Whats the difference between BDD and hypochondriasis? I'm a new psychiatry trainee. I've run through the literature available to me (plus google), and the descriptions of body dysmorphic disorder (BDD) and hypochondriasis seem too similar. Here's what I've understood so far:

Hypochondriasis represents a general anxiety of disease/symptoms whereas BDD happens to revolve around a single issue. The former is more about somatic symptoms whereas the latter is about physical appearances. But I wonder if this understanding is correct and whether there is a clearer differentiation.
DSM-5 is the go to book for psychiatric dignoses and I would strongly recommend you pick up a copy if you are studying Psychiatry.

I am going to split my answer in 3 as you are talking about two different disgnoses and the 3rd part will be making a comparison between the two.

Hypochondriasis

The more recently approved Diagnostic and Statistical Manual of Mental Disorders (DSM-5) contains many revisions, but few are as sweeping as those involving somatoform disorders. In the updated edition, hypochondriasis and several related conditions have been replaced by two new, empirically derived concepts: Somatic Symptom Disorder and Illness Anxiety Disorder. They differ markedly from the somatoform disorders in DSM-IV.

To meet the diagnostic criteria for Somatic Symptom Disorder under DSM-5 (Page 311), patients must have:



The Somatic Symptom Disorder is considered persistant if it is characterised by severe symptoms, marked impairment, and long duration (more than 6 months).

The DSM also goes on to say that there are 3 levels of severity, which are:


Mild if only one of the symptoms specified in Criterion B is fulfilled.
Moderate If two or more of the symptoms specified in Criterion B are fulfilled.
Severe If two or more of the symptoms specified in Criterion B are fulfilled, plus there are multiple somatic complaints (or one very severe somatic symptom).


Patients with illness anxiety disorder may or may not have a medical condition but have heightened bodily sensations, are intensely anxious about the possibility of an undiagnosed illness, or devote excessive time and energy to health concerns, often obsessively researching them. Like people with somatic symptom disorder, they are not easily reassured. Illness anxiety disorder can cause considerable distress and life disruption, even at moderate levels.

In DSM-5 Page 315 it states that the diagnostic critera for Illness Anxiety Disorder is


  
  Preoccupation with having or acquiring a serious illness.
  Somatic symptoms are not present or, if present, are only mild in intensity. If another medical condition is present or there is a high
  risk for developing a medical condition (e.g., strong family history
  is present), the preoccupation is clearly excessive or
  disproportionate.
  There is a high level of anxiety about health, and the individual is easily alarmed about personal health status.
  The individual performs excessive health-related behaviours (e.g., repeatedly checks his or her body for signs of illness) or exhibits
  maladaptive avoidance (e.g., avoids doctor appointments and
  hospitals).
  Illness preoccupation has been present for at least 6 months, but the specific illness that is feared may change over that period of
  time.
  The illness-related preoccupation is not better explained by another mental disorder, such as somatic symptom disorder, panic
  disorder, generalised anxiety disorder, body dysmorphic disorder,
  obsessive-compulsive disorder, or delusional disorder, somatic type.
  


The DSM goes on to say that there are two types of Illness Anxiety Disorder


  
  Care-seeking type: Medical care, including physician visits or undergoing tests and procedures, is frequently used.
  Care-avoidant type: Medical care is rarely used.
  
  
  Diagnostic Features
  
  Most individuals with hypochondriasis are now classified as having
  somatic symptom disorder; however, in a minority of cases, the
  diagnosis of illness anxiety disorder applies instead...


Body Dysmorphic Disorder (BDD)

In DSM-5, BDD is described under Obsessive Compulsive Disorders.

BDD as described in DSM-5


  
  Preoccupation with one or more perceived defects or flaws in physical appearance that are not observable or appear slight to others.
  At some point during the course of the disorder, the individual has performed repetitive behaviors (e.g., mirror checking, excessive
  grooming, skin picking, reassurance seeking) or mental acts (e.g.,
  comparing his or her appearance with that of others) in response to
  the appearance concerns.
  The preoccupation causes clinically significant distress or impairment in social, occupational, or other important areas of
  functioning.
  The appearance preoccupation is not better explained by concerns with body fat or weight in an individual whose symptoms meet
  diagnostic criteria for an eating disorder.
  
  
  Diagnostic Features
  
  Individuals with body dysmorphic disorder (formerly known as
  dysmorphophobia) are preoccupied with one or more perceived defects or
  flaws in their physical appearance, which they believe look ugly,
  unattractive, abnormal, or deformed...


Comparison between Hypochondriasis and BDD

When you compare the two within the DSM they do differ as you suggest where Hypochondriasis is more about somatic symptoms as it is about perception of illness whereas BDD is about physical appearances.  However when you look at the International Statistical Classification of Diseases (ICD) produced by the World Health Organisation, which is linked to in the DSM with the ICD codes at the top of each Diagnostic Criteria (for example 300.82 (F45.1) for Somatic Symptom Disorder - see image above), the 10th revision ICD code - ICD-10 - which is in brackets, refers to Section F45 of the ICD and this adds to the definitions.


  F45 - Somatoform disorders
  
  The main feature is repeated presentation of physical symptoms
  together with persistent requests for medical investigations, in spite
  of repeated negative findings and reassurances by doctors that the
  symptoms have no physical basis. If any physical disorders are
  present, they do not explain the nature and extent of the symptoms or
  the distress and preoccupation of the patient.


...


  F45.2 - Hypochondriacal disorder
  
  The essential feature is a persistent preoccupation with the possibility of having one or more serious and progressive physical
  disorders. Patients manifest persistent somatic complaints or a
  persistent preoccupation with their physical appearance. Normal or
  commonplace sensations and appearances are often interpreted by
  patients as abnormal and distressing, and attention is usually focused
  upon only one or two organs or systems of the body. Marked depression
  and anxiety are often present, and may justify additional diagnoses.
  
  Body dysmorphic disorder
  Dysmorphophobia (nondelusional)
  Hypochondriacal neurosis
  Hypochondriasis
  Nosophobia
  
  Excl.:
  delusional dysmorphophobia (F22.8)
  fixed delusions about bodily functions or shape (F22.-)


When reading this it confuses things as F45.2 considers BDD to be a somatic disorder as it is about perception rather than fact, however in my view, as psychiatry uses the DSM to distinguish between the two, in a psychiatric sense you get a definitive answer which you already have.

With psychosomatic disorders your mind can make your body act like it actually has the medical problem or make an actual problem like pain worse, where as in BDD your mind cannot alter the physical shape of the body.