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What is Aphagia and explain in detail?
Aphagia is the inability or refusal to swallow. The word is derived from the Ancient Greek prefix α, meaning "not" or "without," and the suffix φαγία, derived from the verb φαγεῖν, meaning "to eat." It is related to dysphagia which is difficulty swallowing (Greek prefix δυσ, dys, meaning difficult, or defective), and odynophagia, painful swallowing (from ὀδύνη, odyn(o), meaning "pain"). Aphagia may be temporary or long term, depending on the affected organ. It is an extreme, life-threatening case of dysphagia.  Depending on the cause, untreated dysphagia may develop into aphagia.

Behavioural classification
The following behavioural classifications result from studies performed on rats, in which lesions were made on the lateral hypothalamus region in the brain. 

Passive aphagia: An animal with passive aphagia will not respond to food if it is presented. However, if food is inserted into the mouth, the animal will chew.
Active aphagia: Active aphagia is a complete rejection of food. The animal will physically push food away or move its head from it. One might first sample the food by smelling or tasting, then spit out anything in the mouth. Afterwards the animal will show a complete repulsion to the food. The animal reacts to food as if it is bitter or foul.
Mixed aphagia: When presented with food, the animal initially does not react positively or negatively. However, when food is placed in the mouth, the animal demonstrates active aphagia, spitting out the food and refusing to eat thereafter.These studies point to the function of the hypothalamus in regulating food intake. Animals in this study also demonstrated several other types of eating behaviour: "weak eating," in which the animal slowly approaches, chews, and swallows small observable amounts of food for a brief period; "good eating," in which the animal reaches normally for the food and eats reliably; and "vigorous eating," in which the animal gluttonously reaches for and devours the food. In these cases, there was either minor or no damage to the lateral hypothalamus.Aphagia not classified under behavioral aphagia typically has a structural cause, see causes.

Causes
Aphagia is usually the result of many different diseases as well as different medical treatments. The most common causes of aphagia are:

Aesophageal cancer – there are two types of aesophageal cancer. The squamous cell cancer from the squamous cells of the tongue or the adenocarcinoma from glandular cells present at the junction of the esophagus and stomach. This leads to a local tumour growth with spreading later. This spreading may lead to larger tumors that would result in the inability to swallow.
Aesophageal webs – thin membranes located in the esophagus. Abnormalities can cause constrictions within the esophagus.
Globus pharyngis – commonly referred to as lumps in ones throat
Myasthenia gravis – the thymus gland is thought to be necessary for the deletion of auto-reactive T cells, and seems to have an important role in the pathogenesis of myasthenia gravis. In patients the thymus is typically enlarged, and contains many germinal centres with T and B cell areas very similar to those seen in lymph nodes. The tumour in thymoma associated disease is typically epithelial in nature.
Facioscapulohumeral muscular dystrophy – even though there is not a clear correlation between the facioscapulohumeral muscular dystrophy and the pharyngeal and upper aesophageal striated muscle. Minor, and nonspecific, primary aesophageal dysmotility was present as seen in the 2008 study by Joerg-patrick Stübgen.
Multiple sclerosis – may lead to aesophageal dysmotility Chemotherapy – radiation from cancer therapy may cause a stricture of the throat leading to the inability to swallow.
Stroke – swallowing problems can cause stroke victims to aspirate food or liquid into the lungs and cause pneumonia mostly in elderly people.
Parkinsons disease – the mechanism of swallowing disorders in Parkinsons disease may be related to extrapyramidal and autonomic system disorders. The cardinal symptoms of Parkinsons disease: tremor, bradykinesia, and rigidity are initially responsible for swallowing, which is mainly observed in the advanced stages of the disease
Damage to the lateral hypothalamus can also lead to aphagia, as seen in the 1978 study by Timothy Schallert and Ian Whishaw.
Other causes might be due to depression, cervical spine disease and conversion disorders.It is important to note that all these causes (except due to the damage of the lateral hypothalamus) are indirect.

Diagnosis
Modified barium swallow – videofluoroscopic swallow (fluoroscopy)