Skin rash

Question 1

When taking a history from a patient with a skin rash, what general information do we want to know?

Answer 1

Personal history of similar rash

How and when symptoms started

How it has evolved

Any exacerbating/ relieving factors

Medications – Immunosuppression, herbal

Any allergies - atopy/ antibiotics

FHx - auto-immune disease/ allergy

This list is not comprehensive

Question 1

What are some basic methods of describing a skin rash?

Answer 1

Location/ distribution of rash

Number of lesions

Size of lesions

Morphology - e.g macule/ papule

Colour

Question 2

When taking a history from a patient with a skin rash, what specific exposure information do we want to know?

Answer 2

Unwell contacts with rash

Water exposure

Animals exposure/ bites

Foreign travel

Question 3

When examining a patient with a skin rash, what other features can help us identify a more severe infection?

Answer 3

local or regional lymphadenopathy - suggest the infection has spread past the skin, with a stronger immune response stimualted

Heart rate >90bpm

Respiratory rate >20/min

Temperature >38.3degC

New altered mental state

Systemic Inflammatory Response Syndrome (SIRS) is an older model of identifying severe infection. It can be useful as a prompt to consider severe infection/ sepsis

Question 4

A skin rash can be caused by direct invasion of a pathogen

What types of pathogens usually do this?

Answer 4

Usually bacteria cause cellulitis/ skin abscess

Viruses can cause blistering

Question 5

A skin rash can be caused by direct invasion of a pathogen

What are risks if this is not managed properly?

Answer 5

Can invade deeper from skin to muscle and bone.

Can also spread in bloodstream to other organs, such as the heart valves causing Infective Endocarditis

Question 6

A skin rash can be caused by antibody-antigen deposition

What types of pathogens usually do this?

Answer 6

Usually viruses - such as Measles or Parvo B19

Rash will usually be widespread and macular (flat)

Question 7

A skin rash can be caused by antibody-antigen deposition

What is the mechanism for this?

Answer 7

Antibodies bind to and neutralise a virus

These antibody-antigen lattices can end up cross-linking forming longer chains

These longer complexes can end up being stuck in the small capillaries of the skin, presenting as a skin rash

Question 8

A skin rash can be caused by systemic spread

What is the mechanism for this?

Answer 8

A pathogen such as a bacteria, can enter the body through another means e.g respiratory tract

From there, it can enter the bloodstream

1. It can then end up deposited in the smaller capillaries of the skin presenting as a rash

or

2. It can stimulate an immune response, resulting in abnormal coagulation, abnormal platelet function, and endothelial dysfunction - e.g Meningococcal infection. This presents as a purpuric rash

Question 9

A rash can be toxin mediated

What is the mechanism for this?

Answer 9

A toxin itself can cause direct damage

The toxin can also act as an immune super-stimulator

An over-zealous immune response to this super-stimulator results in dysregulated host response, which can cause organ dysfunction. This is due to mass release of various interleukins

Question 10

What are the 4 main origins of a skin rash?

Answer 10

1. Direct invasion
2. Antibody-antigen
3. Systemic spread
4. Toxin-mediated

Note an infection can sometimes have multiple ways it causes a rash.

For example Staphylococcus aureus can enter through the skin, causing a cellulitis at the entry site.
It can spread in the bloodstream, and deposit on the heart valves causing endocarditis.
This can then throw off micro-emboli, which are deposited in the skin