amygdala is needed for the conditioning of fear, or the creation of a fight-or-flight response. A fight-or-flight response is created when a stimulus is associated with a grievous situation. Cheng believes that a phobias roots are in this fight-or-flight response. In generating a fight-or-flight response, the amygdala acts in the following way: 
The amygdalas anterior nuclei associated with fear each other. Nuclei send out impulses to other nuclei, which influence respiratory rate, physical arousal, the release of adrenaline, blood pressure, heart rate, behavioral fear response, and defensive responses, which may include freezing up.
Diagnosis
Pemphigus defines a group of autoimmune intraepithelial blistering diseases that are characterized by loss of normal cell-cell adhesion (acantholysis), and by the presence of pathogenic (predominantly IgG) autoantibodies reacting against epithelial adhesion molecules. Pemphigus is further divided in two major subtypes: pemphigus vulgaris (PV) and pemphigus foliaceus (PF). However, several other disorders such as IgA pemphigus, IgE pemphigus, pemphigus herpetiformis, drug-induced pemphigus, Senear Usher syndrome, and endemic pemphigus foliaceus exist, and are recognized by a dermatologist from the appearance and distribution of the skin lesions. It is also commonly diagnosed by specialists practicing otolaryngology- head and neck surgery, periodontists, oral and maxillofacial surgeons, and eye doctors, as lesions can affect the eyes and mucous membranes of the oral cavity. Intraorally, it resembles the more common diseases lichen planus and mucous membrane pemphigoid. Definitive diagnosis requires examination of a skin or mucous membrane biopsy by a dermatopathologist or oral pathologist. The skin biopsy is taken from the edge of a blister, prepared for histopathology and examined with a microscope. The pathologist looks for an intraepidermal vesicle caused by the breaking apart of epidermal cells (acantholysis). Thus, the superficial (upper) portion of the epidermis sloughs off, leaving the bottom layer of cells on the "floor" of the blister. This bottom layer of cells is said to have a "tombstone" appearance. Definitive diagnosis also requires the demonstration of antidesmoglein autoantibodies by direct immunofluorescence on the skin biopsy. These antibodies appear as IgG deposits along the desmosomes between epidermal cells, a pattern reminiscent of chicken wire.