Pupillary Evaluation & Differential Diagnosis of Coma

pupil measurement

What is a state of comatose?

The current definition of coma defines it as a state of unarousable unresponsiveness. A patient in an unconscious state cannot be aroused or provoked for a visible or detectable physical response. While patients in deep coma are completely unresponsive, even to pain stimuli, patients under light coma can exhibit protective reflexes under painful or damaging stimuli.

The Glasgow Coma Scale (GCS) is a mathematical measurement of the level or magnitude of comatose a patient is under. This scale is based on neurological findings and is used to define states of altered mentation. Currently, this is the leading scale to diagnose coma patients accurately, and a GCS score of or less than seven is considered the starting threshold of coma.

Its Causes or Aetiology

A variety of disorders can cause coma, but these can be categorized as or attributed to either structural or metabolic origins. This distinction is crucial as neurosurgical interventions can be needed in case of a structural coma. The two aetiologies are listed below in brief detail.

Structural Coma

A structural coma is caused by direct brain injuries that in some way or the other distort or damage the ascending arousal system, which is responsible for arousal and awareness in the human body. The two ways in which the ascending arousal system is harmed to the point of coma are:

  • Compressive lesionscause compression or displacement of the concerned brain tissues.
  • Destructive lesionscause direct damage to the brain tissues concerned.

Metabolic Coma

A metabolic coma is caused by a metabolic encephalopathy which causes global cerebral dysfunction- encompassing several clinical disorders. Some common metabolic causes for a coma include:

  • Hypoglycemia
  • Hyperglycemia
  • Excessive alcohol intake
  • Medication overdose
  • Illicit drug abuse

What’s a bedside diagnosis?

A bedside diagnosis is an initial diagnosis of the patient based on an initial analysis of the common visible symptoms as exhibited by the patient in concern. These can also be influenced by symptomatic accounts of the patient, which are supposed to enhance this initial analysis further.

How can pupillary evaluation lead to a differential diagnosis of coma?

Pupillary evaluation, such as light reflex or anisocoria, has long been associated with differentiating the etiology of a coma.

A doctor or even a qualified nurse can employ a bedside evaluation to determine whether the pupils are responsive to light to measure pupil size.

A presence or absence of a light reflex in the pupils can go a long way in potentially differentiating between a structural and a metabolic coma. Before an official diagnosis, a basic pupil measurement can reveal whether pupils respond to the light stimulus. This can determine whether their coma is metabolic or structural since, under metabolic disorders, the pupillary pathways remain relatively resistant and function normally.

Suppose the pupil diameter measurement reveals a normal size, shape, and response to light. In that case, the midbrain is still functional and this in turn usually excludes midbrain damage or trauma as the cause of coma. A unilateral, dilated, or unreactive pupil is most commonly a result of compression of the third cranial nerve. Which usually occurs in the case of a transtentorial herniation.

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