A patient has been suffering from stomach pain for the past six to sev

A patient has been suffering from stomach pain for the past six to seven months. When a scan was done, it was found that he had kidney inflammation and that he had a small stone measuring four millimeters. Now he cannot feel the back of his left thigh. What could be the reason for this?

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kidney stone retroperitoneal posterior femoral cutaneous nerve numbness posterior thigh

I now have enough information to give a comprehensive, well-sourced answer. Let me synthesize everything.

Here is the full clinical explanation:

What Could Cause Loss of Sensation in the Back of the Left Thigh?

This is a fascinating clinical puzzle that ties together the kidney pathology and the neurological symptom. There are two main mechanisms to consider, and one leading explanation that best connects all three findings (chronic abdominal/flank pain, left kidney inflammation with a stone, and numbness in the back of the left thigh).

The Nerve Responsible

The loss of sensation in the back (posterior) of the thigh is supplied by the posterior femoral cutaneous nerve (PFCN), a purely sensory nerve arising from the sacral plexus (S1-S3). It exits the pelvis through the greater sciatic notch and runs down the posterior thigh supplying the skin there, the popliteal fossa, the gluteal fold, and parts of the perineum.

Localization in Clinical Neurology, 8e states: "The posterior femoral cutaneous nerve, a purely sensory nerve, arises from the anterior primary rami of the first through the third sacral segments. It leaves the pelvis through the greater sciatic notch and descends into the buttock... It supplies the skin of the posterior thigh and popliteal fossa."

Most Likely Explanation: Retroperitoneal / Perinephric Inflammation Compressing the Sacral Plexus or PFCN

The kidney inflammation (pyelonephritis or perinephric abscess) combined with the 4 mm stone causing obstruction and a 6-7 month history points to chronic left retroperitoneal inflammation. The left kidney and ureter sit in the retroperitoneal space in close proximity to the lumbosacral plexus, including the sacral plexus roots that give rise to the PFCN.

Mechanisms include:

Retroperitoneal inflammatory mass/perinephric collection compressing the sacral plexus - Chronic inflammation around the kidney can form a phlegmon or collection in the retroperitoneal space that presses on the S1-S3 sacral roots or the PFCN itself as it courses near the greater sciatic notch.
Psoas/iliacus muscle involvement - Retroperitoneal inflammation can track inferiorly into the psoas muscle space. As noted in Bradley and Daroff's Neurology in Clinical Practice: retroperitoneal collections cause plexus compression, and when they extend from iliacus into psoas, "other components of the plexus, the obturator and lateral femoral cutaneous nerves, are involved" - a similar mechanism can involve sacral plexus branches.
Direct ureteral/pelvic inflammation irritating the sacral plexus - The left ureter runs retroperitoneally and in its pelvic portion comes into anatomical proximity with the sacral plexus. Chronic obstruction and inflammation can cause neural irritation or compression.

The Referred Pain / Nerve Pathway Connection

Ureteric colic and renal pain are known to be referred to the groin, scrotum/labia, and inner/posterior thigh via the genitofemoral nerve and shared spinal cord segments (T10-L1, plus contributions from sacral segments). S Das Manual on Clinical Surgery confirms: "Renal colic is referred from the loin to the groin, testis, and inner side of the thigh." However, the posterior thigh specifically implicates the sacral plexus (S1-S3), not just referred ureteral pain.

Other Possibilities to Rule Out

Cause	Why It Fits or Doesn't
PFCN direct compression (prolonged sitting, injections)	Possible but wouldn't explain the kidney findings
Sciatic nerve involvement (piriformis, retroperitoneal mass)	Would also cause weakness + involves more territory
L5-S1 disc herniation	Would cause more than just posterior thigh numbness; no spinal history given
Perinephric/retroperitoneal abscess	Highly plausible - can track along fascial planes to compress sacral roots
Psoas abscess with plexus involvement	A complication of obstructive uropathy with infection

Summary

The most likely explanation is that the chronic left kidney inflammation (likely from obstructive uropathy due to the 4 mm stone, possibly superimposed infection/perinephric inflammation) has created a retroperitoneal inflammatory process that is compressing or irritating the left sacral plexus (S1-S3), specifically the posterior femoral cutaneous nerve, causing numbness in the back of the left thigh.

Key next steps for this patient:

MRI or CT of the abdomen/pelvis to evaluate for perinephric collection, psoas involvement, or retroperitoneal mass compressing the sacral plexus
Urological management of the obstructing stone (ureteroscopy, shock wave lithotripsy, or stenting depending on degree of obstruction)
Nerve conduction studies / EMG of the left lower extremity to localize the neurological lesion
Neurology consultation to exclude primary sacral plexopathy or disc disease
Treating the underlying kidney inflammation/infection may itself resolve the nerve compression once the retroperitoneal inflammation subsides

The fact that the neurological symptom is on the same side (left) as the kidney stone and inflammation strongly supports a unilateral retroperitoneal compressive neuropathy rather than a coincidental spinal or systemic cause.

Can't we cure the chronic kidney inflammation due to the kidney stone

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I now have comprehensive information to give a thorough answer. Here it is:

Yes - Kidney Inflammation From a Stone Is Treatable and Often Curable

The good news: a 4 mm kidney stone causing inflammation is one of the more manageable urological problems. Here is a full breakdown of how it is treated.

Step 1 - Understand Why the Inflammation Is Happening

The 4 mm stone is likely partially obstructing the ureter, causing urine to back up into the kidney (hydronephrosis). This pressure, combined with possible bacterial infection tracking up the blocked ureter, is what causes the kidney inflammation (obstructive pyelonephritis/nephritis). Treat the stone, and the inflammation resolves.

Step 2 - The Stone Itself: Good News for 4 mm

A 4 mm stone is actually in a favorable size range.

According to Goldman-Cecil Medicine: "Kidney stones 5 mm or smaller have about a 70% probability of passing spontaneously." Since this stone is 4 mm, there is a strong chance it can pass on its own - especially if it is in the lower ureter closer to the bladder.

However, this patient has had symptoms for 6-7 months, which is too long for watchful waiting. At this point, intervention is likely needed.

Step 3 - Treatment Options (From Least to Most Invasive)

Option A: Medical Expulsive Therapy (MET)

Tamsulosin (0.4 mg daily) - an alpha-blocker that relaxes the smooth muscle of the ureter, helping the stone pass on its own
Also: tadalafil (10 mg/day) or silodosin (4 mg/day) as alternatives
Goldman-Cecil Medicine states MET "can reduce ureteral spasm and increase spontaneous passage rates by about 50%"
Suitable if: pain is controlled, kidney function is not severely compromised, no serious infection
Not suitable here because 6-7 months is already far beyond the recommended 4-6 week observation window

Option B: Extracorporeal Shock Wave Lithotripsy (ESWL)

External sound waves focused onto the stone break it into tiny fragments that pass in urine
Non-invasive, done as an outpatient
Works best for stones in the kidney or upper ureter
No cuts, no scope inserted into the body

Option C: Ureteroscopy with Laser Lithotripsy (Most Common for This Stone)

A thin flexible scope is passed through the urethra, bladder, and into the ureter
A laser (holmium laser) fragments the stone directly
Goldman-Cecil Medicine recommends: "ureteral stones should be treated with ureteroscopy or shock wave lithotripsy"
Very high success rate for 4 mm stones
Usually done under general or regional anesthesia, often as a day procedure

Option D: Ureteral Stenting

If there is active infection or severe obstruction, a ureteral stent (a thin plastic tube) is placed first to relieve the obstruction and drain the kidney
This resolves the acute inflammation quickly
The stone is then dealt with in a separate procedure once the infection settles

Option E: Percutaneous Nephrolithotomy (PCNL)

A scope is passed directly into the kidney through the back via a small incision
Reserved for large stones (>1 cm), staghorn calculi, or when other methods have failed
Not needed for a 4 mm stone

Step 4 - Treating the Kidney Inflammation Itself

While managing the stone, the inflammation/infection must also be addressed:

Antibiotics - if infection is present (guided by urine culture), typically fluoroquinolones or cephalosporins for 7-14 days
NSAIDs (ibuprofen, ketorolac) - for pain and inflammation; Goldman-Cecil notes these are "as or more effective compared with opiates for renal colic"
Adequate hydration - important to protect kidney function; however, high-volume IV fluids do NOT speed up stone passage
Monitoring kidney function - serum creatinine, eGFR to ensure the inflammation hasn't damaged the kidney permanently

Step 5 - Preventing Recurrence (Long-Term Cure)

Once the stone is removed and inflammation resolves, the goal is preventing new stones:

Stone Type	Prevention Strategy
Calcium oxalate (most common)	Drink 2-3 L water/day, reduce salt and animal protein, potassium citrate supplements
Uric acid stones	Alkalinize urine (potassium citrate to pH 6.5-7.0), low purine diet, allopurinol if needed
Struvite (infection stones)	Full surgical removal + long-term antibiotics
Cystine stones	High fluid intake, alkalinize urine, chelating agents

Goldman-Cecil Medicine notes: "The recurrence rate of calcium oxalate nephrolithiasis is about 25% at 3-5 years and 50% at 5-10 years" - so prevention is important.

What This Patient Specifically Needs

Given 6-7 months of symptoms + confirmed kidney inflammation + 4 mm stone:

Urgent urology consultation - the chronic duration means intervention is no longer optional
CT urogram or KUB X-ray to locate the stone precisely and plan treatment
Urine culture to check for infection
Antibiotics if infected + possible ureteral stenting to drain the kidney
Ureteroscopy with laser lithotripsy - most likely definitive treatment
Once stone is removed, the kidney inflammation will heal, and with time the nerve compression causing the posterior thigh numbness may also improve

The prognosis is very good. A 4 mm stone treated promptly with ureteroscopy has a success rate exceeding 90%, and the kidney inflammation typically resolves within days to weeks of relieving the obstruction and clearing any infection.

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