It’s playbook in mTBI cases for defense doctors to highlight: “cranial nerves grossly intact.”

The cranial nerve exam primarily evaluates:

• Brainstem integrity

• Focal peripheral nerve function

• Gross sensory and motor pathways

It’s very good at detecting:

• Brainstem strokes

• Mass lesions

• Large focal injuries

But here's the problem: Most mTBI usually isn’t a focal brainstem problem. Most mTBIs involve:

• Diffuse axonal injury at a microscopic level

• Metabolic and neurochemical dysfunction

• Network-level disruption

These processes do not interrupt cranial nerve pathways. So the exam is always going to be “normal,” and the nerves will be “grossly intact.”

(Here's a good way to think about it: With mTBI, the wiring is still connected, but the signal timing is off.)

Symptoms caused by mTBI live outside the boundaries of the cranial nerve exam.

Classic mTBI symptoms include:

• Headache

• Dizziness

• Cognitive slowing

• Word-finding difficulty

• Fatigue

• Photophobia

• Noise sensitivity

• Emotional lability

• Sleep disturbance

None of those are connected to or can be evaluated based on:

• Pupillary reactions

• Extraocular movements

• Facial symmetry

• Tongue deviation

• Gag reflex

A patient can pass every cranial nerve test and still struggle to read an email or tolerate grocery-store lighting.

When you hear defense doctors talk about “normal cranial nerve exam,” think about the term “category error.”

(You probably already know this, but a category error is a reasoning mistake where someone applies a test, concept, or criterion from one category of things to a completely different category - then treats the mismatch as meaningful.

In other words, defense doctors are answering the wrong kind of question (was there a brainstem injury) and acting like they’ve solved the right one (was there a mild traumatic brain injury).)