Sepsis in the post-splenectomy patient

The asplenic state is a favourite topic of the examiners. Septic complications of asplenia featured in  Question 9 from the first paper of 2013. Question 12 from the second paper of 2008 asked about pneumococcal meningitis in the post-splenectomy patient.

Why do I need a spleen

The functions of the spleen are briefly visited in this 2011 paper, and explored to great depth in this comprehensive review article from Nature.

In brief:

• IgM memory B-cell generation and survival
  • These are responsible for generating pentameric IgM, which facilitates the phagocytosis of encapsulated organisms.
  • There is a large reduction of IgM memory B-cells after removal of the spleen. They rely on it for their survival, hanging out in its marginal zone.
• Antibody synthesis
• Opsonin synthesis
  • Properdin and tuftsin are a couple of the molecules produced by the spleen, which coat the bacterial surfaces and then interact with macrophage receptors
• Phagocytic filter: removes undesirables from the blood stream, including:
  • Opsonised bacteria (though these can also be removed by the liver)
  • Poorly opsonised encapsulated bacteria (eg.), which are not susceptible to opsonisation by complement, are removed by the IgM in the spleen; examples include the following:
    • S.pneumoniae
    • Neisseria meningitidis
    • Haemophilus influenzae (type B)
  • Damaged red blood cells
  • Solid particles from erythrocytes
• Haematopoiesis (though this is mainly a phenomenon of infancy)
• Erythrocyte reserve - about a unit of packed cells worth is stored in the adult spleen, which becomes available in haemorrhage and strenuous exercise. Humans usually do no rely on this as much as some other species; for instance the rainbow trout's spleen can contribute an extra 20% to the circulating RBC volume (shrinking by 70% of its volume as it does so).

Why am I missing a spleen

You probably had it removed.

Reasons for this include:

• Trauma
• Spontaneous rupture
• Infarction
• Intentional surgery (splenic reduction, eg. in thalassaemia)

Alternatively, it may be congenitally absent.

How much spleen do you need for normal phagocytic function? Well. Apparently, 80-90% of an enlarged spleen can be removed without any adverse immunological consequence; the remnants (or heterotopic bits of a fragmented spleen) can regrow in a disorganised fashion around the peritoneum in a process termed "splenosis". It seems only about 30ml of splenic tissue is required for reasonably normal function.

One can also have a normal-looking spleeen and be functionally asplenic. The linked article contains a nice table, listing various differentials:

Causes of Functional Aspenism

Sickle cell disease Malignant histiocytosis Celiac sprue Dermatitis herpetiformis Ulcerative colitis Liver disease Portal hypertension Acute alcoholism Systemic lupus erythematosus Rheumatoid arthritis Graves' disease Polyarteritis nodosa (splenic infarct)

Amyloidosis Sarcoidosis Bartonellosis HIV infection Graft versus host disease Post bone marrow transplantation Total parenteral nutrition Cancer therapy High-dose steroid therapy Splenic irradiation Thorium dioxide administration

In case you were wondering, thorium dioxide was at one stage used as a radiological contrast medium.

The septic complications of asplenia

The asplenic man is prone to infections by encapsulated organsisms.

Again, the usual culprits include the following:

• S.pneumoniae (50-90% of cases)
• Neisseria meningitidis
• Haemophilus influenzae (type B)

Unusual culprits also include:

• Capnocytophaga canimorsus - after a dog bite
• group B streptococci
• Enterococcus species
• Bacteroides species
• Salmonella
• Bartonella
• Plesiomonas shigelloides
• Eubacterium plautii
• Pseudomonas pseudomallei

Key features of this immunocompromised condition:

• The risk of overwhelming sepsis is 50 times greater than that of the general population
• It does not matter how long ago the splenectomy was performed
• The clinical course if very rapid (hours rather than days) and most of the mortality is within the first 24 hours
• Features are usually nonspecific.
• A weird specific complication seems to be Waterhouse-Friderichsen syndrome (bilateral adrenal haemorrhage).

Empirical therapy consists of:

• Ceftriaxone
• Vancomycin
• plus-minus gentamicin
• Dexamethasone is also indicated if there are features of pneumococcal meningitis

Guidelines for vaccination of post-splenectomy patients

The vaccinations prevent severe infection by encapsulated organisms - because the encapsulated organisms are poorly opsonised by complement, and the spleen was the only organ which could remove them. Thus, one must protect this patient from these bugs. There has been a 2011 revision of the guidelines for such prophylaxis:

• The asplenic patients should carry an identifying card
• They should receive the following vaccinations:
  • Pneumococcal vaccination
  •  Haemophilus influenzae type b conjugate vaccine
  • Meningococcal conjugate vaccine (B, C)
  • Influenza immunization
• There may be some role for lifelong prophylactic antibiotics
• The patient should have a supply of antibiotics for emergency use at home

Features of splenectomy on the FBC

• Howel-Jolly bodies
• Anisocytosis
• Thrombocytosis
• Acanthocytosis
• Target cells
• Pappenheimer bodies
• Platelet aggregates