College Answer

Indications
1.  Any doubt about the primary diagnosis of the cause of coma.
2.  Possible drug or metabolic cause of coma.
3.  Cranial nerves can not be tested adequately e.g. periorbital oedema, eye injuries, ruptured tympanum
4.  Apnoea test can not be performed e.g. cervical cord injury, cardiorespiratory instability.
5.  confirmation of brain death in some countries (not ANZ)

Advantages
•    Highly specific
•    Does not require preconditions as for clinical testing –ie patient can be cold, hypoxic, sedated, undiagnosed, etc
•    Can be done at the bedside – if portable gamma camera
•    Safe – non-toxic marker (Te99m  HMPAO) can be delivered via peripheral vein
•    Quick – answer can be given within 30 minutes
•    Provides a hard copy – clear permanent documentation of brain death

Disadvantages

•    Requires specialized equipment, marker and staff (nuclear medicine specialist) usually only available in major centres
•    Requires patient transport – if no portable camera
•    Can show minimal flow ( e.g. from meningeal vessels), cannot easily be repeated, and not very soon after first test

Discussion

This question refers specifically to the Tc-99 HMPAO SPECT scan, which (after the four-vessel DSA) is viewed by the ANZICS Statement on Death and Organ Donation as the second best way of confirming that there is no blood flow to the noggin.

The indications for imaging to confirm brain death, broadly, are as follows:

• Inability to perform clinical brain death testing
  • No access to at least one eye and one ear
  • Severe hypoxia
  • Hemodynamic instability
  • High spinal cord injury
• Presence of persisting confounding factors which invalidate the clinical testing for brain death (eg. severe metabolic disturbances, organ system dysfunction)
• Absence of a clear cause for the coma, which - for the puspose of braindeath testing - is defined as "sufficient intracranial pathology" to explain brain death.

The indications for choosing a radionuclide scan, rather than a DSA, may be as follows:

• Allergy to IV contrast
• Injury to the carotids or vertebral arteries, which precluded fluoroscopic access

Advantages of the HMPAO-SPECT:

• Equivalent to DSA in terms of false positive rate (0%)
• Does not require the precodnitions for brain death to be met
• Rapid return of results
• Safe non-toxic contrast agent
• Visually effective representation of absent brain perfusion

Disadvantages of HMPAO-SPECT:

• Expensive
• Requires specialised equipment
• Not available in all but the best-equipped centers
• Portable gamma-cameras are infrequently available, exposing the patient to the risk of transport.
• In very young infants with open cranial sutures, HMPAO-SPECT may return false positive results if used in a single imaging plane.
• In adults, it may return false positives due to minimal flow in meningeal vessels.